Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

[Frederic Weisbecker <fweisbec@gmail.com>]

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