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

[Arjan van de Ven]

On 8/20/2013 1:44 AM, 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.
>
> 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 :-)

I'm quite ok with removing that.

however note that "top" also reports per cpu iowait...
and that's a userspace expectation

>

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