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

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Message-ID: <d86d463c-14ea-47cf-adc6-d3e9caa4ebf1@efficios.com>
Date: Fri, 15 Nov 2024 09:13:35 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Prakash Sangappa <prakash.sangappa@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "rostedt@...dmis.org" <rostedt@...dmis.org>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [RFC PATCH 0/4] Scheduler time slice extension

On 2024-11-14 14:42, Prakash Sangappa wrote:
> 
> 
>> On Nov 14, 2024, at 2:28 AM, Peter Zijlstra <peterz@...radead.org> wrote:

[...]

>>
>> See:
>>
>>   https://lkml.kernel.org/r/20220113233940.3608440-4-posk@google.com
>>
>> for a more elaborate scheme.
>>
>>>> Peter, was there anything fundamentally wrong with your approach based
>>>> on rseq ? https://lore.kernel.org/lkml/20231030132949.GA38123@noisy.programming.kicks-ass.net
>>>>
>>>> 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.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com