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

[Peter Zijlstra]

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.

What again are we solving any why?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/