diff mbox series

[RFC] mm, slub: change percpu partial accounting from objects to pages

Message ID 20210913170148.10992-1-vbabka@suse.cz (mailing list archive)
State New
Headers show
Series [RFC] mm, slub: change percpu partial accounting from objects to pages | expand

Commit Message

Vlastimil Babka Sept. 13, 2021, 5:01 p.m. UTC
With CONFIG_SLUB_CPU_PARTIAL enabled, SLUB keeps a percpu list of partial
slabs that can be promoted to cpu slab when the previous one is depleted,
without accessing the shared partial list. A slab can be added to this list
by 1) refill of an empty list from get_partial_node() - once we really have to
access the shared partial list, we acquire multiple slabs to amortize the cost
of locking, and 2) first free to a previously full slab - instead of putting
the slab on a shared partial list, we can more cheaply freeze it and put it on
the per-cpu list.

To control how large a percpu partial list can grow for a kmem cache,
set_cpu_partial() calculates a target number of free objects on each cpu's
percpu partial list, and this can be also set by the sysfs file cpu_partial.

However, the tracking of actual number of objects is imprecise, in order to
limit overhead from cpu X freeing an objects to a slab on percpu partial
list of cpu Y. Basically, the percpu partial slabs form a single linked list,
and when we add a new slab to the list with current head "oldpage", we set in
the struct page of the slab we're adding:

page->pages = oldpage->pages + 1; // this is precise
page->pobjects = oldpage->pobjects + (page->objects - page->inuse);
page->next = oldpage;

Thus the real number of free objects in the slab (objects - inuse) is only
determined at the moment of adding the slab to the percpu partial list, and
further freeing doesn't update the pobjects counter nor propagate it to the
current list head. As Jann reports [1], this can easily lead to large
inaccuracies, where the target number of objects (up to 30 by default) can
translate to the same number of (empty) slab pages on the list. In case 2)
above, we put a slab with 1 free object on the list, thus only increase
page->pobjects by 1, even if there are subsequent frees on the same slab. Jann
has noticed this in practice and so did we [2] when investigating significant
increase of kmemcg usage after switching from SLAB to SLUB.

While this is no longer a problem in kmemcg context thanks to the accounting
rewrite in 5.9, the memory waste is still not ideal and it's questionable
whether it makes sense to perform free object count based control when object
counts can easily become so much inaccurate. So this patch converts the
accounting to be based on number of pages only (which is precise) and removes
the page->pobjects field completely. This is also ultimately simpler.

To retain the existing set_cpu_partial() heuristic, first calculate the target
number of objects as previously, but then convert it to target number of pages
by assuming the pages will be half-filled on average. This assumption might
obviously also be inaccurate in practice, but cannot degrade to actual number of
pages being equal to the target number of objects.

We could also skip the intermediate step with target number of objects and
rewrite the heuristic in terms of pages. However we still have the sysfs file
cpu_partial which uses number of objects and could break existing users if it
suddenly becomes number of pages, so this patch doesn't do that.

In practice, after this patch the heuristics limit the size of percpu partial
list up to 2 pages. In case of a reported regression (which would mean some
workload has benefited from the previous imprecise object based counting), we
can tune the heuristics to get a better compromise within the new scheme, while
still avoid the unexpectedly long percpu partial lists.

[1] https://lore.kernel.org/linux-mm/CAG48ez2Qx5K1Cab-m8BdSibp6wLTip6ro4=-umR7BLsEgjEYzA@mail.gmail.com/
[2] https://lore.kernel.org/all/2f0f46e8-2535-410a-1859-e9cfa4e57c18@suse.cz/

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 include/linux/mm_types.h |  2 -
 include/linux/slub_def.h | 13 +-----
 mm/slub.c                | 89 ++++++++++++++++++++++++++--------------
 3 files changed, 61 insertions(+), 43 deletions(-)

Comments

David Rientjes Sept. 15, 2021, 5:32 a.m. UTC | #1
On Mon, 13 Sep 2021, Vlastimil Babka wrote:

> With CONFIG_SLUB_CPU_PARTIAL enabled, SLUB keeps a percpu list of partial
> slabs that can be promoted to cpu slab when the previous one is depleted,
> without accessing the shared partial list. A slab can be added to this list
> by 1) refill of an empty list from get_partial_node() - once we really have to
> access the shared partial list, we acquire multiple slabs to amortize the cost
> of locking, and 2) first free to a previously full slab - instead of putting
> the slab on a shared partial list, we can more cheaply freeze it and put it on
> the per-cpu list.
> 
> To control how large a percpu partial list can grow for a kmem cache,
> set_cpu_partial() calculates a target number of free objects on each cpu's
> percpu partial list, and this can be also set by the sysfs file cpu_partial.
> 
> However, the tracking of actual number of objects is imprecise, in order to
> limit overhead from cpu X freeing an objects to a slab on percpu partial
> list of cpu Y. Basically, the percpu partial slabs form a single linked list,
> and when we add a new slab to the list with current head "oldpage", we set in
> the struct page of the slab we're adding:
> 
> page->pages = oldpage->pages + 1; // this is precise
> page->pobjects = oldpage->pobjects + (page->objects - page->inuse);
> page->next = oldpage;
> 
> Thus the real number of free objects in the slab (objects - inuse) is only
> determined at the moment of adding the slab to the percpu partial list, and
> further freeing doesn't update the pobjects counter nor propagate it to the
> current list head. As Jann reports [1], this can easily lead to large
> inaccuracies, where the target number of objects (up to 30 by default) can
> translate to the same number of (empty) slab pages on the list. In case 2)
> above, we put a slab with 1 free object on the list, thus only increase
> page->pobjects by 1, even if there are subsequent frees on the same slab. Jann
> has noticed this in practice and so did we [2] when investigating significant
> increase of kmemcg usage after switching from SLAB to SLUB.
> 
> While this is no longer a problem in kmemcg context thanks to the accounting
> rewrite in 5.9, the memory waste is still not ideal and it's questionable
> whether it makes sense to perform free object count based control when object
> counts can easily become so much inaccurate. So this patch converts the
> accounting to be based on number of pages only (which is precise) and removes
> the page->pobjects field completely. This is also ultimately simpler.
> 

Thanks for the very detailed explanation, this is very timely for us.

I'm wondering if we should be concerned about the memory waste even being 
possible, though, now that we have the kmemcg accounting change?

IIUC, because we're accounting objects and not pages, then it *seems* like 
we could have a high number of pages but very few objects charged per 
page so this memory waste could go unconstrained from any kmemcg 
limitation.

> To retain the existing set_cpu_partial() heuristic, first calculate the target
> number of objects as previously, but then convert it to target number of pages
> by assuming the pages will be half-filled on average. This assumption might
> obviously also be inaccurate in practice, but cannot degrade to actual number of
> pages being equal to the target number of objects.
> 

I think that's a fair heuristic.

> We could also skip the intermediate step with target number of objects and
> rewrite the heuristic in terms of pages. However we still have the sysfs file
> cpu_partial which uses number of objects and could break existing users if it
> suddenly becomes number of pages, so this patch doesn't do that.
> 
> In practice, after this patch the heuristics limit the size of percpu partial
> list up to 2 pages. In case of a reported regression (which would mean some
> workload has benefited from the previous imprecise object based counting), we
> can tune the heuristics to get a better compromise within the new scheme, while
> still avoid the unexpectedly long percpu partial lists.
> 

Curious if you've tried netperf TCP_RR with this change?  This benchmark 
was the most significantly improved benchmark that I recall with the 
introduction of per-cpu partial slabs for SLUB.  If there are any 
regressions to be introduced by such an approach, I'm willing to bet that 
it would be surfaced with that benchmark.

Let me know if we can help in benchmarking.

> [1] https://lore.kernel.org/linux-mm/CAG48ez2Qx5K1Cab-m8BdSibp6wLTip6ro4=-umR7BLsEgjEYzA@mail.gmail.com/
> [2] https://lore.kernel.org/all/2f0f46e8-2535-410a-1859-e9cfa4e57c18@suse.cz/
> 
> Reported-by: Jann Horn <jannh@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  include/linux/mm_types.h |  2 -
>  include/linux/slub_def.h | 13 +-----
>  mm/slub.c                | 89 ++++++++++++++++++++++++++--------------
>  3 files changed, 61 insertions(+), 43 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 7f8ee09c711f..68ffa064b7a8 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -124,10 +124,8 @@ struct page {
>  					struct page *next;
>  #ifdef CONFIG_64BIT
>  					int pages;	/* Nr of pages left */
> -					int pobjects;	/* Approximate count */
>  #else
>  					short int pages;
> -					short int pobjects;
>  #endif
>  				};
>  			};
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index 85499f0586b0..0fa751b946fa 100644
> --- a/include/linux/slub_def.h
> +++ b/include/linux/slub_def.h
> @@ -99,6 +99,8 @@ struct kmem_cache {
>  #ifdef CONFIG_SLUB_CPU_PARTIAL
>  	/* Number of per cpu partial objects to keep around */
>  	unsigned int cpu_partial;
> +	/* Number of per cpu partial pages to keep around */
> +	unsigned int cpu_partial_pages;
>  #endif
>  	struct kmem_cache_order_objects oo;
>  
> @@ -141,17 +143,6 @@ struct kmem_cache {
>  	struct kmem_cache_node *node[MAX_NUMNODES];
>  };
>  
> -#ifdef CONFIG_SLUB_CPU_PARTIAL
> -#define slub_cpu_partial(s)		((s)->cpu_partial)
> -#define slub_set_cpu_partial(s, n)		\
> -({						\
> -	slub_cpu_partial(s) = (n);		\
> -})
> -#else
> -#define slub_cpu_partial(s)		(0)
> -#define slub_set_cpu_partial(s, n)
> -#endif /* CONFIG_SLUB_CPU_PARTIAL */
> -
>  #ifdef CONFIG_SYSFS
>  #define SLAB_SUPPORTS_SYSFS
>  void sysfs_slab_unlink(struct kmem_cache *);
> diff --git a/mm/slub.c b/mm/slub.c
> index 3d2025f7163b..3757f31c5d97 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -414,6 +414,29 @@ static inline unsigned int oo_objects(struct kmem_cache_order_objects x)
>  	return x.x & OO_MASK;
>  }
>  
> +#ifdef CONFIG_SLUB_CPU_PARTIAL
> +static void slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
> +{
> +	unsigned int nr_pages;
> +
> +	s->cpu_partial = nr_objects;
> +
> +	/*
> +	 * We take the number of objects but actually limit the number of
> +	 * pages on the per cpu partial list, in order to limit excessive
> +	 * growth of the list. For simplicity we assume that the pages will
> +	 * be half-full.
> +	 */
> +	nr_pages = DIV_ROUND_UP(nr_objects * 2, oo_objects(s->oo));
> +	s->cpu_partial_pages = nr_pages;
> +}
> +#else
> +static inline void
> +slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
> +{
> +}
> +#endif /* CONFIG_SLUB_CPU_PARTIAL */
> +
>  /*
>   * Per slab locking using the pagelock
>   */
> @@ -2045,7 +2068,7 @@ static inline void remove_partial(struct kmem_cache_node *n,
>   */
>  static inline void *acquire_slab(struct kmem_cache *s,
>  		struct kmem_cache_node *n, struct page *page,
> -		int mode, int *objects)
> +		int mode)
>  {
>  	void *freelist;
>  	unsigned long counters;
> @@ -2061,7 +2084,6 @@ static inline void *acquire_slab(struct kmem_cache *s,
>  	freelist = page->freelist;
>  	counters = page->counters;
>  	new.counters = counters;
> -	*objects = new.objects - new.inuse;
>  	if (mode) {
>  		new.inuse = page->objects;
>  		new.freelist = NULL;
> @@ -2099,9 +2121,8 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
>  {
>  	struct page *page, *page2;
>  	void *object = NULL;
> -	unsigned int available = 0;
>  	unsigned long flags;
> -	int objects;
> +	unsigned int partial_pages = 0;
>  
>  	/*
>  	 * Racy check. If we mistakenly see no partial slabs then we
> @@ -2119,11 +2140,10 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
>  		if (!pfmemalloc_match(page, gfpflags))
>  			continue;
>  
> -		t = acquire_slab(s, n, page, object == NULL, &objects);
> +		t = acquire_slab(s, n, page, object == NULL);
>  		if (!t)
>  			break;
>  
> -		available += objects;
>  		if (!object) {
>  			*ret_page = page;
>  			stat(s, ALLOC_FROM_PARTIAL);
> @@ -2131,10 +2151,15 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
>  		} else {
>  			put_cpu_partial(s, page, 0);
>  			stat(s, CPU_PARTIAL_NODE);
> +			partial_pages++;
>  		}
> +#ifdef CONFIG_SLUB_CPU_PARTIAL
>  		if (!kmem_cache_has_cpu_partial(s)
> -			|| available > slub_cpu_partial(s) / 2)
> +			|| partial_pages > s->cpu_partial_pages / 2)
>  			break;
> +#else
> +		break;
> +#endif
>  
>  	}
>  	spin_unlock_irqrestore(&n->list_lock, flags);
> @@ -2539,14 +2564,13 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>  	struct page *page_to_unfreeze = NULL;
>  	unsigned long flags;
>  	int pages = 0;
> -	int pobjects = 0;
>  
>  	local_lock_irqsave(&s->cpu_slab->lock, flags);
>  
>  	oldpage = this_cpu_read(s->cpu_slab->partial);
>  
>  	if (oldpage) {
> -		if (drain && oldpage->pobjects > slub_cpu_partial(s)) {
> +		if (drain && oldpage->pages >= s->cpu_partial_pages) {
>  			/*
>  			 * Partial array is full. Move the existing set to the
>  			 * per node partial list. Postpone the actual unfreezing
> @@ -2555,16 +2579,13 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>  			page_to_unfreeze = oldpage;
>  			oldpage = NULL;
>  		} else {
> -			pobjects = oldpage->pobjects;
>  			pages = oldpage->pages;
>  		}
>  	}
>  
>  	pages++;
> -	pobjects += page->objects - page->inuse;
>  
>  	page->pages = pages;
> -	page->pobjects = pobjects;
>  	page->next = oldpage;
>  
>  	this_cpu_write(s->cpu_slab->partial, page);
> @@ -3980,6 +4001,8 @@ static void set_min_partial(struct kmem_cache *s, unsigned long min)
>  static void set_cpu_partial(struct kmem_cache *s)
>  {
>  #ifdef CONFIG_SLUB_CPU_PARTIAL
> +	unsigned int nr_objects;
> +
>  	/*
>  	 * cpu_partial determined the maximum number of objects kept in the
>  	 * per cpu partial lists of a processor.
> @@ -3989,24 +4012,22 @@ static void set_cpu_partial(struct kmem_cache *s)
>  	 * filled up again with minimal effort. The slab will never hit the
>  	 * per node partial lists and therefore no locking will be required.
>  	 *
> -	 * This setting also determines
> -	 *
> -	 * A) The number of objects from per cpu partial slabs dumped to the
> -	 *    per node list when we reach the limit.
> -	 * B) The number of objects in cpu partial slabs to extract from the
> -	 *    per node list when we run out of per cpu objects. We only fetch
> -	 *    50% to keep some capacity around for frees.
> +	 * For backwards compatibility reasons, this is determined as number
> +	 * of objects, even though we now limit maximum number of pages, see
> +	 * slub_set_cpu_partial()
>  	 */
>  	if (!kmem_cache_has_cpu_partial(s))
> -		slub_set_cpu_partial(s, 0);
> +		nr_objects = 0;
>  	else if (s->size >= PAGE_SIZE)
> -		slub_set_cpu_partial(s, 2);
> +		nr_objects = 2;
>  	else if (s->size >= 1024)
> -		slub_set_cpu_partial(s, 6);
> +		nr_objects = 6;
>  	else if (s->size >= 256)
> -		slub_set_cpu_partial(s, 13);
> +		nr_objects = 13;
>  	else
> -		slub_set_cpu_partial(s, 30);
> +		nr_objects = 30;
> +
> +	slub_set_cpu_partial(s, nr_objects);
>  #endif
>  }
>  
> @@ -5379,7 +5400,12 @@ SLAB_ATTR(min_partial);
>  
>  static ssize_t cpu_partial_show(struct kmem_cache *s, char *buf)
>  {
> -	return sysfs_emit(buf, "%u\n", slub_cpu_partial(s));
> +	unsigned int nr_partial = 0;
> +#ifdef CONFIG_SLUB_CPU_PARTIAL
> +	nr_partial = s->cpu_partial;
> +#endif
> +
> +	return sysfs_emit(buf, "%u\n", nr_partial);
>  }
>  
>  static ssize_t cpu_partial_store(struct kmem_cache *s, const char *buf,
> @@ -5450,12 +5476,12 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
>  
>  		page = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));
>  
> -		if (page) {
> +		if (page)
>  			pages += page->pages;
> -			objects += page->pobjects;
> -		}
>  	}
>  
> +	/* Approximate half-full pages , see slub_set_cpu_partial() */
> +	objects = (pages * oo_objects(s->oo)) / 2;
>  	len += sysfs_emit_at(buf, len, "%d(%d)", objects, pages);
>  
>  #ifdef CONFIG_SMP
> @@ -5463,9 +5489,12 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
>  		struct page *page;
>  
>  		page = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));
> -		if (page)
> +		if (page) {
> +			pages = READ_ONCE(page->pages);
> +			objects = (pages * oo_objects(s->oo)) / 2;
>  			len += sysfs_emit_at(buf, len, " C%d=%d(%d)",
> -					     cpu, page->pobjects, page->pages);
> +					     cpu, objects, pages);
> +		}
>  	}
>  #endif
>  	len += sysfs_emit_at(buf, len, "\n");
> -- 
> 2.33.0
> 
>
Vlastimil Babka Sept. 15, 2021, 8:42 a.m. UTC | #2
On 9/15/21 07:32, David Rientjes wrote:
> On Mon, 13 Sep 2021, Vlastimil Babka wrote:
> 
>> While this is no longer a problem in kmemcg context thanks to the accounting
>> rewrite in 5.9, the memory waste is still not ideal and it's questionable
>> whether it makes sense to perform free object count based control when object
>> counts can easily become so much inaccurate. So this patch converts the
>> accounting to be based on number of pages only (which is precise) and removes
>> the page->pobjects field completely. This is also ultimately simpler.
>> 
> 
> Thanks for the very detailed explanation, this is very timely for us.
> 
> I'm wondering if we should be concerned about the memory waste even being 
> possible, though, now that we have the kmemcg accounting change?
> 
> IIUC, because we're accounting objects and not pages, then it *seems* like 
> we could have a high number of pages but very few objects charged per 
> page so this memory waste could go unconstrained from any kmemcg 
> limitation.

So the main problem before 5.9 was that there were separate kmem caches per
memcg with their own percpu partial lists, so the memory used was determined
by caches x cpus x memcgs, now they are shared so it's just caches x cpus.
What you're saying would be also true, but relatively much smaller issue
than what it was before 5.9.

>> To retain the existing set_cpu_partial() heuristic, first calculate the target
>> number of objects as previously, but then convert it to target number of pages
>> by assuming the pages will be half-filled on average. This assumption might
>> obviously also be inaccurate in practice, but cannot degrade to actual number of
>> pages being equal to the target number of objects.
>> 
> 
> I think that's a fair heuristic.
> 
>> We could also skip the intermediate step with target number of objects and
>> rewrite the heuristic in terms of pages. However we still have the sysfs file
>> cpu_partial which uses number of objects and could break existing users if it
>> suddenly becomes number of pages, so this patch doesn't do that.
>> 
>> In practice, after this patch the heuristics limit the size of percpu partial
>> list up to 2 pages. In case of a reported regression (which would mean some
>> workload has benefited from the previous imprecise object based counting), we
>> can tune the heuristics to get a better compromise within the new scheme, while
>> still avoid the unexpectedly long percpu partial lists.
>> 
> 
> Curious if you've tried netperf TCP_RR with this change?  This benchmark 
> was the most significantly improved benchmark that I recall with the 
> introduction of per-cpu partial slabs for SLUB.  If there are any 
> regressions to be introduced by such an approach, I'm willing to bet that 
> it would be surfaced with that benchmark.

I'll try, thanks for the tip.
Vlastimil Babka Oct. 1, 2021, 12:35 p.m. UTC | #3
On 9/15/21 07:32, David Rientjes wrote:
> On Mon, 13 Sep 2021, Vlastimil Babka wrote:
>> We could also skip the intermediate step with target number of objects and
>> rewrite the heuristic in terms of pages. However we still have the sysfs file
>> cpu_partial which uses number of objects and could break existing users if it
>> suddenly becomes number of pages, so this patch doesn't do that.
>>
>> In practice, after this patch the heuristics limit the size of percpu partial
>> list up to 2 pages. In case of a reported regression (which would mean some
>> workload has benefited from the previous imprecise object based counting), we
>> can tune the heuristics to get a better compromise within the new scheme, while
>> still avoid the unexpectedly long percpu partial lists.
>>
> 
> Curious if you've tried netperf TCP_RR with this change?  This benchmark 
> was the most significantly improved benchmark that I recall with the 
> introduction of per-cpu partial slabs for SLUB.  If there are any 
> regressions to be introduced by such an approach, I'm willing to bet that 
> it would be surfaced with that benchmark.
> 
> Let me know if we can help in benchmarking.

Mel was kind enough to run it through mmtests machinery for netperf
(localhost) and hackbench and I finally processed the results, see
below. So there are some apparent regressions, especially with
hackbench, which I think ultimately boils down to having shorter percpu
partial lists on average and some benchmarks benefiting from longer
ones. Monitoring slab usage also indicated less memory usage by slab. So
maybe I could try bumping the defaults somewhat as part of the patch,
but certainly not such that we would limit the percpu partial lists to
30 pages just because previously a specific alloc/free pattern could
lead to the limit of 30 objects translate to a limit to 30 pages - that
would make little sense. This is a correctness patch, and if a workload
benefits from larger lists, the sysfs tuning knobs are still there to use.

Netperf

2-socket Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz (20 cores, 40 threads
per socket), 384GB RAM
TCP-RR:
hmean before 127045.79 after 121092.94 (-4.69%, worse)
stddev before  2634.37 after   1254.08
UDP-RR:
hmean before 166985.45 after 160668.94 ( -3.78%, worse)
stddev before 4059.69 after 1943.63

2-socket Intel(R) Xeon(R) CPU E5-2698 v4 @ 2.20GHz (20 cores, 40 threads
per socket), 512GB RAM
TCP-RR:
hmean before 84173.25 after 76914.72 ( -8.62%, worse)
UDP-RR:
hmean before 93571.12 after 96428.69 ( 3.05%, better)
stddev before 23118.54 after 16828.14

2-socket Intel(R) Xeon(R) CPU E5-2670 v3 @ 2.30GHz (12 cores, 24 threads
per socket), 64GB RAM
TCP-RR:
hmean before 49984.92 after 48922.27 ( -2.13%, worse)
stddev before 6248.15 after 4740.51
UDP-RR:
hmean before 61854.31 after 68761.81 ( 11.17%, better)
stddev before 4093.54 after 5898.91

other machines - within 2%

Hackbench

2-socket AMD EPYC 7713 (64 cores, 128 threads per core), 256GB RAM
hackbench-process-sockets (less is worse)
Amean 	1 	0.5380	0.5583	( -3.78%)
Amean 	4 	0.7510	0.8150	( -8.52%)
Amean 	7 	0.7930	0.9533	( -20.22%)
Amean 	12 	0.7853	1.1313	( -44.06%)
Amean 	21 	1.1520	1.4993	( -30.15%)
Amean 	30 	1.6223	1.9237	( -18.57%)
Amean 	48 	2.6767	2.9903	( -11.72%)
Amean 	79 	4.0257	5.1150	( -27.06%)
Amean 	110	5.5193	7.4720	( -35.38%)
Amean 	141	7.2207	9.9840	( -38.27%)
Amean 	172	8.4770	12.1963	( -43.88%)
Amean 	203	9.6473	14.3137	( -48.37%)
Amean 	234	11.3960	18.7917	( -64.90%)
Amean 	265	13.9627	22.4607	( -60.86%)
Amean 	296	14.9163	26.0483	( -74.63%)

hackbench-thread-sockets (less is worse)
Amean 	1 	0.5597	0.5877	( -5.00%)
Amean 	4 	0.7913	0.8960	( -13.23%)
Amean 	7 	0.8190	1.0017	( -22.30%)
Amean 	12 	0.9560	1.1727	( -22.66%)
Amean 	21 	1.7587	1.5660	( 10.96%)
Amean 	30 	2.4477	1.9807	( 19.08%)
Amean 	48 	3.4573	3.0630	( 11.41%)
Amean 	79 	4.7903	5.1733	( -8.00%)
Amean 	110	6.1370	7.4220	( -20.94%)
Amean 	141	7.5777	9.2617	( -22.22%)
Amean 	172	9.2280	11.0907	( -20.18%)
Amean 	203	10.2793	13.3470	( -29.84%)
Amean 	234	11.2410	17.1070	( -52.18%)
Amean 	265	12.5970	23.3323	( -85.22%)
Amean 	296	17.1540	24.2857	( -41.57%)

2-socket Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz (20 cores, 40 threads
per socket), 384GB RAM
hackbench-process-sockets (less is worse)
Amean 	1 	0.5760	0.4793	( 16.78%)
Amean 	4 	0.9430	0.9707	( -2.93%)
Amean 	7 	1.5517	1.8843	( -21.44%)
Amean 	12 	2.4903	2.7267	( -9.49%)
Amean 	21 	3.9560	4.2877	( -8.38%)
Amean 	30 	5.4613	5.8343	( -6.83%)
Amean 	48 	8.5337	9.2937	( -8.91%)
Amean 	79 	14.0670	15.2630	( -8.50%)
Amean 	110	19.2253	21.2467	( -10.51%)
Amean 	141	23.7557	25.8550	( -8.84%)
Amean 	172	28.4407	29.7603	( -4.64%)
Amean 	203	33.3407	33.9927	( -1.96%)
Amean 	234	38.3633	39.1150	( -1.96%)
Amean 	265	43.4420	43.8470	( -0.93%)
Amean 	296	48.3680	48.9300	( -1.16%)

hackbench-thread-sockets (less is worse)
Amean 	1 	0.6080	0.6493	( -6.80%)
Amean 	4 	1.0000	1.0513	( -5.13%)
Amean 	7 	1.6607	2.0260	( -22.00%)
Amean 	12 	2.7637	2.9273	( -5.92%)
Amean 	21 	5.0613	4.5153	( 10.79%)
Amean 	30 	6.3340	6.1140	( 3.47%)
Amean 	48 	9.0567	9.5577	( -5.53%)
Amean 	79 	14.5657	15.7983	( -8.46%)
Amean 	110	19.6213	21.6333	( -10.25%)
Amean 	141	24.1563	26.2697	( -8.75%)
Amean 	172	28.9687	30.2187	( -4.32%)
Amean 	203	33.9763	34.6970	( -2.12%)
Amean 	234	38.8647	39.3207	( -1.17%)
Amean 	265	44.0813	44.1507	( -0.16%)
Amean 	296	49.2040	49.4330	( -0.47%)

2-socket Intel(R) Xeon(R) CPU E5-2698 v4 @ 2.20GHz (20 cores, 40 threads
per socket), 512GB RAM
hackbench-process-sockets (less is worse)
Amean 	1 	0.5027	0.5017	( 0.20%)
Amean 	4 	1.1053	1.2033	( -8.87%)
Amean 	7 	1.8760	2.1820	( -16.31%)
Amean 	12 	2.9053	3.1810	( -9.49%)
Amean 	21 	4.6777	4.9920	( -6.72%)
Amean 	30 	6.5180	6.7827	( -4.06%)
Amean 	48 	10.0710	10.5227	( -4.48%)
Amean 	79 	16.4250	17.5053	( -6.58%)
Amean 	110	22.6203	24.4617	( -8.14%)
Amean 	141	28.0967	31.0363	( -10.46%)
Amean 	172	34.4030	36.9233	( -7.33%)
Amean 	203	40.5933	43.0850	( -6.14%)
Amean 	234	46.6477	48.7220	( -4.45%)
Amean 	265	53.0530	53.9597	( -1.71%)
Amean 	296	59.2760	59.9213	( -1.09%)

hackbench-thread-sockets (less is worse)
Amean 	1 	0.5363	0.5330	( 0.62%)
Amean 	4 	1.1647	1.2157	( -4.38%)
Amean 	7 	1.9237	2.2833	( -18.70%)
Amean 	12 	2.9943	3.3110	( -10.58%)
Amean 	21 	4.9987	5.1880	( -3.79%)
Amean 	30 	6.7583	7.0043	( -3.64%)
Amean 	48 	10.4547	10.8353	( -3.64%)
Amean 	79 	16.6707	17.6790	( -6.05%)
Amean 	110	22.8207	24.4403	( -7.10%)
Amean 	141	28.7090	31.0533	( -8.17%)
Amean 	172	34.9387	36.8260	( -5.40%)
Amean 	203	41.1567	43.0450	( -4.59%)
Amean 	234	47.3790	48.5307	( -2.43%)
Amean 	265	53.9543	54.6987	( -1.38%)
Amean 	296	60.0820	60.2163	( -0.22%)

1-socket Intel(R) Xeon(R) CPU E3-1240 v5 @ 3.50GHz (4 cores, 8 threads),
32 GB RAM
hackbench-process-sockets (less is worse)
Amean 	1 	1.4760	1.5773	( -6.87%)
Amean 	3 	3.9370	4.0910	( -3.91%)
Amean 	5 	6.6797	6.9357	( -3.83%)
Amean 	7 	9.3367	9.7150	( -4.05%)
Amean 	12	15.7627	16.1400	( -2.39%)
Amean 	18	23.5360	23.6890	( -0.65%)
Amean 	24	31.0663	31.3137	( -0.80%)
Amean 	30	38.7283	39.0037	( -0.71%)
Amean 	32	41.3417	41.6097	( -0.65%)

hackbench-thread-sockets (less is worse)
Amean 	1 	1.5250	1.6043	( -5.20%)
Amean 	3 	4.0897	4.2603	( -4.17%)
Amean 	5 	6.7760	7.0933	( -4.68%)
Amean 	7 	9.4817	9.9157	( -4.58%)
Amean 	12	15.9610	16.3937	( -2.71%)
Amean 	18	23.9543	24.3417	( -1.62%)
Amean 	24	31.4400	31.7217	( -0.90%)
Amean 	30	39.2457	39.5467	( -0.77%)
Amean 	32	41.8267	42.1230	( -0.71%)

2-socket Intel(R) Xeon(R) CPU E5-2670 v3 @ 2.30GHz (12 cores, 24 threads
per socket), 64GB RAM
hackbench-process-sockets (less is worse)
Amean 	1 	1.0347	1.0880	( -5.15%)
Amean 	4 	1.7267	1.8527	( -7.30%)
Amean 	7 	2.6707	2.8110	( -5.25%)
Amean 	12 	4.1617	4.3383	( -4.25%)
Amean 	21 	7.0070	7.2600	( -3.61%)
Amean 	30 	9.9187	10.2397	( -3.24%)
Amean 	48 	15.6710	16.3923	( -4.60%)
Amean 	79 	24.7743	26.1247	( -5.45%)
Amean 	110	34.3000	35.9307	( -4.75%)
Amean 	141	44.2043	44.8010	( -1.35%)
Amean 	172	54.2430	54.7260	( -0.89%)
Amean 	192	60.6557	60.9777	( -0.53%)

hackbench-thread-sockets (less is worse)
Amean 	1 	1.0610	1.1353	( -7.01%)
Amean 	4 	1.7543	1.9140	( -9.10%)
Amean 	7 	2.7840	2.9573	( -6.23%)
Amean 	12 	4.3813	4.4937	( -2.56%)
Amean 	21 	7.3460	7.5350	( -2.57%)
Amean 	30 	10.2313	10.5190	( -2.81%)
Amean 	48 	15.9700	16.5940	( -3.91%)
Amean 	79 	25.3973	26.6637	( -4.99%)
Amean 	110	35.1087	36.4797	( -3.91%)
Amean 	141	45.8220	46.3053	( -1.05%)
Amean 	172	55.4917	55.7320	( -0.43%)
Amean 	192	62.7490	62.5410	( 0.33%)
diff mbox series

Patch

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 7f8ee09c711f..68ffa064b7a8 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -124,10 +124,8 @@  struct page {
 					struct page *next;
 #ifdef CONFIG_64BIT
 					int pages;	/* Nr of pages left */
-					int pobjects;	/* Approximate count */
 #else
 					short int pages;
-					short int pobjects;
 #endif
 				};
 			};
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index 85499f0586b0..0fa751b946fa 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -99,6 +99,8 @@  struct kmem_cache {
 #ifdef CONFIG_SLUB_CPU_PARTIAL
 	/* Number of per cpu partial objects to keep around */
 	unsigned int cpu_partial;
+	/* Number of per cpu partial pages to keep around */
+	unsigned int cpu_partial_pages;
 #endif
 	struct kmem_cache_order_objects oo;
 
@@ -141,17 +143,6 @@  struct kmem_cache {
 	struct kmem_cache_node *node[MAX_NUMNODES];
 };
 
-#ifdef CONFIG_SLUB_CPU_PARTIAL
-#define slub_cpu_partial(s)		((s)->cpu_partial)
-#define slub_set_cpu_partial(s, n)		\
-({						\
-	slub_cpu_partial(s) = (n);		\
-})
-#else
-#define slub_cpu_partial(s)		(0)
-#define slub_set_cpu_partial(s, n)
-#endif /* CONFIG_SLUB_CPU_PARTIAL */
-
 #ifdef CONFIG_SYSFS
 #define SLAB_SUPPORTS_SYSFS
 void sysfs_slab_unlink(struct kmem_cache *);
diff --git a/mm/slub.c b/mm/slub.c
index 3d2025f7163b..3757f31c5d97 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -414,6 +414,29 @@  static inline unsigned int oo_objects(struct kmem_cache_order_objects x)
 	return x.x & OO_MASK;
 }
 
+#ifdef CONFIG_SLUB_CPU_PARTIAL
+static void slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
+{
+	unsigned int nr_pages;
+
+	s->cpu_partial = nr_objects;
+
+	/*
+	 * We take the number of objects but actually limit the number of
+	 * pages on the per cpu partial list, in order to limit excessive
+	 * growth of the list. For simplicity we assume that the pages will
+	 * be half-full.
+	 */
+	nr_pages = DIV_ROUND_UP(nr_objects * 2, oo_objects(s->oo));
+	s->cpu_partial_pages = nr_pages;
+}
+#else
+static inline void
+slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
+{
+}
+#endif /* CONFIG_SLUB_CPU_PARTIAL */
+
 /*
  * Per slab locking using the pagelock
  */
@@ -2045,7 +2068,7 @@  static inline void remove_partial(struct kmem_cache_node *n,
  */
 static inline void *acquire_slab(struct kmem_cache *s,
 		struct kmem_cache_node *n, struct page *page,
-		int mode, int *objects)
+		int mode)
 {
 	void *freelist;
 	unsigned long counters;
@@ -2061,7 +2084,6 @@  static inline void *acquire_slab(struct kmem_cache *s,
 	freelist = page->freelist;
 	counters = page->counters;
 	new.counters = counters;
-	*objects = new.objects - new.inuse;
 	if (mode) {
 		new.inuse = page->objects;
 		new.freelist = NULL;
@@ -2099,9 +2121,8 @@  static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
 {
 	struct page *page, *page2;
 	void *object = NULL;
-	unsigned int available = 0;
 	unsigned long flags;
-	int objects;
+	unsigned int partial_pages = 0;
 
 	/*
 	 * Racy check. If we mistakenly see no partial slabs then we
@@ -2119,11 +2140,10 @@  static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
 		if (!pfmemalloc_match(page, gfpflags))
 			continue;
 
-		t = acquire_slab(s, n, page, object == NULL, &objects);
+		t = acquire_slab(s, n, page, object == NULL);
 		if (!t)
 			break;
 
-		available += objects;
 		if (!object) {
 			*ret_page = page;
 			stat(s, ALLOC_FROM_PARTIAL);
@@ -2131,10 +2151,15 @@  static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
 		} else {
 			put_cpu_partial(s, page, 0);
 			stat(s, CPU_PARTIAL_NODE);
+			partial_pages++;
 		}
+#ifdef CONFIG_SLUB_CPU_PARTIAL
 		if (!kmem_cache_has_cpu_partial(s)
-			|| available > slub_cpu_partial(s) / 2)
+			|| partial_pages > s->cpu_partial_pages / 2)
 			break;
+#else
+		break;
+#endif
 
 	}
 	spin_unlock_irqrestore(&n->list_lock, flags);
@@ -2539,14 +2564,13 @@  static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 	struct page *page_to_unfreeze = NULL;
 	unsigned long flags;
 	int pages = 0;
-	int pobjects = 0;
 
 	local_lock_irqsave(&s->cpu_slab->lock, flags);
 
 	oldpage = this_cpu_read(s->cpu_slab->partial);
 
 	if (oldpage) {
-		if (drain && oldpage->pobjects > slub_cpu_partial(s)) {
+		if (drain && oldpage->pages >= s->cpu_partial_pages) {
 			/*
 			 * Partial array is full. Move the existing set to the
 			 * per node partial list. Postpone the actual unfreezing
@@ -2555,16 +2579,13 @@  static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 			page_to_unfreeze = oldpage;
 			oldpage = NULL;
 		} else {
-			pobjects = oldpage->pobjects;
 			pages = oldpage->pages;
 		}
 	}
 
 	pages++;
-	pobjects += page->objects - page->inuse;
 
 	page->pages = pages;
-	page->pobjects = pobjects;
 	page->next = oldpage;
 
 	this_cpu_write(s->cpu_slab->partial, page);
@@ -3980,6 +4001,8 @@  static void set_min_partial(struct kmem_cache *s, unsigned long min)
 static void set_cpu_partial(struct kmem_cache *s)
 {
 #ifdef CONFIG_SLUB_CPU_PARTIAL
+	unsigned int nr_objects;
+
 	/*
 	 * cpu_partial determined the maximum number of objects kept in the
 	 * per cpu partial lists of a processor.
@@ -3989,24 +4012,22 @@  static void set_cpu_partial(struct kmem_cache *s)
 	 * filled up again with minimal effort. The slab will never hit the
 	 * per node partial lists and therefore no locking will be required.
 	 *
-	 * This setting also determines
-	 *
-	 * A) The number of objects from per cpu partial slabs dumped to the
-	 *    per node list when we reach the limit.
-	 * B) The number of objects in cpu partial slabs to extract from the
-	 *    per node list when we run out of per cpu objects. We only fetch
-	 *    50% to keep some capacity around for frees.
+	 * For backwards compatibility reasons, this is determined as number
+	 * of objects, even though we now limit maximum number of pages, see
+	 * slub_set_cpu_partial()
 	 */
 	if (!kmem_cache_has_cpu_partial(s))
-		slub_set_cpu_partial(s, 0);
+		nr_objects = 0;
 	else if (s->size >= PAGE_SIZE)
-		slub_set_cpu_partial(s, 2);
+		nr_objects = 2;
 	else if (s->size >= 1024)
-		slub_set_cpu_partial(s, 6);
+		nr_objects = 6;
 	else if (s->size >= 256)
-		slub_set_cpu_partial(s, 13);
+		nr_objects = 13;
 	else
-		slub_set_cpu_partial(s, 30);
+		nr_objects = 30;
+
+	slub_set_cpu_partial(s, nr_objects);
 #endif
 }
 
@@ -5379,7 +5400,12 @@  SLAB_ATTR(min_partial);
 
 static ssize_t cpu_partial_show(struct kmem_cache *s, char *buf)
 {
-	return sysfs_emit(buf, "%u\n", slub_cpu_partial(s));
+	unsigned int nr_partial = 0;
+#ifdef CONFIG_SLUB_CPU_PARTIAL
+	nr_partial = s->cpu_partial;
+#endif
+
+	return sysfs_emit(buf, "%u\n", nr_partial);
 }
 
 static ssize_t cpu_partial_store(struct kmem_cache *s, const char *buf,
@@ -5450,12 +5476,12 @@  static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
 
 		page = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));
 
-		if (page) {
+		if (page)
 			pages += page->pages;
-			objects += page->pobjects;
-		}
 	}
 
+	/* Approximate half-full pages , see slub_set_cpu_partial() */
+	objects = (pages * oo_objects(s->oo)) / 2;
 	len += sysfs_emit_at(buf, len, "%d(%d)", objects, pages);
 
 #ifdef CONFIG_SMP
@@ -5463,9 +5489,12 @@  static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
 		struct page *page;
 
 		page = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));
-		if (page)
+		if (page) {
+			pages = READ_ONCE(page->pages);
+			objects = (pages * oo_objects(s->oo)) / 2;
 			len += sysfs_emit_at(buf, len, " C%d=%d(%d)",
-					     cpu, page->pobjects, page->pages);
+					     cpu, objects, pages);
+		}
 	}
 #endif
 	len += sysfs_emit_at(buf, len, "\n");