From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Nicolas Saenz Julienne <nsaenzju@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>,
Michal Hocko <mhocko@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue
Date: Thu, 19 May 2022 13:13:41 +0100 [thread overview]
Message-ID: <20220519121341.GV3441@techsingularity.net> (raw)
In-Reply-To: <5a0c9b7c-3620-3e0f-7510-d0fc3fa6ceda@suse.cz>
On Thu, May 19, 2022 at 12:57:01PM +0200, Vlastimil Babka wrote:
> On 5/12/22 10:50, Mel Gorman wrote:
> > The VM_BUG_ON check for a valid page can be avoided with a simple
> > change in the flow. The ZONE_BOOSTED_WATERMARK is unlikely in general
> > and even more unlikely if the page allocation failed so mark the
> > branch unlikely.
>
> Hm, so that makes a DEBUG_VM config avoid the check. On the other hand,
> it puts it on the path returning from rmqueue_pcplist() for all configs,
> and that should be the fast path. So unless things further change in the
> following patches, it doesn't seem that useful?
>
You're right -- the fast path ends up with both a if
(page) and if (!page) checks. Andrew, can you drop the patch
mm-page_alloc-remove-unnecessary-page-==-null-check-in-rmqueue.patch from
your tree please?
Originally the flow was important when I was writing the patch and later
became unnecessary. However, it reminded me of another problem I thought
of when writing this and then forgotten to note it in the changelog. If
the page allocation fails then ZONE_BOOSTED_WATERMARK should still be
tested and cleared before waking kswapd. It could happen if an allocation
attempt tried to fallback to another migratetype and still fail to find
a suitable page. This is true whether going through the PCP lists or not.
So what do you think of me adding this patch to a follow-up series?
--8<--
mm/page_alloc: Remove mistaken page == NULL check in rmqueue
If a page allocation fails, the ZONE_BOOSTER_WATERMARK should be tested,
cleared and kswapd woken whether the allocation attempt was via the PCP
or directly via the buddy list.
Remove the page == NULL so the ZONE_BOOSTED_WATERMARK bit is checked
unconditionally. As it is unlikely that ZONE_BOOSTED_WATERMARK is set,
mark the branch accordingly.
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/page_alloc.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1c4c54503a5d..61d5bc2efffe 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3765,12 +3765,10 @@ struct page *rmqueue(struct zone *preferred_zone,
page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags,
migratetype);
- if (unlikely(!page))
- return NULL;
out:
/* Separate test+clear to avoid unnecessary atomics */
- if (test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags)) {
+ if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
wakeup_kswapd(zone, 0, 0, zone_idx(zone));
}
next prev parent reply other threads:[~2022-05-19 12:14 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-12 8:50 [PATCH 0/6] Drain remote per-cpu directly v3 Mel Gorman
2022-05-12 8:50 ` [PATCH 1/6] mm/page_alloc: Add page->buddy_list and page->pcp_list Mel Gorman
2022-05-13 11:59 ` Nicolas Saenz Julienne
2022-05-19 9:36 ` Vlastimil Babka
2022-05-12 8:50 ` [PATCH 2/6] mm/page_alloc: Use only one PCP list for THP-sized allocations Mel Gorman
2022-05-19 9:45 ` Vlastimil Babka
2022-05-12 8:50 ` [PATCH 3/6] mm/page_alloc: Split out buddy removal code from rmqueue into separate helper Mel Gorman
2022-05-13 12:01 ` Nicolas Saenz Julienne
2022-05-19 9:52 ` Vlastimil Babka
2022-05-23 16:09 ` Qais Yousef
2022-05-24 11:55 ` Mel Gorman
2022-05-25 11:23 ` Qais Yousef
2022-05-12 8:50 ` [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue Mel Gorman
2022-05-13 12:03 ` Nicolas Saenz Julienne
2022-05-19 10:57 ` Vlastimil Babka
2022-05-19 12:13 ` Mel Gorman [this message]
2022-05-19 12:26 ` Vlastimil Babka
2022-05-12 8:50 ` [PATCH 5/6] mm/page_alloc: Protect PCP lists with a spinlock Mel Gorman
2022-05-13 12:22 ` Nicolas Saenz Julienne
2022-05-12 8:50 ` [PATCH 6/6] mm/page_alloc: Remotely drain per-cpu lists Mel Gorman
2022-05-12 19:37 ` Andrew Morton
2022-05-13 15:04 ` Mel Gorman
2022-05-13 15:19 ` Nicolas Saenz Julienne
2022-05-13 18:23 ` Mel Gorman
2022-05-17 12:57 ` Mel Gorman
2022-05-12 19:43 ` [PATCH 0/6] Drain remote per-cpu directly v3 Andrew Morton
2022-05-13 14:23 ` Mel Gorman
2022-05-13 19:38 ` Andrew Morton
2022-05-16 10:53 ` Mel Gorman
2022-05-13 12:24 ` Nicolas Saenz Julienne
2022-05-17 23:35 ` Qian Cai
2022-05-18 12:51 ` Mel Gorman
2022-05-18 16:27 ` Qian Cai
2022-05-18 17:15 ` Paul E. McKenney
2022-05-19 13:29 ` Qian Cai
2022-05-19 19:15 ` Paul E. McKenney
2022-05-19 21:05 ` Qian Cai
2022-05-19 21:29 ` Paul E. McKenney
2022-05-18 17:26 ` Marcelo Tosatti
2022-05-18 17:44 ` Marcelo Tosatti
2022-05-18 18:01 ` Nicolas Saenz Julienne
2022-05-26 17:19 ` Qian Cai
2022-05-27 8:39 ` Mel Gorman
2022-05-27 12:58 ` Qian Cai
-- strict thread matches above, loose matches on Subject: below --
2022-05-09 13:07 [RFC PATCH 0/6] Drain remote per-cpu directly v2 Mel Gorman
2022-05-09 13:08 ` [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue Mel Gorman
2022-04-20 9:59 [RFC PATCH 0/6] Drain remote per-cpu directly Mel Gorman
2022-04-20 9:59 ` [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue Mel Gorman
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