On Fri, Aug 15, 2014 at 11:37:33AM +0200, Mike Galbraith wrote: > 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().
Urgh, yes times().. Now I don't think we do very accurate accounting of those particular numbers, so we could fudge some of that. Typically we only do TICK_NSEC granularity accounting on user/system divide anyhow, seeing how putting timestamp reads in the kernel<>user switch is _expensive_ -- see NOHZ_FULL.
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