linux-kernel - Re: [PATCH 2/9] mm: workingset: tell cache transitions from workingset thrashing

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Message-ID: <20180802122852.GA17974@cmpxchg.org>
Date:   Thu, 2 Aug 2018 08:28:52 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Daniel Drake <drake@...lessm.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        linux-mm <linux-mm@...ck.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/9] mm: workingset: tell cache transitions from
 workingset thrashing

Hi Suren,

On Wed, Aug 01, 2018 at 02:56:27PM -0700, Suren Baghdasaryan wrote:
> On Wed, Aug 1, 2018 at 8:19 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> >         /*
> > -        * The unsigned subtraction here gives an accurate distance
> > -        * across inactive_age overflows in most cases.
> > +        * Calculate the refault distance
> >          *
> > -        * There is a special case: usually, shadow entries have a
> > -        * short lifetime and are either refaulted or reclaimed along
> > -        * with the inode before they get too old.  But it is not
> > -        * impossible for the inactive_age to lap a shadow entry in
> > -        * the field, which can then can result in a false small
> > -        * refault distance, leading to a false activation should this
> > -        * old entry actually refault again.  However, earlier kernels
> > -        * used to deactivate unconditionally with *every* reclaim
> > -        * invocation for the longest time, so the occasional
> > -        * inappropriate activation leading to pressure on the active
> > -        * list is not a problem.
> > +        * The unsigned subtraction here gives an accurate distance
> > +        * across inactive_age overflows in most cases. There is a
> > +        * special case: usually, shadow entries have a short lifetime
> > +        * and are either refaulted or reclaimed along with the inode
> > +        * before they get too old.  But it is not impossible for the
> > +        * inactive_age to lap a shadow entry in the field, which can
> > +        * then can result in a false small refault distance, leading
> 
> "which can then can" - please remove one of the "can".

Good catch, will fix.