Re: [PATCH v3] sched_ext: Replace rq_lock() to raw_spin_rq_lock() in scx_ops_bypass()

[Andrea Righi <arighi@nvidia.com>]

On Thu, Jan 09, 2025 at 12:08:06AM +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().
> 
> Without this change, we observe the following warning:
> 
> ===== START =====
> [    6.615205] rq->balance_callback && rq->balance_callback != &balance_push_callback
> [    6.615208] WARNING: CPU: 2 PID: 0 at kernel/sched/sched.h:1730 __schedule+0x1130/0x1c90
> =====  END  =====
> 
> Fixes: 0e7ffff1b811 ("scx: Fix raciness in scx_ops_bypass()")
> Signed-off-by: Changwoo Min <changwoo@igalia.com>

Looks good to me.

Acked-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
> 
> ChangeLog v2 -> v3:
>   - Trim long warning messages for readability.
>   - Properly add the Fixes tag.
> 
> ChangeLog v1 -> v2:
>   - Add warning messages without this change.
>   - Use resched_curr() instead of resched_cpu() to avoid redundant locking (Andrea)
> 
>  kernel/sched/ext.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 8fe64c27004e..cb6eb49d16be 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;
>  		}
>  
> @@ -4842,10 +4841,11 @@ static void scx_ops_bypass(bool bypass)
>  			sched_enq_and_set_task(&ctx);
>  		}
>  
> -		rq_unlock(rq, &rf);
> -
>  		/* resched to restore ticks and idle state */
> -		resched_cpu(cpu);
> +		if (cpu_online(cpu) || cpu == smp_processor_id())
> +			resched_curr(rq);
> +
> +		raw_spin_rq_unlock(rq);
>  	}
>  
>  	atomic_dec(&scx_ops_breather_depth);
> -- 
> 2.47.1
>