linux-kernel - Re: [PATCH] sched: Add _TIF_NEED_RESCHED_LAZY to __resched

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Message-ID: <c8864652-d763-48a3-9292-24c29c25838d@amd.com>
Date: Mon, 29 Sep 2025 10:07:33 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Jemmy Wong <jemmywong512@...il.com>, Ingo Molnar <mingo@...hat.com>,
	"Peter Zijlstra" <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
	"Vincent Guittot" <vincent.guittot@...aro.org>, Dietmar Eggemann
	<dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall
	<bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Valentin Schneider
	<vschneid@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski
	<luto@...nel.org>, Kees Cook <kees@...nel.org>, Sebastian Andrzej Siewior
	<bigeasy@...utronix.de>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Add _TIF_NEED_RESCHED_LAZY to __resched_curr check

Hello Jemmy,

On 9/28/2025 8:44 PM, Jemmy Wong wrote:
> The TIF_NEED_RESCHED_LAZY flag can be set multiple times in a single
> call path. For example:
> 
> entity_tick()
>     update_curr(cfs_rq);
>         resched_curr_lazy(rq);
>     resched_curr_lazy(rq_of(cfs_rq));
> 
> Add a check in resched_curr_lazy() to return early if the flag is
> already set, avoiding redundant operations.

That would have been a decent idea but then you decided to put that
check in __resched_curr() which makes it plain wrong.

[..snip..]

> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -67,6 +67,8 @@ enum syscall_work_bit {
>  #define _TIF_NEED_RESCHED_LAZY _TIF_NEED_RESCHED
>  #endif
> 
> +#define _TIF_NEED_RESCHED_MUSK (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)

s/MUSK/MASK/g

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1108,7 +1108,7 @@ static void __resched_curr(struct rq *rq, int tif)
>  	if (is_idle_task(curr) && tif == TIF_NEED_RESCHED_LAZY)
>  		tif = TIF_NEED_RESCHED;
> 
> -	if (cti->flags & ((1 << tif) | _TIF_NEED_RESCHED))
> +	if (cti->flags & ((1 << tif) | _TIF_NEED_RESCHED_MUSK))
>  		return;

__resched_curr() is used to set both TIF_NEED_RESCHED_LAZY and
TIF_NEED_RESCHED.

By putting this check here, any effort to set NEED_RESCHED and force an
early preemption will bail out if NEED_RESCHED_LAZY is already set which
will delay the preemption.

An example:

    /* New fair task wakes up. */
    check_preempt_wakeup_fair()
        resched_curr_lazy()
            __resched_curr(TIF_NEED_RESCHED_LAZY)

    /* New RT task wakes up. */
    wakeup_preempt()
        resched_curr()
            __resched_curr(TIF_NEED_RESCHED)
                /* Sees NEED_RESCHED_LAZY is already set. */
                /* Does not do a set_preempt_need_resched() */

    ... /* Added latency */
    sched_tick()
        if (tif_test_bit(TIF_NEED_RESCHED_LAZY))
            resched_curr()
                __resched_curr(TIF_NEED_RESCHED)
                    /* Again bails out early! */

    ... /* More latency! */


So, the tick doesn't even upgrade the LAZY flag to a full NEED_RESCHED
and the only time you actually schedule is either at exit to user mode
or if a kthread decides to yield.

Going back to your commit message, something like:

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7f1e5cb94c53..3275abce9ca2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1164,6 +1164,9 @@ static __always_inline int get_lazy_tif_bit(void)
 
 void resched_curr_lazy(struct rq *rq)
 {
+	if (task_thread_info(rq->curr)->flags & TIF_NEED_RESCHED_MASK)
+		return;
+
 	__resched_curr(rq, get_lazy_tif_bit());
 }
 
probably fits the bill better.

> 
>  	cpu = cpu_of(rq);
> --
> 2.50.1 (Apple Git-155)
> 

-- 
Thanks and Regards,
Prateek