Re: [RFC PATCH] ata: libata-scsi: Move long delayed work on system_dfl_long_wq

[Niklas Cassel <cassel@kernel.org>]

On Thu, Apr 30, 2026 at 11:29:47AM +0200, Marco Crivellari wrote:
> Currently the code enqueue work items using {queue|mod}_delayed_work(),
> using system_long_wq. This workqueue should be used when long works are
> expected, but it is a per-cpu workqueue.
> 
> This is important because queue_delayed_work() queue the work using:
> 
>    queue_delayed_work_on(WORK_CPU_UNBOUND, ...);
> 
> Note that WORK_CPU_UNBOUND = NR_CPUS.
> 
> This would end up calling __queue_delayed_work() that does:
> 
>     if (housekeeping_enabled(HK_TYPE_TIMER)) {
>     //      [....]
>     } else {
>             if (likely(cpu == WORK_CPU_UNBOUND))
>                     add_timer_global(timer);
>             else
>                     add_timer_on(timer, cpu);
>     }
> 
> So when cpu == WORK_CPU_UNBOUND the timer is global and is
> not using a specific CPU. Later, when __queue_work() is called:
> 
>     if (req_cpu == WORK_CPU_UNBOUND) {
>             if (wq->flags & WQ_UNBOUND)
>                     cpu = wq_select_unbound_cpu(raw_smp_processor_id());
>             else
>                     cpu = raw_smp_processor_id();
>     }
> 
> Because the wq is not unbound, it takes the CPU where the timer
> fired and enqueue the work on that CPU.
> The consequence of all of this is that the work can run anywhere,
> depending on where the timer fired.
> 
> Recently, a new unbound workqueue specific for long running work has
> been added:
> 
>     c116737e972e ("workqueue: Add system_dfl_long_wq for long unbound works")
> 
> So change system_long_wq with system_dfl_long_wq so that the work may
> benefit from scheduler task placement.
> 
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
> ---
>  drivers/ata/libata-scsi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
> index f44612e269a4..6733f2b14521 100644
> --- a/drivers/ata/libata-scsi.c
> +++ b/drivers/ata/libata-scsi.c
> @@ -4753,7 +4753,7 @@ void ata_scsi_scan_host(struct ata_port *ap, int sync)
>  			     "WARNING: synchronous SCSI scan failed without making any progress, switching to async\n");
>  	}
>  
> -	queue_delayed_work(system_long_wq, &ap->hotplug_task,
> +	queue_delayed_work(system_dfl_long_wq, &ap->hotplug_task,
>  			   round_jiffies_relative(HZ));
>  }
>  
> -- 
> 2.53.0
> 

Looks good to me.

Any particular reason that you sent this as an RFC?

I can see similar patches queued up in linux-next already.

Do you want us to pick this one up?


Kind regards,
Niklas