Re: [PATCH 3/3] sched: fair: Fix wrong idle timestamp usage

[Daniel Lezcano]

On 04/15/2015 07:10 PM, Morten Rasmussen wrote:
> On Wed, Apr 15, 2015 at 04:43:17PM +0100, Daniel Lezcano wrote:
> > On 04/15/2015 02:18 PM, Peter Zijlstra wrote:
> > > On Wed, Apr 15, 2015 at 12:00:24PM +0200, Daniel Lezcano wrote:
> > > > The find_idlest_cpu is assuming the rq->idle_stamp information reflects when
> > > > the cpu entered the idle state. This is wrong as the cpu may exit and enter
> > > > the idle state several times without the rq->idle_stamp being updated.
> > >
> > > Sure, but you forgot to tell us why it matters.
> >
> > Yes, right. Thanks for pointing this out.
> >
> > Assuming we are in the situation where there are several idle cpus in
> > the same idle state.
> >
> > With the current code, the function find_idlest_cpu will choose a cpu
> > with the shortest idle duration. This information is based on the
> > rq->idle_stamp variable and is correct until one of the idle cpu is
> > exiting the cpuidle_enter function and re-entering it again. As soon as
> > this happen, the rq->idle_stamp value is no longer a reliable information.
> >
> > Example:
> >
> >    * CPU0 and CPU1 are running
> >    * CPU2 and CPU3 are in the C3 state.
> >    * CPU2 entered idle at T2
> >    * CPU3 entered idle at T3
> >    * T2 < T3
> >
> > The function find_idlest_cpu will choose CPU3 because it has a shorter
> > idle duration.
> >
> > Then CPU3 is woken up by an interrupt, process it and re-enter idle C3.
> >
> > The information will still give the out to date information T2 < T3 and
> > find_idlest_cpu will choose CPU2 instead of CPU3.
>
> I can't get the example to match your description of how
> find_idlest_cpu() is supposed to work :-(
>
> Did you mean CPU2 (not CPU3) getting woken up by an interrupt and
> find_busiest_cpu() choosing CPU3 instead of CPU2 after the interrupt?
>
> In your example find_busiest_cpu() should return CPU3 before the
> interrupt as it went to sleep last and the interrupt on CPU3 should not
> affect that choice as CPU3 is still the last cpu to go to sleep
> (regardless of your patch). No?

Yes you are right. I meant CPU2 is woken up by the interrupt.


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