Re: [PATCH 17/24] xen: credit2: soft-affinity awareness in runq_tickle()

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

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On Thu, 2016-09-01 at 11:52 +0100, anshul makkar wrote:
> On 17/08/16 18:19, Dario Faggioli wrote:
> > 
> > +    /*
> > +     * We're doing soft-affinity, and we know that the current
> > vcpu on cpu
> > +     * has a soft affinity. We now want to know whether cpu itself
> > is in
> Please can you explain the above statment. If the vcpu has soft
> affinity 
> and its currently executing, doesn;t it always means that its running
> on 
> one of the pcpu which is there in its soft affinity or hard affinity?
>
A vcpu will always run on a pcpu from its own hard-affinity (that's the
definition of hard-affinity).

On the other hand, a vcpu will, most of the time, run on a cpu from its
own soft affinity, but can run on a cpu that is in its hard-affinity,
but *IS NOT* in its soft-affinity.

That's the definition of soft-affinity: the scheduler will try to run
it there, but it that can't happen, it will run it will run it outside
of it (but still within its hard-affinity, of course).

So, yes, we know already that it's running in a cpu at least from its
hard affinity, what is it exactly that you are not understanding?

> > +     * such affinity. In fact, since we now that new (in
> > runq_tickle()) is
> Typo:   * such affinity. In fact, since now we know that new (in 
> runq_tickle()) is
>
Thanks. :-)

> > +     *  - if cpu is not in cur's soft-affinity, we should indeed
> > check to
> > +     *    see whether new should preempt cur. If that will be the
> > case, that
> > +     *    would be an improvement wrt respecting soft affinity;
> > +     *  - if cpu is in cur's soft-affinity, we leave it alone and
> > (in
> > +     *    runq_tickle()) move on to another cpu. In fact, we don't
> > want to
> > +     *    be too harsh with someone which is running within its
> > soft-affinity.
> > +     *    This is safe because later, if we don't fine anyone else
> > during the
> > +     *    soft-affinity step, we will check cpu for preemption
> > anyway, when
> > +     *    doing hard-affinity.
> > +     */
>
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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