linux-kernel - Re: [PATCH 2/2 RESEND] mm: vmpressure: fix sending wrong events on underflow

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Date:   Mon, 6 Feb 2017 14:24:11 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     vinayak menon <vinayakm.list@...il.com>
Cc:     Vinayak Menon <vinmenon@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        mgorman@...hsingularity.net, vbabka@...e.cz,
        Rik van Riel <riel@...hat.com>, vdavydov.dev@...il.com,
        anton.vorontsov@...aro.org, Minchan Kim <minchan@...nel.org>,
        shashim@...eaurora.org, "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2 RESEND] mm: vmpressure: fix sending wrong events on
 underflow

On Mon 06-02-17 18:39:03, vinayak menon wrote:
> On Mon, Feb 6, 2017 at 6:10 PM, Michal Hocko <mhocko@...nel.org> wrote:
> > On Mon 06-02-17 17:54:10, Vinayak Menon wrote:
> > [...]
> >> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> >> index 149fdf6..3281b34 100644
> >> --- a/mm/vmpressure.c
> >> +++ b/mm/vmpressure.c
> >> @@ -112,8 +112,10 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
> >>                                                   unsigned long reclaimed)
> >>  {
> >>       unsigned long scale = scanned + reclaimed;
> >> -     unsigned long pressure;
> >> +     unsigned long pressure = 0;
> >>
> >> +     if (reclaimed >= scanned)
> >> +             goto out;
> >
> > This deserves a comment IMHO. Besides that, why shouldn't we normalize
> > the result already in vmpressure()? Please note that the tree == true
> > path will aggregate both scanned and reclaimed and that already skews
> > numbers.
> Sure. Will add a comment.
> IIUC, normalizing in vmpressure() means something like this which you
> mentioned in one
> of your previous emails right ?
> 
> + if (reclaimed > scanned)
> +          reclaimed = scanned;

yes or scanned = reclaimed.

> Considering a scan window of 512 pages and without above piece of
> code, if the first scanning is of a THP page
> Scan=1,Reclaimed=512
> If the next 511 scans results in 0 reclaimed pages
> total_scan=512,Reclaimed=512 => vmpressure 0

I am not sure I understand. What do you mean by next scans? We do not
modify counters outside of vmpressure? If you mean next iteration of
shrink_node's loop then this changeshouldn't make a difference, no?

> 
> Now with the above piece of code in place
> Scan=1,Reclaimed=1, then
> Scan=511, Reclaimed=0
> total_scan=512,Reclaimed=1 => critical vmpressure

-- 
Michal Hocko
SUSE Labs