Re: [Xen-devel] [PATCH 19/19] xen: credit2: use cpumask_first instead of cpumask_any when choosing cpu

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Tue, 2016-06-21 at 11:42 +0100, David Vrabel wrote:
> On 18/06/16 00:13, Dario Faggioli wrote:
> > 
> > because it is cheaper, and there is no much point in
> > randomizing which cpu gets selected anyway, as such
> > choice will be overridden shortly after, in runq_tickle().
> > 
> > If we really feel the need (e.g., we prove it worth with
> > benchmarking), we can record the last cpu which was used
> > by csched2_cpu_pick() and migrate() in a per-runq variable,
> > and then use cpumask_cycle()... but this really does not
> > look necessary.
> Isn't this backwards?  Surely you should demonstrate that this change
> is
> beneficial before proposing it?
> 
Right. I think it's my fault having presented things this way.

This patch get rid of something that is pure overhead, and getting rid
of overhead is, in general, a good thing.

There is only one possible situation under which we may actually end up
favouring lower pCPU IDs, and it is unlikely enough that it is IMO, of
no concern.

But in any case, let's just drop this patch. I'm rerunning the
benchmarks anyway, I'll consider doing a set of runs with and without
this patch, and check if it does make any difference.

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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