Re: [PATCH V3 RESEND RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task

Ingo Molnar
2013-01-25T10:47:37Z

* Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:

> * Ingo Molnar <mingo@kernel.org> [2013-01-24 11:32:13]:
>
> >
> > * Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:
> >
> > > From: Peter Zijlstra <peterz@infradead.org>
> > >
> > > In case of undercomitted scenarios, especially in large guests
> > > yield_to overhead is significantly high. when run queue length of
> > > source and target is one, take an opportunity to bail out and return
> > > -ESRCH. This return condition can be further exploited to quickly come
> > > out of PLE handler.
> > >
> > > (History: Raghavendra initially worked on break out of kvm ple handler upon
> > > seeing source runqueue length = 1, but it had to export rq length).
> > > Peter came up with the elegant idea of return -ESRCH in scheduler core.
> > >
> > > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> > > Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
> > > Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> > > Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> > > Acked-by: Andrew Jones <drjones@redhat.com>
> > > Tested-by: Chegu Vinod <chegu_vinod@hp.com>
> > > ---
> > >
> > > kernel/sched/core.c | 25 +++++++++++++++++++------
> > > 1 file changed, 19 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 2d8927f..fc219a5 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -4289,7 +4289,10 @@ EXPORT_SYMBOL(yield);
> > > * It's the caller's job to ensure that the target task struct
> > > * can't go away on us before we can do any checks.
> > > *
> > > - * Returns true if we indeed boosted the target task.
> > > + * Returns:
> > > + * true (>0) if we indeed boosted the target task.
> > > + * false (0) if we failed to boost the target.
> > > + * -ESRCH if there's no task to yield to.
> > > */
> > > bool __sched yield_to(struct task_struct *p, bool preempt)
> > > {
> > > @@ -4303,6 +4306,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
> > >
> > > again:
> > > p_rq = task_rq(p);
> > > + /*
> > > + * If we're the only runnable task on the rq and target rq also
> > > + * has only one task, there's absolutely no point in yielding.
> > > + */
> > > + if (rq->nr_running == 1 && p_rq->nr_running == 1) {
> > > + yielded = -ESRCH;
> > > + goto out_irq;
> > > + }
> >
> > Looks good to me in principle.
> >
> > Would be nice to get more consistent benchmark numbers. Once
> > those are unambiguously showing that this is a win:
> >
> > Acked-by: Ingo Molnar <mingo@kernel.org>
> >
>
> I ran the test with kernbench and sysbench again on 32 core mx3850
> machine with 32 vcpu guests. Results shows definite improvements.
>
> ebizzy and dbench show similar improvement for 1x overcommit
> (note that stdev for 1x in dbench is lesser improvemet is now seen at
> only 20%)
>
> [ all the experiments are taken out of 8 run averages ].
>
> The patches benefit large guest undercommit scenarios, so I believe
> with large guest performance improvemnt is even significant. [ Chegu
> Vinod results show performance near to no ple cases ]. Unfortunately I
> do not have a machine to test larger guest (>32).
>
> Ingo, Please let me know if this is okay to you.
>
> base kernel = 3.8.0-rc4
>
> +-----------+-----------+-----------+------------+-----------+
> kernbench (time in sec lower is better)
> +-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x 46.6028 1.8672 42.4494 1.1390 8.91234
> 2x 99.9074 9.1859 90.4050 2.6131 9.51121
> +-----------+-----------+-----------+------------+-----------+
> +-----------+-----------+-----------+------------+-----------+
> sysbench (time in sec lower is better)
> +-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x 18.7402 0.3764 17.7431 0.3589 5.32065
> 2x 13.2238 0.1935 13.0096 0.3152 1.61981
> +-----------+-----------+-----------+------------+-----------+
>
> +-----------+-----------+-----------+------------+-----------+
> ebizzy (records/sec higher is better)
> +-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x 2421.9000 19.1801 5883.1000 112.7243 142.91259
> +-----------+-----------+-----------+------------+-----------+
>
> +-----------+-----------+-----------+------------+-----------+
> dbench (throughput MB/sec higher is better)
> +-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x 11675.9900 857.4154 14103.5000 215.8425 20.79061
> +-----------+-----------+-----------+------------+-----------+

The numbers look pretty convincing, thanks. The workloads were
CPU bound most of the time, right?

Thanks,

Ingo