linux-kernel - Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO

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Message-ID: <20180718122207.GX2512@hirez.programming.kicks-ass.net>
Date:   Wed, 18 Jul 2018 14:22:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO

On Wed, Jul 18, 2018 at 02:03:18PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:
> > +	for (to = 0; (bo = ffs(set)); to += bo, set >>= bo)
> > +		tasks[to + (bo - 1)]++;
> 
> You want to benchmark this, but since it's only 3 consecutive bits, it
> might actually be faster to not use ffs() and simply test all 3 bits:
> 
> 	for (to = set, bo = 0; to; to &= ~(1 << bo), bo++)

		if (to & (1 << bo))

> 		tasks[bo]++;