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Date:   Wed, 18 Jul 2018 14:22:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Christopher Lameter <cl@linux.com>,
        Mike Galbraith <efault@gmx.de>,
        Shakeel Butt <shakeelb@google.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO
Message-ID: <20180718122207.GX2512@hirez.programming.kicks-ass.net>
References: <20180712172942.10094-1-hannes@cmpxchg.org>
 <20180712172942.10094-9-hannes@cmpxchg.org>
 <20180718120318.GC2476@hirez.programming.kicks-ass.net>
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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]++;