Re: [RFC V2 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU isn't found

[Subhra Mazumdar <subhra.mazumdar@oracle.com>]

On 5/14/19 9:03 AM, Steven Sistare wrote:
> On 5/13/2019 7:35 AM, Peter Zijlstra wrote:
>> On Mon, May 13, 2019 at 03:04:18PM +0530, Viresh Kumar wrote:
>>> On 10-05-19, 09:21, Peter Zijlstra wrote:
>>>> I don't hate his per se; but the whole select_idle_sibling() thing is
>>>> something that needs looking at.
>>>>
>>>> There was the task stealing thing from Steve that looked interesting and
>>>> that would render your apporach unfeasible.
>>> I am surely missing something as I don't see how that patchset will
>>> make this patchset perform badly, than what it already does.
>> Nah; I just misremembered. I know Oracle has a patch set poking at
>> select_idle_siblings() _somewhere_ (as do I), and I just found the wrong
>> one.
>>
>> Basically everybody is complaining select_idle_sibling() is too
>> expensive for checking the entire LLC domain, except for FB (and thus
>> likely some other workloads too) that depend on it to kill their tail
>> latency.
>>
>> But I suppose we could still do this, even if we scan only a subset of
>> the LLC, just keep track of the last !idle CPU running only SCHED_IDLE
>> tasks and pick that if you do not (in your limited scan) find a better
>> candidate.
> Subhra posted a patch that incrementally searches for an idle CPU in the LLC,
> remembering the last CPU examined, and searching a fixed number of CPUs from there.
> That technique is compatible with the one that Viresh suggests; the incremental
> search would stop if a SCHED_IDLE cpu was found.
This was the last version of patchset I sent:
https://lkml.org/lkml/2018/6/28/810

Also select_idle_core is a net -ve for certain workloads like OLTP. So I
had put a SCHED_FEAT to be able to disable it.

Thanks,
Subhra
>
> I also fiddled with select_idle_sibling, maintaining a per-LLC bitmap of idle CPUs,
> updated with atomic operations.  Performance was basically unchanged for the
> workloads I tested, and I inserted timers around the idle search showing it was
> a very small fraction of time both before and after my changes.  That led me to
> ignore the push side and optimize the pull side with task stealing.
>
> I would be very interested in hearing from folks that have workloads that demonstrate
> that select_idle_sibling is too expensive.
>
> - Steve