linux-kernel - Re: [PATCH 2/2] sched/fair: Always propagate runnable_load

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: Windows password security audit tool. GUI, reports in PDF.

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

Date:   Wed, 3 May 2017 11:37:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
        Chris Mason <clm@...com>, kernel-team@...com
Subject: Re: [PATCH 2/2] sched/fair: Always propagate runnable_load_avg

On Wed, May 03, 2017 at 09:34:51AM +0200, Vincent Guittot wrote:

> We use load_avg for calculating a stable share and we want to use it
> more and more.  So breaking it because it's easier doesn't seems to be
> the right way to do IMHO

So afaict we calculate group se->load.weight (aka shares, see
calc_cfs_shares), using cfs_rq->avg.load_avg, which feeds into
tg->load_avg through cfs_rq->tg_load_avg_contrib and
cfs_rq->load.weight.

And cfs_rq->avg.load_avg is computed from cfs_rq->load.weight, which
is \Sum se->load.weight.

OTOH group se->avg.load_avg isn't used much, which is TJ's point.

The only cases where group se->avg.load_avg are relevant to
cfs_rq->avg.load are the cases I listed yesterday, group creation and
group destruction. There we use the group se->avg.load_avg to manage the
boundary conditions.

So with the proposed change to se->avg.load_avg we can have some
(temporary) boundary effect when you destroy a lot of (previously
active) cgroups.

Of course, it could be I overlooked something, in which case, please
tell :-)

That said, I agree it would be nice to entirely get rid of runnable_avg,
but that is a much larger change and would require a lot more work. I
don't immediately see why we can't fix the thing now and then work on
removing runnable_load_avg later.

Of course, we should not regress either, I'll go read up on that part.