@@ -3317,10 +3317,15 @@ static bool free_unref_page_prepare(struct page *page, unsigned long pfn,
return true;
}
-static int nr_pcp_free(struct per_cpu_pages *pcp, int high, int batch)
+static int nr_pcp_free(struct per_cpu_pages *pcp, int high, int batch,
+ bool free_high)
{
int min_nr_free, max_nr_free;
+ /* Free everything if batch freeing high-order pages. */
+ if (unlikely(free_high))
+ return pcp->count;
+
/* Check for PCP disabled or boot pageset */
if (unlikely(high < batch))
return 1;
@@ -3341,11 +3346,12 @@ static int nr_pcp_free(struct per_cpu_pages *pcp, int high, int batch)
return batch;
}
-static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone)
+static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
+ bool free_high)
{
int high = READ_ONCE(pcp->high);
- if (unlikely(!high))
+ if (unlikely(!high || free_high))
return 0;
if (!test_bit(ZONE_RECLAIM_ACTIVE, &zone->flags))
@@ -3365,17 +3371,27 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn,
struct per_cpu_pages *pcp;
int high;
int pindex;
+ bool free_high;
__count_vm_event(PGFREE);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
pindex = order_to_pindex(migratetype, order);
list_add(&page->lru, &pcp->lists[pindex]);
pcp->count += 1 << order;
- high = nr_pcp_high(pcp, zone);
+
+ /*
+ * As high-order pages other than THP's stored on PCP can contribute
+ * to fragmentation, limit the number stored when PCP is heavily
+ * freeing without allocation. The remainder after bulk freeing
+ * stops will be drained from vmstat refresh context.
+ */
+ free_high = (pcp->free_factor && order && order <= PAGE_ALLOC_COSTLY_ORDER);
+
+ high = nr_pcp_high(pcp, zone, free_high);
if (pcp->count >= high) {
int batch = READ_ONCE(pcp->batch);
- free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch), pcp, pindex);
+ free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch, free_high), pcp, pindex);
}
}
When a PCP is mostly used for frees then high-order pages can exist on PCP lists for some time. This is problematic when the allocation pattern is all allocations from one CPU and all frees from another resulting in colder pages being used. When bulk freeing pages, limit the number of high-order pages that are stored on the PCP lists. Netperf running on localhost exhibits this pattern and while it does not matter for some machines, it does matter for others with smaller caches where cache misses cause problems due to reduced page reuse. Pages freed directly to the buddy list may be reused quickly while still cache hot where as storing on the PCP lists may be cold by the time free_pcppages_bulk() is called. Using perf kmem:mm_page_alloc, the 5 most used page frames were 5.17-rc3 13041 pfn=0x111a30 13081 pfn=0x5814d0 13097 pfn=0x108258 13121 pfn=0x689598 13128 pfn=0x5814d8 5.17-revert-highpcp 192009 pfn=0x54c140 195426 pfn=0x1081d0 200908 pfn=0x61c808 243515 pfn=0xa9dc20 402523 pfn=0x222bb8 5.17-full-series 142693 pfn=0x346208 162227 pfn=0x13bf08 166413 pfn=0x2711e0 166950 pfn=0x2702f8 The spread is wider as there is still time before pages freed to one PCP get released with a tradeoff between fast reuse and reduced zone lock acquisition. From the machine used to gather the traces, the headline performance was equivalent. netperf-tcp 5.17.0-rc3 5.17.0-rc3 5.17.0-rc3 vanilla mm-reverthighpcp-v1r1 mm-highpcplimit-v2 Hmean 64 839.93 ( 0.00%) 840.77 ( 0.10%) 841.02 ( 0.13%) Hmean 128 1614.22 ( 0.00%) 1622.07 * 0.49%* 1636.41 * 1.37%* Hmean 256 2952.00 ( 0.00%) 2953.19 ( 0.04%) 2977.76 * 0.87%* Hmean 1024 10291.67 ( 0.00%) 10239.17 ( -0.51%) 10434.41 * 1.39%* Hmean 2048 17335.08 ( 0.00%) 17399.97 ( 0.37%) 17134.81 * -1.16%* Hmean 3312 22628.15 ( 0.00%) 22471.97 ( -0.69%) 22422.78 ( -0.91%) Hmean 4096 25009.50 ( 0.00%) 24752.83 * -1.03%* 24740.41 ( -1.08%) Hmean 8192 32745.01 ( 0.00%) 31682.63 * -3.24%* 32153.50 * -1.81%* Hmean 16384 39759.59 ( 0.00%) 36805.78 * -7.43%* 38948.13 * -2.04%* From a 1-socket skylake machine with a small CPU cache that suffers more if cache misses are too high netperf-tcp 5.17.0-rc3 5.17.0-rc3 5.17.0-rc3 vanilla mm-reverthighpcp-v1 mm-highpcplimit-v2 Hmean 64 938.95 ( 0.00%) 941.50 * 0.27%* 943.61 * 0.50%* Hmean 128 1843.10 ( 0.00%) 1857.58 * 0.79%* 1861.09 * 0.98%* Hmean 256 3573.07 ( 0.00%) 3667.45 * 2.64%* 3674.91 * 2.85%* Hmean 1024 13206.52 ( 0.00%) 13487.80 * 2.13%* 13393.21 * 1.41%* Hmean 2048 22870.23 ( 0.00%) 23337.96 * 2.05%* 23188.41 * 1.39%* Hmean 3312 31001.99 ( 0.00%) 32206.50 * 3.89%* 31863.62 * 2.78%* Hmean 4096 35364.59 ( 0.00%) 36490.96 * 3.19%* 36112.54 * 2.11%* Hmean 8192 48497.71 ( 0.00%) 49954.05 * 3.00%* 49588.26 * 2.25%* Hmean 16384 58410.86 ( 0.00%) 60839.80 * 4.16%* 62282.96 * 6.63%* Note that this was a machine that did not benefit from caching high-order pages and performance is almost restored with the series applied. It's not fully restored as cache misses are still higher. This is a trade-off between optimising for a workload that does all allocs on one CPU and frees on another or more general workloads that need high-order pages for SLUB and benefit from avoiding zone->lock for every SLUB refill/drain. Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Reviewed-by: Vlastimil Babka <vbabka@suse.cz> --- mm/page_alloc.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)