linux-kernel - Re: [PATCH 2/2] sched: Implement interface for cgroup unified hierarchy

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Message-ID: <20170802181600.GJ2311718@devbig577.frc2.facebook.com>
Date:   Wed, 2 Aug 2017 11:16:00 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     lizefan@...wei.com, hannes@...xchg.org, mingo@...hat.com,
        longman@...hat.com, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, efault@....de, torvalds@...ux-foundation.org,
        guro@...com
Subject: Re: [PATCH 2/2] sched: Implement interface for cgroup unified
 hierarchy

Hello, Peter.

On Wed, Aug 02, 2017 at 06:05:11PM +0200, Peter Zijlstra wrote:
> > * The stat file is sampling based and the usage files are calculated
> >   from actual scheduling events.  Is this because the latter is more
> >   accurate?
> 
> So I actually don't know the history of this stuff too well. But I would
> think so. This all looks rather dodgy.

I see.

> > * Why do we have user/sys breakdown in usage numbers?  It tries to
> >   distinguish user or sys by looking at task_pt_regs().  I can't see
> >   how this would work (e.g. interrupt handlers never schedule) and w/o
> >   kernel preemption, the sys part is always zero.  What is this number
> >   supposed to mean?
> 
> For normal scheduler stuff we account the total runtime in ns and use
> the user/kernel tick samples to divide it into user/kernel time parts.
> See cputime_adjust().
> 
> But looking at the cpuacct I have no clue, that looks wonky at best.
> 
> Ideally we'd reuse the normal cputime code and do the same thing
> per-cgroup, but clearly that isn't happening now.
> 
> I never really looked further than that cpuacct_charge() doing _another_
> cgroup iteration, even though we already account that delta to each
> cgroup (modulo scheduling class crud).

Yeah, it's kinda silly.  I'll see if I can just kill cpuacct for
cgroup2.

Thanks.

-- 
tejun