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

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Date:   Mon, 6 Feb 2017 18:39:03 +0530
From:   vinayak menon <vinayakm.list@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
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, 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;

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

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

With the slab issue fixed separately, we need to fix only the
underflow right ? And if we do it in vmpressure_calc_level,
the check needs to done only once at the end of a scan window.

Thanks,
Vinayak