linux-kernel - Re: [mm] 755d6edc1a: will-it-scale.per_process

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Date:   Tue, 6 Aug 2019 20:00:24 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Miguel de Dios <migueldedios@...gle.com>,
        Wei Wang <wvw@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>, lkp@...org
Subject: Re: [mm]  755d6edc1a:  will-it-scale.per_process_ops -4.1% regression

On Tue, Aug 06, 2019 at 10:04:15AM +0200, Michal Hocko wrote:
> On Tue 06-08-19 15:05:47, kernel test robot wrote:
> > Greeting,
> > 
> > FYI, we noticed a -4.1% regression of will-it-scale.per_process_ops due to commit:
> 
> I have to confess I cannot make much sense from numbers because they
> seem to be too volatile and the main contributor doesn't stand up for
> me. Anyway, regressions on microbenchmarks like this are not all that
> surprising when a locking is slightly changed and the critical section
> made shorter. I have seen that in the past already.

I guess if it's multi process workload. The patch will give more chance
to be scheduled out so TLB miss ratio would be bigger than old.
I see it's natural trade-off for latency vs. performance so only thing
I could think is just increase threshold from 32 to 64 or 128?

> 
> That being said I would still love to get to bottom of this bug rather
> than play with the lock duration by a magic. In other words
> http://lkml.kernel.org/r/20190730125751.GS9330@dhcp22.suse.cz

Yes, if we could remove mark_page_accessed there, it would be best.
I added a commen in the thread.