linux-kernel - Re: [PATCH] mm: avoid blocking lock

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Date:   Tue, 28 Jan 2020 09:22:43 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

On Mon 27-01-20 16:46:16, Cong Wang wrote:
> On Mon, Jan 27, 2020 at 6:49 AM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > On Sun 26-01-20 11:53:55, Cong Wang wrote:
> > > On Tue, Jan 21, 2020 at 1:00 AM Michal Hocko <mhocko@...nel.org> wrote:
> > > >
> > > > On Mon 20-01-20 14:48:05, Cong Wang wrote:
> > > > > It got stuck somewhere along the call path of mem_cgroup_try_charge(),
> > > > > and the trace events of mm_vmscan_lru_shrink_inactive() indicates this
> > > > > too:
> > > >
> > > > So it seems that you are condending on the page lock. It is really
> > > > unexpected that the reclaim would take that long though. Please try to
> > > > enable more vmscan tracepoints to see where the time is spent.
> > >
> > > Sorry for the delay. I have been trying to collect more data in one shot.
> > >
> > > This is a a typical round of the loop after enabling all vmscan tracepoints:
> > >
> > >            <...>-455450 [007] .... 4048595.842992:
> > > mm_vmscan_memcg_reclaim_begin: order=0 may_writepage=1
> > > gfp_flags=GFP_NOFS|__GFP_HIGHMEM|__GFP_HARDWALL|__GFP_MOVABLE
> > > classzone_idx=4
> > >            <...>-455450 [007] .... 4048595.843012:
> > > mm_vmscan_memcg_reclaim_end: nr_reclaimed=0
> >
> > This doesn't tell us much though. This reclaim round has taken close to
> > no time. See timestamps.
> >
> > > The whole trace output is huge (33M), I can provide it on request.
> >
> > Focus on reclaim rounds that take a long time and see where it gets you.
> 
> I reviewed the tracing output with my eyes, they all took little time.
> But of course I can't review all of them given the size is huge.

Sure, I would simply use a script to check for exessive reclaims rounds.
Also it is important to find out whether the page of your interest is
locked outside of the reclaim or inside.
-- 
Michal Hocko
SUSE Labs