Re: [RFC PATCH 0/4] Scheduler time slice extension

[Prakash Sangappa]

> On Nov 15, 2024, at 6:13 AM, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> 
> On 2024-11-14 14:42, Prakash Sangappa wrote:
>>> On Nov 14, 2024, at 2:28 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> [...]
> 
>>> 
>>> See:
>>> 
>>>  https://lkml.kernel.org/r/20220113233940.3608440-4-posk@xxxxxxxxxx
>>> 
>>> for a more elaborate scheme.
>>> 
>>>>> Peter, was there anything fundamentally wrong with your approach based
>>>>> on rseq ? https://lore.kernel.org/lkml/20231030132949.GA38123@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>>>>> 
>>>>> The main thing I wonder is whether loading the rseq delay resched flag
>>>>> on return to userspace is too late in your patch. Also, I'm not sure it is
>>>>> realistic to require that no system calls should be done within time extension
>>>>> slice. If we have this scenario:
>>>> 
>>>> I am also not sure if we need to prevent system calls in this scenario.
>>>> Was that restriction mainly because of restartable sequence API implements it?
>>> 
>>> No, the whole premise of delaying resched was because people think that
>>> syscalls are too slow. If you do not think this, then you shouldn't be
>>> using this.
>> Agree.
> 
> I only partially agree with Peter here. I agree that we don't want to
> add system calls on the delay-resched critical section fast path,
> because this would have a significant performance hit.
> 
> But there are scenarios where issuing system calls from within that
> critical section would be needed, even though those would not belong
> to the fast path:
> 
> 1) If a signal handler nests over a delay-resched critical section.
>   That signal handler is allowed to issue system calls.
> 
> 2) If the critical section fast-path is calling GNU C library API and/or
>   a vDSO, which is typically fast, but can end up calling a system call
>   as fallback. e.g. clock_gettime, sched_getcpu. Preventing use of a
>   system call by killing the application punches a hole in the
>   abstractions meant to be provided by GNU libc and vDSO.
> 
> I would recommend that we allow issuing system calls while the
> delay-resched bit is set. However, we may not strictly need to honor
> the delay-resched hint from a system call context, as those would
> be expected to be either infrequent or a portability fallback,
> which means the enhanced performance provided by delay-resched
> really won't matter.
> 
> Another scenario to keep in mind are page faults happening within a
> delay-resched critical section. This is a scenario where page fault
> handling can explicitly reschedule. If this happens, I suspect we
> really don't care about the delay-resched hint, but we should consider
> whether this hint should be left as-is or cleared.
> 

There are no explicit checks right now that a system call occurred in the critical section, 
except under DEBUG_RSEQ, which probably will not apply for the delay-resched case.
 
But the question was should there be checks implemented, probably not 
due to scenarios you mentioned above.


> Thoughts ?
> 
> Thanks,
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>