From: Mel Gorman <mgorman@techsingularity.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@redhat.com>,
Davidlohr Bueso <dave@stgolabs.net>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: Cgroup memory barrier usage and call frequency from scheduler
Date: Thu, 9 Apr 2020 18:21:11 +0100 [thread overview]
Message-ID: <20200409172111.GL3818@techsingularity.net> (raw)
In-Reply-To: <20200409164919.GF20713@hirez.programming.kicks-ass.net>
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
next prev parent reply other threads:[~2020-04-09 17:21 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-09 15:44 Cgroup memory barrier usage and call frequency from scheduler Mel Gorman
2020-04-09 16:49 ` Peter Zijlstra
2020-04-09 17:21 ` Mel Gorman [this message]
2020-04-09 17:56 ` Tejun Heo
2020-04-09 18:20 ` Mel Gorman
2020-04-09 19:27 ` Tejun Heo
2020-04-09 19:08 ` [PATCH cgroup/for-5.7-fixes] Revert "cgroup: Add memory barriers to plug cgroup_rstat_updated() race window" Tejun Heo
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