From: Aaron Lu <aaron.lu@intel.com>
To: David Rientjes <rientjes@google.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Huang Ying <ying.huang@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Kemi Wang <kemi.wang@intel.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
Andi Kleen <ak@linux.intel.com>, Michal Hocko <mhocko@suse.com>,
Vlastimil Babka <vbabka@suse.cz>,
Mel Gorman <mgorman@techsingularity.net>,
Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v3 1/3] mm/free_pcppages_bulk: update pcp->count inside
Date: Tue, 27 Feb 2018 09:56:13 +0800 [thread overview]
Message-ID: <20180227015613.GA9141@intel.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1802261345550.135844@chino.kir.corp.google.com>
On Mon, Feb 26, 2018 at 01:48:14PM -0800, David Rientjes wrote:
> On Mon, 26 Feb 2018, Aaron Lu wrote:
>
> > Matthew Wilcox found that all callers of free_pcppages_bulk() currently
> > update pcp->count immediately after so it's natural to do it inside
> > free_pcppages_bulk().
> >
> > No functionality or performance change is expected from this patch.
> >
> > Suggested-by: Matthew Wilcox <willy@infradead.org>
> > Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> > ---
> > mm/page_alloc.c | 10 +++-------
> > 1 file changed, 3 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index cb416723538f..3154859cccd6 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1117,6 +1117,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> > int batch_free = 0;
> > bool isolated_pageblocks;
> >
> > + pcp->count -= count;
> > spin_lock(&zone->lock);
> > isolated_pageblocks = has_isolate_pageblock(zone);
> >
>
> Why modify pcp->count before the pages have actually been freed?
When count is still count and not zero after pages have actually been
freed :-)
>
> I doubt that it matters too much, but at least /proc/zoneinfo uses
> zone->lock. I think it should be done after the lock is dropped.
Agree that it looks a bit weird to do it beforehand and I just want to
avoid adding one more local variable here.
pcp->count is not protected by zone->lock though so even we do it after
dropping the lock, it could still happen that zoneinfo shows a wrong
value of pcp->count while it should be zero(this isn't a problem since
zoneinfo doesn't need to be precise).
Anyway, I'll follow your suggestion here to avoid confusion.
> Otherwise, looks good.
Thanks for taking a look at this.
next prev parent reply other threads:[~2018-02-27 1:55 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-26 13:53 [PATCH v3 0/3] mm: improve zone->lock scalability Aaron Lu
2018-02-26 13:53 ` [PATCH v3 1/3] mm/free_pcppages_bulk: update pcp->count inside Aaron Lu
2018-02-26 21:48 ` David Rientjes
2018-02-27 1:56 ` Aaron Lu [this message]
2018-02-27 3:12 ` Aaron Lu
2018-02-26 13:53 ` [PATCH v3 2/3] mm/free_pcppages_bulk: do not hold lock when picking pages to free Aaron Lu
2018-02-26 21:53 ` David Rientjes
2018-02-27 2:00 ` Aaron Lu
2018-02-27 3:17 ` Aaron Lu
2018-02-26 13:53 ` [PATCH v3 3/3] mm/free_pcppages_bulk: prefetch buddy while not holding lock Aaron Lu
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