[v4,10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting
diff mbox series

Message ID 20200204234117.2974687-11-jhubbard@nvidia.com
State New
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Series
  • mm/gup: track FOLL_PIN pages
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Commit Message

John Hubbard Feb. 4, 2020, 11:41 p.m. UTC
Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via
unpin_user_pages*(), we need some visibility into whether all of this is
working correctly.

Add two new fields to /proc/vmstat:

    nr_foll_pin_acquired
    nr_foll_pin_released

These are documented in Documentation/core-api/pin_user_pages.rst.
They represent the number of pages (since boot time) that have been
pinned ("nr_foll_pin_acquired") and unpinned ("nr_foll_pin_released"),
via pin_user_pages*() and unpin_user_pages*().

In the absence of long-running DMA or RDMA operations that hold pages
pinned, the above two fields will normally be equal to each other.

Also: update Documentation/core-api/pin_user_pages.rst, to remove an
earlier (now confirmed untrue) claim about a performance problem with
/proc/vmstat.

Also: updated Documentation/core-api/pin_user_pages.rst to rename the
new /proc/vmstat entries, to the names listed here.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 Documentation/core-api/pin_user_pages.rst |  8 ++------
 include/linux/mmzone.h                    |  2 ++
 mm/gup.c                                  | 10 ++++++++++
 mm/vmstat.c                               |  2 ++
 4 files changed, 16 insertions(+), 6 deletions(-)

Comments

Jan Kara Feb. 5, 2020, 9:37 a.m. UTC | #1
On Tue 04-02-20 15:41:15, John Hubbard wrote:
> Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via
> unpin_user_pages*(), we need some visibility into whether all of this is
> working correctly.
> 
> Add two new fields to /proc/vmstat:
> 
>     nr_foll_pin_acquired
>     nr_foll_pin_released
> 
> These are documented in Documentation/core-api/pin_user_pages.rst.
> They represent the number of pages (since boot time) that have been
> pinned ("nr_foll_pin_acquired") and unpinned ("nr_foll_pin_released"),
> via pin_user_pages*() and unpin_user_pages*().
> 
> In the absence of long-running DMA or RDMA operations that hold pages
> pinned, the above two fields will normally be equal to each other.
> 
> Also: update Documentation/core-api/pin_user_pages.rst, to remove an
> earlier (now confirmed untrue) claim about a performance problem with
> /proc/vmstat.
> 
> Also: updated Documentation/core-api/pin_user_pages.rst to rename the
> new /proc/vmstat entries, to the names listed here.
> 
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>

...

> @@ -104,6 +106,9 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>  		if (hpage_pincount_available(page))
>  			hpage_pincount_add(page, refs);
>  
> +		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
> +				    orig_refs);
> +
>  		return page;
>  	}
>  

It seems to me you miss mod_node_page_state() in put_compound_head(), don't
you?

Otherwise I like the new stat names better :).

								Honza
Kirill A. Shutemov Feb. 5, 2020, 11:43 a.m. UTC | #2
On Tue, Feb 04, 2020 at 03:41:15PM -0800, John Hubbard wrote:
> Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via
> unpin_user_pages*(), we need some visibility into whether all of this is
> working correctly.
> 
> Add two new fields to /proc/vmstat:
> 
>     nr_foll_pin_acquired
>     nr_foll_pin_released
> 
> These are documented in Documentation/core-api/pin_user_pages.rst.
> They represent the number of pages (since boot time) that have been
> pinned ("nr_foll_pin_acquired") and unpinned ("nr_foll_pin_released"),
> via pin_user_pages*() and unpin_user_pages*().
> 
> In the absence of long-running DMA or RDMA operations that hold pages
> pinned, the above two fields will normally be equal to each other.
> 
> Also: update Documentation/core-api/pin_user_pages.rst, to remove an
> earlier (now confirmed untrue) claim about a performance problem with
> /proc/vmstat.
> 
> Also: updated Documentation/core-api/pin_user_pages.rst to rename the
> new /proc/vmstat entries, to the names listed here.
> 
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>

Please, clarify semantics for huge page. An user may want to know if we
count huge page as one pin-acquired or by number of pages.

Otherwise looks good (given Jan concern is addressed).
John Hubbard Feb. 5, 2020, 11:13 p.m. UTC | #3
On 2/5/20 1:37 AM, Jan Kara wrote:
> ...
> 
>> @@ -104,6 +106,9 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>>  		if (hpage_pincount_available(page))
>>  			hpage_pincount_add(page, refs);
>>  
>> +		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
>> +				    orig_refs);
>> +
>>  		return page;
>>  	}
>>  
> 
> It seems to me you miss mod_node_page_state() in put_compound_head(), don't
> you?


Yes, that was definitely missing. I've added this for the next version:


diff --git a/mm/gup.c b/mm/gup.c
index 7c543849181b..ae503c51bc7f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2268,6 +2268,8 @@ static int record_subpages(struct page *page, unsigned long addr,
 
 static void put_compound_head(struct page *page, int refs, unsigned int flags)
 {
+       int orig_refs = refs;
+
        if (flags & FOLL_PIN) {
                if (hpage_pincount_available(page))
                        hpage_pincount_sub(page, refs);
@@ -2283,6 +2285,8 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
        if (refs > 1)
                page_ref_sub(page, refs - 1);
        put_page(page);
+
+       mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, orig_refs);
 }
 
 #ifdef CONFIG_ARCH_HAS_HUGEPD



> 
> Otherwise I like the new stat names better :).
> 
> 								Honza
> 

Glad to hear that! :)

thanks,
John Hubbard Feb. 5, 2020, 11:58 p.m. UTC | #4
On 2/5/20 3:43 AM, Kirill A. Shutemov wrote:
> On Tue, Feb 04, 2020 at 03:41:15PM -0800, John Hubbard wrote:
>> Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via
>> unpin_user_pages*(), we need some visibility into whether all of this is
>> working correctly.
>>
>> Add two new fields to /proc/vmstat:
>>
>>     nr_foll_pin_acquired
>>     nr_foll_pin_released
>>
>> These are documented in Documentation/core-api/pin_user_pages.rst.
>> They represent the number of pages (since boot time) that have been
>> pinned ("nr_foll_pin_acquired") and unpinned ("nr_foll_pin_released"),
>> via pin_user_pages*() and unpin_user_pages*().
>>
>> In the absence of long-running DMA or RDMA operations that hold pages
>> pinned, the above two fields will normally be equal to each other.
>>
>> Also: update Documentation/core-api/pin_user_pages.rst, to remove an
>> earlier (now confirmed untrue) claim about a performance problem with
>> /proc/vmstat.
>>
>> Also: updated Documentation/core-api/pin_user_pages.rst to rename the
>> new /proc/vmstat entries, to the names listed here.
>>
>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> 
> Please, clarify semantics for huge page. An user may want to know if we
> count huge page as one pin-acquired or by number of pages.


OK, I've added this for the next version:


diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 5776ad1ed5e4..2e939ff10b86 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -211,6 +211,33 @@ since the system was booted, via two new /proc/vmstat entries: ::
     /proc/vmstat/nr_foll_pin_acquired
     /proc/vmstat/nr_foll_pin_released
 
+Under normal conditions, these two values will be equal unless there are any
+long-term [R]DMA pins in place, or during pin/unpin transitions.
+
+* nr_foll_pin_acquired: This is the number of logical pins that have been
+  acquired since the system was powered on. For huge pages, the head page is
+  pinned once for each page (head page and each tail page) within the huge page.
+  This follows the same sort of behavior that get_user_pages() uses for huge
+  pages: the head page is refcounted once for each tail or head page in the huge
+  page, when get_user_pages() is applied to a huge page.
+
+* nr_foll_pin_released: The number of logical pins that have been released since
+  the system was powered on. Note that pages are released (unpinned) on a
+  PAGE_SIZE granularity, even if the original pin was applied to a huge page.
+  Becaused of the pin count behavior described above in "nr_foll_pin_acquired",
+  the accounting balances out, so that after doing this::
+
+    pin_user_pages(huge_page);
+    for (each page in huge_page)
+        unpin_user_page(page);
+
+...the following is expected::
+
+    nr_foll_pin_released == nr_foll_pin_acquired
+
+(...unless it was already out of balance due to a long-term RDMA pin being in
+place.)
+
 Other diagnostics
 =================
 


thanks,

Patch
diff mbox series

diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index dd21ea140ef4..194204d74a50 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -208,12 +208,8 @@  has the following new calls to exercise the new pin*() wrapper functions:
 You can monitor how many total dma-pinned pages have been acquired and released
 since the system was booted, via two new /proc/vmstat entries: ::
 
-    /proc/vmstat/nr_foll_pin_requested
-    /proc/vmstat/nr_foll_pin_requested
-
-Those are both going to show zero, unless CONFIG_DEBUG_VM is set. This is
-because there is a noticeable performance drop in unpin_user_page(), when they
-are activated.
+    /proc/vmstat/nr_foll_pin_acquired
+    /proc/vmstat/nr_foll_pin_released
 
 Other diagnostics
 =================
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f6873905a..4bca42eeb439 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -243,6 +243,8 @@  enum node_stat_item {
 	NR_DIRTIED,		/* page dirtyings since bootup */
 	NR_WRITTEN,		/* page writings since bootup */
 	NR_KERNEL_MISC_RECLAIMABLE,	/* reclaimable non-slab kernel pages */
+	NR_FOLL_PIN_ACQUIRED,	/* via: pin_user_page(), gup flag: FOLL_PIN */
+	NR_FOLL_PIN_RELEASED,	/* pages returned via unpin_user_page() */
 	NR_VM_NODE_STAT_ITEMS
 };
 
diff --git a/mm/gup.c b/mm/gup.c
index 4d0d94405639..7c543849181b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -86,6 +86,8 @@  static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 	if (flags & FOLL_GET)
 		return try_get_compound_head(page, refs);
 	else if (flags & FOLL_PIN) {
+		int orig_refs = refs;
+
 		/*
 		 * When pinning a compound page of order > 1 (which is what
 		 * hpage_pincount_available() checks for), use an exact count to
@@ -104,6 +106,9 @@  static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 		if (hpage_pincount_available(page))
 			hpage_pincount_add(page, refs);
 
+		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
+				    orig_refs);
+
 		return page;
 	}
 
@@ -158,6 +163,8 @@  bool __must_check try_grab_page(struct page *page, unsigned int flags)
 		 * once, so that the page really is pinned.
 		 */
 		page_ref_add(page, refs);
+
+		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
 	}
 
 	return true;
@@ -178,6 +185,7 @@  static bool __unpin_devmap_managed_user_page(struct page *page)
 
 	count = page_ref_sub_return(page, refs);
 
+	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
 	/*
 	 * devmap page refcounts are 1-based, rather than 0-based: if
 	 * refcount is 1, then the page is free and the refcount is
@@ -228,6 +236,8 @@  void unpin_user_page(struct page *page)
 
 	if (page_ref_sub_and_test(page, refs))
 		__put_page(page);
+
+	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
 }
 EXPORT_SYMBOL(unpin_user_page);
 
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 78d53378db99..c9c0d71f917f 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1168,6 +1168,8 @@  const char * const vmstat_text[] = {
 	"nr_dirtied",
 	"nr_written",
 	"nr_kernel_misc_reclaimable",
+	"nr_foll_pin_acquired",
+	"nr_foll_pin_released",
 
 	/* enum writeback_stat_item counters */
 	"nr_dirty_threshold",