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Message-ID: <1408095453.5567.6.camel@marge.simpson.net>
Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with
 seqlock
From: Mike Galbraith <umgwanakikbuti@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Rik van Riel <riel@redhat.com>,
        linux-kernel@vger.kernel.org,
        Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>,
        Frank Mayhar <fmayhar@google.com>,
        Frederic Weisbecker <fweisbec@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sanjay Rao <srao@redhat.com>, Larry Woodman <lwoodman@redhat.com>
Date: Fri, 15 Aug 2014 11:37:33 +0200
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On Fri, 2014-08-15 at 08:28 +0200, Peter Zijlstra wrote: 
> On Fri, Aug 15, 2014 at 07:19:31AM +0200, Mike Galbraith wrote:
> > For the N threads doing this on N cores case, seems rq->lock hammering
> > will still be a source of major box wide pain.  Is there any correctness
> > reason to add up unaccounted ->on_cpu beans, or is that just value
> > added?  
> 
> That delta can be arbitrarily large with nohz_full. And without
> nohz_full the error is nr_cpus*TICK_NSEC, which I bet is larger than the
> reported clock resolution.
> 
> Having a non-constant error bound is annoying for you never quite know
> what to expect.

Ah, yeah, that could get rather large.

> Also; why do we care about PROCESS_CPUTIME? People should really not use
> it. What are the 'valid' usecases you guys care about?

I don't care much, said "don't do that" before I saw a similar big box
problem had popped up with times().

-Mike