Re: [PATCH] sched_ext: Replace rq_lock() to raw_spin_rq_lock() in scx_ops_bypass()

[Andrea Righi <arighi@nvidia.com>]

Hi Changwoo,

On Wed, Jan 08, 2025 at 11:55:21AM +0900, Changwoo Min wrote:
> scx_ops_bypass() iterates all CPUs to re-enqueue all the scx tasks.
> For each CPU, it acquires a lock using rq_lock() regardless of whether
> a CPU is offline or the CPU is currently running a task in a higher
> scheduler class (e.g., deadline). The rq_lock() is supposed to be used
> for online CPUs, and the use of rq_lock() may trigger an unnecessary
> warning in rq_pin_lock(). Therefore, replace rq_lock() to
> raw_spin_rq_lock() in scx_ops_bypass().

Can we include the warning here? In this way people that are hitting the
same warning can search for it and find this fix.

Moreover, we can also add:

Fixes: 0e7ffff1b811 ("scx: Fix raciness in scx_ops_bypass()")

> 
> Signed-off-by: Changwoo Min <changwoo@igalia.com>
> ---
>  kernel/sched/ext.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 8fe64c27004e..741398f3e730 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -4803,10 +4803,9 @@ static void scx_ops_bypass(bool bypass)
>  	 */
>  	for_each_possible_cpu(cpu) {
>  		struct rq *rq = cpu_rq(cpu);
> -		struct rq_flags rf;
>  		struct task_struct *p, *n;
>  
> -		rq_lock(rq, &rf);
> +		raw_spin_rq_lock(rq);
>  
>  		if (bypass) {
>  			WARN_ON_ONCE(rq->scx.flags & SCX_RQ_BYPASSING);
> @@ -4822,7 +4821,7 @@ static void scx_ops_bypass(bool bypass)
>  		 * sees scx_rq_bypassing() before moving tasks to SCX.
>  		 */
>  		if (!scx_enabled()) {
> -			rq_unlock(rq, &rf);
> +			raw_spin_rq_unlock(rq);
>  			continue;
>  		}
>  
> @@ -4842,7 +4841,7 @@ static void scx_ops_bypass(bool bypass)
>  			sched_enq_and_set_task(&ctx);
>  		}

Maybe we can also do this here since we're already holding the rq lock and
irqs are disabled:

		/* resched to restore ticks and idle state */
		if (cpu == smp_processor_id() || cpu_online(cpu))
			resched_curr(rq);

>  
> -		rq_unlock(rq, &rf);
> +		raw_spin_rq_unlock(rq);
>  

And remove the following:

>  		/* resched to restore ticks and idle state */
>  		resched_cpu(cpu);

Thanks,
-Andrea