From mboxrd@z Thu Jan  1 00:00:00 1970
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and
 IO
Date: Mon, 6 Aug 2018 11:23:54 -0400
Message-ID: <20180806152354.GC9888@cmpxchg.org>
References: <20180801151958.32590-1-hannes@cmpxchg.org>
 <20180801151958.32590-9-hannes@cmpxchg.org>
 <20180803170733.GC2494@hirez.programming.kicks-ass.net>
Mime-Version: 1.0
Return-path: <linux-kernel-owner@vger.kernel.org>
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=cmpxchg-org.20150623.gappssmtp.com; s=20150623;
        h=date:from:to:cc:subject:message-id:references:mime-version
         :content-disposition:in-reply-to:user-agent;
        bh=h+72NQHnVkw3JzF7vSCUqCLRqDUBnExu3aTIRjzL7UQ=;
        b=CoCMzqzoYcTFmx000b8rNbHI4R45SGuXMX7WwraKp1WQingX7hWPLQsH2yLNZZNKA4
         VwfNXJqbPKUPqtsle2r4g8kRNsolpAe8qZhoKlTJh9KUJ9X11zGks/ZrNuzDt8y2Ktpz
         0o8CGXsBjD+ds/MEGkt0Q8pKvRqmr+wvvU9RglL0BdEsSpryYENJq1GOva3nzzjlK48L
         PpoZ4Sj0NQ9PFwNHzFaPHWDOgAMa/RrxYewv+FyPfCkT7MyKmYo/wlU19VXehfy0dmpR
         trRqAAHGL7mOcPuSnziZkWO/HqBk7uS5PtodZ0ce4yIbB/wYlYlm5RJVl62ItLPUO1TS
         bbRQ==
Content-Disposition: inline
In-Reply-To: <20180803170733.GC2494@hirez.programming.kicks-ass.net>
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <cgroups.vger.kernel.org>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
To: Peter Zijlstra <peterz@infradead.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>, Daniel Drake <drake@endlessm.com>, Vinayak Menon <vinmenon@codeaurora.org>, Christopher Lameter <cl@linux.com>, Mike Galbraith <efault@gmx.de>, Shakeel Butt <shakeelb@google.com>, Peter Enderborg <peter.enderborg@sony.com>, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com

On Fri, Aug 03, 2018 at 07:07:33PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote:
> > +static bool psi_update_stats(struct psi_group *group)
> > +{
> > +	u64 deltas[NR_PSI_STATES - 1] = { 0, };
> > +	unsigned long missed_periods = 0;
> > +	unsigned long nonidle_total = 0;
> > +	u64 now, expires, period;
> > +	int cpu;
> > +	int s;
> > +
> > +	mutex_lock(&group->stat_lock);
> > +
> > +	/*
> > +	 * Collect the per-cpu time buckets and average them into a
> > +	 * single time sample that is normalized to wallclock time.
> > +	 *
> > +	 * For averaging, each CPU is weighted by its non-idle time in
> > +	 * the sampling period. This eliminates artifacts from uneven
> > +	 * loading, or even entirely idle CPUs.
> > +	 *
> > +	 * We don't need to synchronize against CPU hotplugging. If we
> > +	 * see a CPU that's online and has samples, we incorporate it.
> > +	 */
> > +	for_each_online_cpu(cpu) {
> 
> I'm still puzzled by this.. for 99% of the machines online == possible.
> Why not always iterate possible and leave it at that? This is hardly a
> fast path.

Hmm, you're right, that makes things much simpler. I guess I'm mostly
worried about the 1% where this significantly differs, but it looks
like we're smarter than simply doing CONFIG_NR_CPUS for the possible
map, and we can easily stomach a bit of discrepancy in this path.

I'll change that to possible and delete/update the third paragraph.

Thanks