Re: [PATCH 1/6] mm: page_alloc: remove pcppage migratetype caching

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Sep 27, 2023 at 01:42:25PM +0800, Huang, Ying wrote:
> Johannes Weiner <hannes@cmpxchg.org> writes:
> 
> > The idea behind the cache is to save get_pageblock_migratetype()
> > lookups during bulk freeing. A microbenchmark suggests this isn't
> > helping, though. The pcp migratetype can get stale, which means that
> > bulk freeing has an extra branch to check if the pageblock was
> > isolated while on the pcp.
> >
> > While the variance overlaps, the cache write and the branch seem to
> > make this a net negative. The following test allocates and frees
> > batches of 10,000 pages (~3x the pcp high marks to trigger flushing):
> >
> > Before:
> >           8,668.48 msec task-clock                       #   99.735 CPUs utilized               ( +-  2.90% )
> >                 19      context-switches                 #    4.341 /sec                        ( +-  3.24% )
> >                  0      cpu-migrations                   #    0.000 /sec
> >             17,440      page-faults                      #    3.984 K/sec                       ( +-  2.90% )
> >     41,758,692,473      cycles                           #    9.541 GHz                         ( +-  2.90% )
> >    126,201,294,231      instructions                     #    5.98  insn per cycle              ( +-  2.90% )
> >     25,348,098,335      branches                         #    5.791 G/sec                       ( +-  2.90% )
> >         33,436,921      branch-misses                    #    0.26% of all branches             ( +-  2.90% )
> >
> >          0.0869148 +- 0.0000302 seconds time elapsed  ( +-  0.03% )
> >
> > After:
> >           8,444.81 msec task-clock                       #   99.726 CPUs utilized               ( +-  2.90% )
> >                 22      context-switches                 #    5.160 /sec                        ( +-  3.23% )
> >                  0      cpu-migrations                   #    0.000 /sec
> >             17,443      page-faults                      #    4.091 K/sec                       ( +-  2.90% )
> >     40,616,738,355      cycles                           #    9.527 GHz                         ( +-  2.90% )
> >    126,383,351,792      instructions                     #    6.16  insn per cycle              ( +-  2.90% )
> >     25,224,985,153      branches                         #    5.917 G/sec                       ( +-  2.90% )
> >         32,236,793      branch-misses                    #    0.25% of all branches             ( +-  2.90% )
> >
> >          0.0846799 +- 0.0000412 seconds time elapsed  ( +-  0.05% )
> >
> > A side effect is that this also ensures that pages whose pageblock
> > gets stolen while on the pcplist end up on the right freelist and we
> > don't perform potentially type-incompatible buddy merges (or skip
> > merges when we shouldn't), whis is likely beneficial to long-term
> > fragmentation management, although the effects would be harder to
> > measure. Settle for simpler and faster code as justification here.
> 
> I suspected the PCP allocating/freeing path may be influenced (that is,
> allocating/freeing batch is less than PCP high).  So I tested
> one-process will-it-scale/page_fault1 with sysctl
> percpu_pagelist_high_fraction=8.  So pages will be allocated/freed
> from/to PCP only.  The test results are as follows,
> 
> Before:
> will-it-scale.1.processes                        618364.3      (+-  0.075%)
> perf-profile.children.get_pfnblock_flags_mask         0.13     (+-  9.350%)
> 
> After:
> will-it-scale.1.processes	                 616512.0      (+-  0.057%)
> perf-profile.children.get_pfnblock_flags_mask	      0.41     (+-  22.44%)
> 
> The change isn't large: -0.3%.  Perf profiling shows the cycles% of
> get_pfnblock_flags_mask() increases.

Ah, this is going through the free_unref_page_list() path that
Vlastimil had pointed out as well. I made another change on top that
eliminates the second lookup. After that, both pcp fast paths have the
same number of lookups as before: 1. This fixes the regression for me.

Would you mind confirming this as well?

--

From f5d032019ed832a1a50454347a33b00ca6abeb30 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Fri, 15 Sep 2023 16:03:24 -0400
Subject: [PATCH] mm: page_alloc: optimize free_unref_page_list()

Move direct freeing of isolated pages to the lock-breaking block in
the second loop. This saves an unnecessary migratetype reassessment.

Minor comment and local variable scoping cleanups.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/page_alloc.c | 44 ++++++++++++++++++--------------------------
 1 file changed, 18 insertions(+), 26 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bfffc1af94cd..665930ffe22a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2466,48 +2466,40 @@ void free_unref_page_list(struct list_head *list)
 	struct per_cpu_pages *pcp = NULL;
 	struct zone *locked_zone = NULL;
 	int batch_count = 0;
-	int migratetype;
-
-	/* Prepare pages for freeing */
-	list_for_each_entry_safe(page, next, list, lru) {
-		unsigned long pfn = page_to_pfn(page);
-
-		if (!free_pages_prepare(page, 0, FPI_NONE)) {
-			list_del(&page->lru);
-			continue;
-		}
 
-		/*
-		 * Free isolated pages directly to the allocator, see
-		 * comment in free_unref_page.
-		 */
-		migratetype = get_pfnblock_migratetype(page, pfn);
-		if (unlikely(is_migrate_isolate(migratetype))) {
+	list_for_each_entry_safe(page, next, list, lru)
+		if (!free_pages_prepare(page, 0, FPI_NONE))
 			list_del(&page->lru);
-			free_one_page(page_zone(page), page, pfn, 0, FPI_NONE);
-			continue;
-		}
-	}
 
 	list_for_each_entry_safe(page, next, list, lru) {
 		unsigned long pfn = page_to_pfn(page);
 		struct zone *zone = page_zone(page);
+		int migratetype;
 
 		list_del(&page->lru);
 		migratetype = get_pfnblock_migratetype(page, pfn);
 
 		/*
-		 * Either different zone requiring a different pcp lock or
-		 * excessive lock hold times when freeing a large list of
-		 * pages.
+		 * Zone switch, batch complete, or non-pcp freeing?
+		 * Drop the pcp lock and evaluate.
 		 */
-		if (zone != locked_zone || batch_count == SWAP_CLUSTER_MAX) {
+		if (unlikely(zone != locked_zone ||
+			     batch_count == SWAP_CLUSTER_MAX ||
+			     is_migrate_isolate(migratetype))) {
 			if (pcp) {
 				pcp_spin_unlock(pcp);
 				pcp_trylock_finish(UP_flags);
+				locked_zone = NULL;
 			}
 
-			batch_count = 0;
+			/*
+			 * Free isolated pages directly to the
+			 * allocator, see comment in free_unref_page.
+			 */
+			if (is_migrate_isolate(migratetype)) {
+				free_one_page(zone, page, pfn, 0, FPI_NONE);
+				continue;
+			}
 
 			/*
 			 * trylock is necessary as pages may be getting freed
@@ -2518,10 +2510,10 @@ void free_unref_page_list(struct list_head *list)
 			if (unlikely(!pcp)) {
 				pcp_trylock_finish(UP_flags);
 				free_one_page(zone, page, pfn, 0, FPI_NONE);
-				locked_zone = NULL;
 				continue;
 			}
 			locked_zone = zone;
+			batch_count = 0;
 		}
 
 		/*
-- 
2.42.0