linux-kernel - Re: [PATCH V4 1/6] Sched: Scheduler time slice extension

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Message-ID: <7EF41833-EA7F-4C02-837E-FE2260E3EAFB@oracle.com>
Date: Fri, 23 May 2025 20:34:15 +0000
From: Prakash Sangappa <prakash.sangappa@...cle.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org"
	<rostedt@...dmis.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "kprateek.nayak@....com"
	<kprateek.nayak@....com>
Subject: Re: [PATCH V4 1/6] Sched: Scheduler time slice extension



> On May 23, 2025, at 1:06 PM, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
> On 2025-05-13 17:45, Prakash Sangappa wrote:
> [...]
>> diff --git a/kernel/rseq.c b/kernel/rseq.c
>> index b7a1ec327e81..dba44ca9f624 100644
>> --- a/kernel/rseq.c
>> +++ b/kernel/rseq.c
>> @@ -448,6 +448,62 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
>>   force_sigsegv(sig);
>>  }
>>  +bool rseq_delay_resched(void)
>> +{
>> + struct task_struct *t = current;
>> + u32 flags;
>> +
>> + if (!IS_ENABLED(CONFIG_SCHED_HRTICK))
>> + return false;
>> +
>> + if (!t->rseq)
>> + return false;
>> +
>> + if (t->sched_time_delay)
>> + return false;
>> +
>> + if (copy_from_user_nofault(&flags, &t->rseq->flags, sizeof(flags)))
>> + return false;
> 
> This considers rseq->flags, but not rseq->rseq_cs->flags. Am I missing
> something ?

To use sched time delay feature, rseq->rseq_cs need not be set.

Mainly the rseq->flags member is where the flag(RSEQ_CS_FLAG_DELAY_RESCHED)
to request delaying preemption is set.

In that case should the flag be named ‘RSEQ_FLAG_DELAY_RESCHED’?
Although the enum is called or rseq_cs_flags. Description in structure rseq 
will need to be updated.

-Prakash

> 
> Thanks,
> 
> Mathieu
> 
>> +
>> + if (!(flags & RSEQ_CS_FLAG_DELAY_RESCHED))
>> + return false;
>> +
>> + flags &= ~RSEQ_CS_FLAG_DELAY_RESCHED;
>> + if (copy_to_user_nofault(&t->rseq->flags, &flags, sizeof(flags)))
>> + return false;
>> +
>> + t->sched_time_delay = 1;
>> +
>> + return true;
>> +}
>> +
> 
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com