Re: [RFC PATCH] drm/gm12u320: Replace system_long_wq with system_dfl_long_wq

[Thomas Zimmermann <tzimmermann@suse.de>]

Am 30.04.26 um 11:15 schrieb Marco Crivellari:
> 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>

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

> ---
>   drivers/gpu/drm/tiny/gm12u320.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/tiny/gm12u320.c b/drivers/gpu/drm/tiny/gm12u320.c
> index d73dfebb4353..4ad074337af0 100644
> --- a/drivers/gpu/drm/tiny/gm12u320.c
> +++ b/drivers/gpu/drm/tiny/gm12u320.c
> @@ -388,7 +388,7 @@ static void gm12u320_fb_update_work(struct work_struct *work)
>   	 * We must draw a frame every 2s otherwise the projector
>   	 * switches back to showing its logo.
>   	 */
> -	queue_delayed_work(system_long_wq, &gm12u320->fb_update.work,
> +	queue_delayed_work(system_dfl_long_wq, &gm12u320->fb_update.work,
>   			   msecs_to_jiffies(IDLE_TIMEOUT));
>   
>   	return;
> @@ -427,7 +427,8 @@ static void gm12u320_fb_mark_dirty(struct drm_framebuffer *fb,
>   	mutex_unlock(&gm12u320->fb_update.lock);
>   
>   	if (wakeup)
> -		mod_delayed_work(system_long_wq, &gm12u320->fb_update.work, 0);
> +		mod_delayed_work(system_dfl_long_wq,
> +				 &gm12u320->fb_update.work, 0);
>   
>   	if (old_fb)
>   		drm_framebuffer_put(old_fb);

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)