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

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Message-ID: <CF032474-7725-48C1-BA31-A8728C6C06E7@oracle.com>
Date: Thu, 14 Nov 2024 19:42:00 +0000
From: Prakash Sangappa <prakash.sangappa@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "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 Nov 14, 2024, at 2:28 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Nov 13, 2024 at 08:10:52PM +0000, Prakash Sangappa wrote:
>> 
>> 
>>> On Nov 13, 2024, at 11:36 AM, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
>>> 
>>> On 2024-11-13 13:50, Peter Zijlstra wrote:
>>>> On Wed, Nov 13, 2024 at 12:01:22AM +0000, Prakash Sangappa wrote:
>>>>> This patch set implements the above mentioned 50us extension time as posted
>>>>> by Peter. But instead of using restartable sequences as API to set the flag
>>>>> to request the extension, this patch proposes a new API with use of a per
>>>>> thread shared structure implementation described below. This shared structure
>>>>> is accessible in both users pace and kernel. The user thread will set the
>>>>> flag in this shared structure to request execution time extension.
>>>> But why -- we already have rseq, glibc uses it by default. Why add yet
>>>> another thing?
>>> 
>>> Indeed, what I'm not seeing in this RFC patch series cover letter is an
>>> explanation that justifies adding yet another per-thread memory area
>>> shared between kernel and userspace when we have extensible rseq
>>> already.
>> 
>> It mainly provides pinned memory, can be useful for  future use cases
>> where updating user memory in kernel context can be fast or needs to
>> avoid pagefaults.
> 
> 'might be useful' it not good enough a justification. Also, I don't
> think you actually need this.

Will get back with database benchmark results using rseq API for scheduler time extension.

> 
> 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.

Thanks,
-Prakash