From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@techsingularity.net>
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 18:49:19 +0200 [thread overview]
Message-ID: <20200409164919.GF20713@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200409154413.GK3818@techsingularity.net>
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?
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index ca19b4c8acf5..bc3125949b4b 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -36,7 +36,7 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu)
> * Paired with the one in cgroup_rstat_cpu_pop_upated(). Either we
> * see NULL updated_next or they see our updated stat.
> */
> - smp_mb();
> + smp_rmb();
>
> /*
> * Because @parent's updated_children is terminated with @parent
> @@ -139,7 +139,7 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
> * Either they see NULL updated_next or we see their
> * updated stat.
> */
> - smp_mb();
> + smp_wmb();
>
> return pos;
> }
next prev parent reply other threads:[~2020-04-09 16:49 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 [this message]
2020-04-09 17:21 ` Mel Gorman
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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