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

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Message-ID: <20200409164919.GF20713@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Apr 2020 18:49:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
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 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;
>  	}