Re: [PATCH 3/9] xen: sched: make locking for {insert, remove}_vcpu consistent

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Thu, 2015-10-08 at 18:23 +0100, Andrew Cooper wrote:
> On 08/10/15 17:46, George Dunlap wrote:
> > On 08/10/15 16:20, Andrew Cooper wrote:

> > > There are a lot of codepaths, but I cant see one which is
> > > definitely
> > > called with interrupts disables.  (OTOH, I can see several where
> > > interrupts are definitely enabled).
> > Oh, I think I misunderstood you.  You meant, "No codepaths *calling
> > into* generic scheduling code should need interrupts disabled at
> > all".
> > I can certainly believe that to be true in most cases; there's no
> > sense
> > in saving the flags if we don't need to.
> 
> My original statement came from the observation that schedule() runs
> with interrupts disabled, and takes between 2.2 and 4 microseconds to
> run (as measured during the ticketlock performance analysis).
> 
Ok, this part, I got, I think.

BTW, do you still have the scripts/harness/whatever to perform such
analysis? I'm asking because I'm about to give it a try at reducing the
amount of time during which the scheduler keeps interrupt disabled.

If I manage, it'd be nice to retry it, an get updated numbers.

> I am going to go out on a limb and say that the majority of that time
> does not need to be spent with interrupts disabled.  I might easily
> be
> wrong, but I suspect I am not.
> 
I'll have a look and let you know what I find.

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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