linux-kernel - Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20161018110954.GX3142@twins.programming.kicks-ass.net>
Date:   Tue, 18 Oct 2016 13:09:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Matt Fleming <matt@...eblueprint.co.uk>,
        Wanpeng Li <kernellwp@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Yuyang Du <yuyang.du@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

On Wed, Oct 12, 2016 at 09:41:36AM +0200, Vincent Guittot wrote:

> ok. In fact, I have noticed another regression with tip/sched/core and
> hackbench while looking at yours.
> I have bisect to :
> 10e2f1acd0 ("sched/core: Rewrite and improve select_idle_siblings")
> 
> hackbench -P -g 1
> 
>        v4.8        tip/sched/core  tip/sched/core+revert 10e2f1acd010
> and 1b568f0aabf2
> min 0.051       0,052               0.049
> avg 0.057(0%)   0,062(-7%)   0.056(+1%)
> max 0.070       0,073      0.067
> stdev  +/-8%       +/-10%    +/-9%
> 
> The issue seems to be that it prevents some migration at wake up at
> the end of hackbench test so we have last tasks that compete for the
> same CPU whereas other CPUs are idle in the same MC domain. I haven't
> to look more deeply which part of the patch do the regression yet

So select_idle_cpu(), which does the LLC wide CPU scan is now throttled
by a comparison between avg_cost and avg_idle; where avg_cost is a
historical measure of how costly it was to scan the entire LLC domain
and avg_idle is our current idle time guestimate (also a historical
average).

The problem was that a number of workloads were spending quite a lot of
time here scanning CPUs while they could be doing useful work (esp.
since newer parts have silly amounts of CPUs per LLC).

The toggle is a heuristic with a random number in.. we could see if
there's anything better we can do. I know some people take the toggle
out entirely, but that will regress other workloads.