On Tue, Oct 01, 2013 at 05:00:37PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 01, 2013 at 04:05:27PM +0200, Frederic Weisbecker wrote:
>
> > struct cpu_idletime {
> > nr_iowait,
> > seqlock,
> > idle_start,
> > idle_time,
> > iowait_time,
> > } __cacheline_aligned_in_smp;
> >
> > DEFINE_PER_CPU(struct cpu_idletime, cpu_idletime);
> >
> > io_schedule()
> > {
> > int prev_cpu;
> >
> > preempt_disable();
> > prev_cpu_idletime = __this_cpu_ptr(&cpu_idletime);
> > atomic_inc(prev_cpu_idletime->nr_iowait);
> > WARN_ON_ONCE(is_idle_task(current));
> > preempt_enable_no_resched();
> >
> > schedule();
> >
> > write_seqlock(prev_cpu_idletime->seqlock)
> > if (!atomic_dec_return(prev_cpu_idletime->nr_iowait))
> > flush_cpu_idle_time(prev_cpu_idletime, 1)
> > write_sequnlock(prev_cpu_idletime->seqlock)
> >
> > }
>
> This is at least 3 atomic ops and a whole bunch of branches extra. It
> used to be 2 atomics and no branches.
Yeah. If somebody has a better proposition, I'm all for it.
Of course the best would be to remove these stats if we can. I think we
already concluded that the idea of per CPU iowait stats is broken since
sleeping tasks aren't assigned a particular CPU.
>
> What again are we solving any why?
Fernando summerized it better than I could
* https://lkml.org/lkml/2013/3/18/962
* and http://marc.info/?l=linux-kernel&m=137273800916899&w=2
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