linux-kernel - Re: [PATCH V2 1/3] Sched: Scheduler time slice extension

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Message-ID: <20250424141345.7F42GFmZ@linutronix.de>
Date: Thu, 24 Apr 2025 16:13:45 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Prakash Sangappa <prakash.sangappa@...cle.com>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org, rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com, tglx@...utronix.de
Subject: Re: [PATCH V2 1/3] Sched: Scheduler time slice extension

On 2025-04-18 19:34:08 [+0000], Prakash Sangappa wrote:
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
…
> @@ -930,6 +931,9 @@ struct task_struct {
>  	struct plist_node		pushable_tasks;
>  	struct rb_node			pushable_dl_tasks;
>  #endif
> +#ifdef CONFIG_RSEQ
> +	unsigned			rseq_sched_delay:1;
> +#endif

There should be somewhere a bitfield already which you could use without
the ifdef. Then you could use IS_ENABLED() if you want to save some code
if RSEQ is not enabled.

>  
>  	struct mm_struct		*mm;
>  	struct mm_struct		*active_mm;
> --- a/include/uapi/linux/rseq.h
> +++ b/include/uapi/linux/rseq.h
…
> @@ -128,6 +131,8 @@ struct rseq {
>  	 * - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
>  	 *     Inhibit instruction sequence block restart on migration for
>  	 *     this thread.
> +	 * - RSEQ_CS_DELAY_RESCHED
> +	 *     Try delay resched...

Delay resched up to $time for $kind-of-stats under $conditions.

>  	 */
>  	__u32 flags;
>  
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 6b7ff1bc1b9b..944027d14198 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
…
> @@ -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 | _TIF_NEED_RESCHED_LAZY))
> -			schedule();
> +		if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) {

couldn't we restrict this to _TIF_NEED_RESCHED_LAZY? That way we would
still schedule immediately for any SCHED_FIFO/RR/DL tasks and do this
delay only for everything else such as SCHED_OTHER/…

> +		       if (irq && rseq_delay_resched())
> +			       clear_tsk_need_resched(current);
> +		       else
> +			       schedule();
> +		}
>  
>  		if (ti_work & _TIF_UPROBE)
>  			uprobe_notify_resume(regs);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 165c90ba64ea..cee50e139723 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -823,6 +823,7 @@ void update_rq_clock(struct rq *rq)
>  
>  static void hrtick_clear(struct rq *rq)
>  {
> +	rseq_delay_resched_tick();

This is called from __schedule(). If you set the need-resched flag here,
it gets removed shortly after. Do I miss something?

>  	if (hrtimer_active(&rq->hrtick_timer))
>  		hrtimer_cancel(&rq->hrtick_timer);
>  }

Sebastian