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

[Peter Zijlstra]

On Wed, Aug 21, 2013 at 02:33:11PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 21, 2013 at 01:35:51PM +0200, Oleg Nesterov wrote:
> > On 08/21, Peter Zijlstra wrote:
> 
> > > Yes its the right rq, but the wrong time.
> > 
> > Hmm. Just in case, it is not that I think this patch really makes sense,
> > but I'd like to understand why do you think it is wrong.
>  
> > But it is not "after it switches out", it is after it switched back.
> 
> D'uh I was being particularly dense it seems :/
> 
> Yes I think it is correct. You're now trading two atomic ops on a
> different cacheline than rq->lock for one atomic op on the rq->lock.
> 
> Might be a win, esp. since hopefully there's a fair chance its the same
> runqueue.

The other consideration is that this adds two branches to the normal
schedule path. I really don't know what the regular ratio between
schedule() and io_schedule() is -- and I suspect it can very much depend
on workload -- but it might be a net loss due to that, even if it makes
io_schedule() 'lots' cheaper.


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