[RFC PATCH 2/3] mm/mglru: skip the flags cmpxchg when nothing changes

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

folio_update_gen() and folio_set_lru_refs() always end with a
try_cmpxchg() on folio->flags, even when the new value equals the old
one. The locked instruction still takes the struct page cacheline in
exclusive state.

That is common in practice: a folio mapped by many processes is visited
once per mm during a page table walk, and after the first visit it is
already in the youngest generation with refs cleared and PG_workingset
set. isolate_folio() and the eviction paths also call
folio_set_lru_refs(folio, 0) on folios whose refs are already zero.

Break out of the loop when the flags would not change. The result is
the same as a successful no-op cmpxchg. The only difference is that the
full barrier implied by the locked instruction is gone on that path.
No caller relies on it: the eviction and isolation callers run under
the LRU lock, and the others (page table walks, folio_mark_accessed(),
lru_gen_clear_refs()) only care about the resulting flags value, which
is already in place. Note that lru_gen_inc_refs() already returns
without any atomic when refs is at LRU_REFS_MAX and the folio is in
the workingset.

Tested on an i7-13700HX with turbo disabled and the performance
governor. N processes map the same 2GiB tmpfs file (huge=never) and
read every page, then one aging pass is triggered on their memcg
through /sys/kernel/debug/lru_gen ("+ <memcg_id> 0 <max_seq> 200 1").
Time per aging pass in ms, median of 5 runs of 10 passes each, averaged
over 2 boots of each kernel:

  processes    base   patched
          1    32.9      31.7   -3.5%
          8   195.1     160.2  -17.9%
         32   755.8     592.2  -21.7%
         64  1597.2    1260.6  -21.1%

With one process, every visit changes the generation, so there is
nothing to skip. Before the patch, perf annotate attributes 53-58% of
the walk_update_folio() samples to the lock cmpxchg in the inlined
folio_update_gen() in this test.

Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 include/linux/mm_inline.h | 2 ++
 mm/vmscan.c               | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index b3d85fe91385..4ba9079f4dc8 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -244,6 +244,8 @@ static inline void folio_set_lru_refs(struct folio *folio, unsigned int refs)
 	do {
 		new_flags = old_flags;
 		lru_set_refs_flags(&new_flags, refs);
+		if (new_flags == old_flags)
+			break;
 	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
 }
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 50c352e9ffef..3fa76715382d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3398,6 +3398,8 @@ static int folio_update_gen(struct folio *folio, int new_gen, int *type,
 		lru_set_gen_flags(&new_flags, new_gen);
 		lru_set_refs_flags(&new_flags, 0);
 		new_flags |= BIT(PG_workingset);
+		if (new_flags == old_flags)
+			break;
 	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
 
 	*type = folio_flags_is_file_lru(&old_flags);
-- 
2.34.1