linux-kernel - Re: [PATCH 3/3] cgroup: Implement cgroup2 basic CPU usage accounting

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Message-ID: <20170813194421.GB1438922@devbig577.frc2.facebook.com>
Date:   Sun, 13 Aug 2017 12:44:21 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     lizefan@...wei.com, hannes@...xchg.org, peterz@...radead.org,
        mingo@...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 3/3] cgroup: Implement cgroup2 basic CPU usage accounting

Hello, Waiman.

On Fri, Aug 11, 2017 at 04:19:07PM -0400, Waiman Long wrote:
> > * CPU usage stats are collected and shown in "cgroup.stat" with "cpu."
> >   prefix.  Total usage is collected from scheduling events.  User/sys
> >   breakdown is sourced from tick sampling and adjusted to the usage
> >   using cputime_adjuts().
> 
> Typo: cputime_adjust()

Oops, will fix.

> > +static DEFINE_MUTEX(cgroup_stat_mutex);
> > +static DEFINE_PER_CPU(raw_spinlock_t, cgroup_cpu_stat_lock);
> 
> If the hierarchy is large enough and the stat data hasn't been read
> recently, it may take a while to accumulate all the stat data even for
> one cpu in cgroup_stat_flush_locked(). So I think it will make more
> sense to use regular spinlock_t instead of raw_spinlock_t.

They need to be raw spinlocks because the accounting side may grab
them while scheduling.  If the accumulating latency becomes
problematic, we can test for need_resched and spin_needbreak and break
out as necessary.  The iteration logic is built to allow that and an
earlier revision actually did that but I wasn't sure whether it's
actually necessary and removed it for simplicity.

If the latency ever becomes a problem, resurrecting that isn't
difficult at all.

> > +static struct cgroup *cgroup_cpu_stat_next_updated(struct cgroup *pos,
> > +						   struct cgroup *root, int cpu)
> 
> This function is trying to unwind one cgrp from the updated_children and
> updated_next linkage. It is somewhat like the opposite of
> cgroup_cpu_stat_updated(). I just feel like its name isn't intuitive
> enough to convey what it is doing. Maybe use name like
> cgroup_cpu_stat_unlink_one() to match cgroup_cpu_stat_flush_one().

Hmm... the name comes from it being an iterator - most interators are
named _next.  But, yeah, the name doesn't signifiy that it unlinks as
it goes along.  I'll rename it to cgroup_cpu_stat_pop_updated().

Thanks.

-- 
tejun