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

Ingo Molnar
2013-01-25T18:49:17Z

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

> On 01/25/2013 04:17 PM, Ingo Molnar wrote:
> >
> >* 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?
>
> Yes. CPU bound most of the time. I also used tmpfs to reduce
> io overhead (for dbbench).

Ok, cool.

Which tree will this be upstreamed through - the KVM tree? I'd
suggest the KVM tree because KVM will be the one exposed to the
effects of this change.

Thanks,

Ingo