linux-kernel - Re: Cgroup memory barrier usage and call frequency from scheduler

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Message-ID: <20200409172111.GL3818@techsingularity.net>
Date:   Thu, 9 Apr 2020 18:21:11 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Cgroup memory barrier usage and call frequency from scheduler

On Thu, Apr 09, 2020 at 06:49:19PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 09, 2020 at 04:44:13PM +0100, Mel Gorman wrote:
> 
> > For 1, the use of a full barrier seems unnecessary when it appears that
> > you could have used a read barrier and a write barrier. The following
> > patch drops the profile overhead to 0.1%
> 
> Yikes. And why still .1% the below should be a barrier() on x86. Is the
> compiler so contrained by that?
> 

The 0.1% is still doing all the work up until just after the barrier with
this check;

	if (cgroup_rstat_cpu(cgrp, cpu)->updated_next)
		return;

That must often be true as samples were not gathered in the rest of the
function. As this function is called on every update_curr(), it gets
called a lot.

-- 
Mel Gorman
SUSE Labs