Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

Mike Galbraith
2014-08-15T09:37:39Z

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