linux-kernel - Re: [PATCH v2] mm/page_alloc: call check_new_pages() while zone spinlock is not held

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Date:   Tue, 8 Mar 2022 15:49:48 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v2] mm/page_alloc: call check_new_pages() while zone
 spinlock is not held

On Mon, Mar 7, 2022 at 1:15 AM Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Fri, Mar 04, 2022 at 09:02:15AM -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > For high order pages not using pcp, rmqueue() is currently calling
> > the costly check_new_pages() while zone spinlock is held,
> > and hard irqs masked.
> >
> > This is not needed, we can release the spinlock sooner to reduce
> > zone spinlock contention.
> >
> > Note that after this patch, we call __mod_zone_freepage_state()
> > before deciding to leak the page because it is in bad state.
> >
> > v2: We need to keep interrupts disabled to call __mod_zone_freepage_state()
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Mel Gorman <mgorman@...hsingularity.net>
> > Cc: Vlastimil Babka <vbabka@...e.cz>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: Shakeel Butt <shakeelb@...gle.com>
> > Cc: Wei Xu <weixugc@...gle.com>
> > Cc: Greg Thelen <gthelen@...gle.com>
> > Cc: Hugh Dickins <hughd@...gle.com>
> > Cc: David Rientjes <rientjes@...gle.com>
>
> Ok, this is only more expensive in the event pages on the free list have
> been corrupted whch is already very unlikely so thanks!
>
> Acked-by: Mel Gorman <mgorman@...hsingularity.net>
>

One remaining question is:

After your patch ("mm/page_alloc: allow high-order pages to be stored
on the per-cpu lists"),
do we want to change check_pcp_refill()/check_new_pcp() to check all pages,
and not only the head ?

Or was it a conscious choice of yours ?
(I presume part of the performance gains came from
not having to bring ~7 cache lines per 32KB chunk on x86)

Thanks !