linux-kernel - Re: [RFC PATCH 2/4] Scheduler time extention

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Message-ID: <92F18B23-0B35-486F-BF69-17A73FFB10FD@oracle.com>
Date: Wed, 13 Nov 2024 17:40:54 +0000
From: Prakash Sangappa <prakash.sangappa@...cle.com>
To: K Prateek Nayak <kprateek.nayak@....com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "peterz@...radead.org"
	<peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        Daniel
 Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [RFC PATCH 2/4] Scheduler time extention



> On Nov 12, 2024, at 7:57 PM, K Prateek Nayak <kprateek.nayak@....com> wrote:
> 
> Hello Prakash,
> 
> Full disclaimer: I haven't looked closely at the complete series but ...
> 
> On 11/13/2024 5:31 AM, Prakash Sangappa wrote:
>> [..snip..]
>> @@ -99,8 +100,12 @@ __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
>>     local_irq_enable_exit_to_user(ti_work);
>>  - if (ti_work & _TIF_NEED_RESCHED)
>> - schedule();
>> + if (ti_work & _TIF_NEED_RESCHED) {
>> + if (irq && taskshrd_delay_resched())
>> + clear_tsk_need_resched(current);
> 
> Suppose the current task had requested for a delayed resched but an RT
> task's wakeup sets the TIF_NEED_RESCHED flag via an IPI, doesn't this
> clear the flag indiscriminately and allow the task to run for an
> extended amount of time? Am I missing something?

If the scheduler extension delay has already been granted when the IPI from wakeup occurs, then it would not clear the TIF_NEED_RESCHED flag and so would preempt.

[...]

> 
>>  }
>> @@ -830,6 +831,8 @@ static enum hrtimer_restart hrtick(struct hrtimer *timer)
>>     WARN_ON_ONCE(cpu_of(rq) != smp_processor_id());
>>  + taskshrd_delay_resched_tick();
>> +
>>   rq_lock(rq, &rf);
>>   update_rq_clock(rq);
>>   rq->curr->sched_class->task_tick(rq, rq->curr, 1);
>> @@ -903,6 +906,16 @@ void hrtick_start(struct rq *rq, u64 delay)
>>    #endif /* CONFIG_SMP */
>>  +void hrtick_local_start(u64 delay)
>> +{
>> + struct rq *rq = this_rq();
>> + struct rq_flags rf;
>> +
>> + rq_lock(rq, &rf);
> 
> You can use guard(rq_lock)(rq) and avoid declaring rf.

Will take a look and address it.

[...]

> 
>>  +bool taskshrd_delay_resched(void)
>> +{
>> + struct task_struct *t = current;
>> + struct task_ushrd_struct *shrdp = t->task_ushrd;
>> +
>> + if (!IS_ENABLED(CONFIG_SCHED_HRTICK))
>> + return false;
>> +
>> + if(shrdp == NULL || shrdp->kaddr == NULL)
>> + return false;
>> +
>> + if (t->taskshrd_sched_delay)
>> + return false;
>> +
>> + if (!(shrdp->kaddr->ts.sched_delay))
>> + return false;
>> +
>> + shrdp->kaddr->ts.sched_delay = 0;
>> + t->taskshrd_sched_delay = 1;
>> +
>> + return true;
> 
> Perhaps this needs to also check
> "rq->nr_running == rq->cfs.h_nr_running" since I believe it only makes
> sense for fair tasks to request that extra slice?

From the discussion in 
https://lore.kernel.org/lkml/20231025054219.1acaa3dd@gandalf.local.home/

It was ok to have this behavior for all tasks. It could be changed to work only for fair tasks, if there is agreement.


Thanks,
-Prakash

> 
> -- 
> Thanks and Regards,
> Prateek
> 
>> +}
>> +
>> +void taskshrd_delay_resched_fini(void)
>> +{
>> +#ifdef CONFIG_SCHED_HRTICK
>> + struct task_struct *t = current;
>> + /*
>> + * IRQs off, guaranteed to return to userspace, start timer on this CPU
>> + * to limit the resched-overdraft.
>> + *
>> + * If your critical section is longer than 50 us you get to keep the
>> + * pieces.
>> + */
>> + if (t->taskshrd_sched_delay)
>> + hrtick_local_start(50 * NSEC_PER_USEC);
>> +#endif
>> +}
>> +
>> +void taskshrd_delay_resched_tick(void)
>> +{
>> +#ifdef CONFIG_SCHED_HRTICK
>> + struct task_struct *t = current;
>> +
>> + if (t->taskshrd_sched_delay) {
>> + set_tsk_need_resched(t);
>> + }
>> +#endif
>> +}
>> +
>>    /*
>>   * Get Task Shared structure, allocate if needed and return mapped user address.
>