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SubjectRe: [PATCH v2] sched/core: Preempt current task in favour of bound kthread
* Vincent Guittot <vincent.guittot@linaro.org> [2019-12-10 10:43:46]:

> On Tue, 10 Dec 2019 at 06:43, Srikar Dronamraju
> <srikar@linux.vnet.ibm.com> wrote:
> >
> > This is more prone to happen if the current running task is CPU
> > intensive and the sched_wake_up_granularity is set to larger value.
> > When the sched_wake_up_granularity was relatively small, it was observed
> > that the bound thread would complete before the load balancer would have
> > chosen to move the cache hot task to a different CPU.
> >
> > To deal with this situation, the current running task would yield to a
> > per CPU bound kthread, provided kthread is not CPU intensive.
> >
> > /pboffline/hwcct_prg_old/lib/fsperf -t overwrite --noclean -f 5g -b 4k /pboffline
> >
> > (With sched_wake_up_granularity set to 15ms)
>
> So you increase sched_wake_up_granularity to a high level to ensure
> that current is no preempted by waking thread but then you add a way
> to finally preempt it which is somewhat weird IMO
>

Yes, setting to a smaller value will help mitigate/solve the problem.
There may be folks out who have traditionally set a high wake_up_granularity
(and have seen better performance with it), who may miss out that when using
blk-mq, such settings will cause more harm. And they may continue to see
some performance regressions when they move to a lower wake_up_granularity.

> Have you tried to increase the priority of workqueue thread (decrease
> nice priority) ? This is the right way to reduce the impact of the
> sched_wake_up_granularity on the wakeup of your specific kthread.
> Because what you want at the end is keeping a low wakeup granularity
> for these io workqueues
>

Yes, people can tune the priority of workqueue threads and infact it may be
easier to set wake_up_granularity to a lower value. However the point is how
do we make everyone aware that they are running into a performance issue
with a higher wakeup_granularity?

--
Thanks and Regards
Srikar Dronamraju

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