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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Aug 20, 2013 at 10:44:05AM +0200, Peter Zijlstra wrote:
> On Tue, Aug 20, 2013 at 03:59:36PM +0900, Fernando Luis Vázquez Cao wrote:
> > That said, if deemed acceptable, option A is the one I would
> > choose.
> 
> Right, so I think we can do A without much extra cost mostly because we
> already have 2 atomics in the io_schedule() path. If we replace those
> two atomic operations with locks and modify nr_iowait and the other
> stats under the same lock, and ensure all those variables (including the
> lock) live in the same cacheline we should have the same cost we have
> now.

I can try that :-)

> 
> Of course, if we can get away with completely removing all of that
> (which I think Arjan suggested was a real possibility) then that would
> be ever so much better still :-)

Would be lovely. But I don't know much about cpufreq, I hope somebody who's
familiar with that code can handle this. Then once there are no more users
of get_cpu_iowait_sleep_time() I can simply zap and clean the tick/time related
code.

Surely the overhead that this mess brings to io_schedule() (and it's going
to be worth with a seqlock, whether in the same cacheline than nr_iowait or
not) may be a good motivation to poke at that cpufreq code part.

Thanks.