linux-kernel - Re: [PATCH 23/25 v2] ALSA/dummy: Replace tasklet with softirq hrtimer

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Date:   Tue, 05 Sep 2017 18:02:36 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Takashi Sakamoto <o-takashi@...amocchi.jp>, perex@...ex.cz,
        anna-maria@...utronix.de, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, hch@....de, keescook@...omium.org,
        john.stultz@...aro.org, tglx@...utronix.de
Subject: Re: [PATCH 23/25 v2] ALSA/dummy: Replace tasklet with softirq hrtimer

On Tue, 05 Sep 2017 17:53:51 +0200,
Sebastian Andrzej Siewior wrote:
> 
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> The tasklet is used to defer the execution of snd_pcm_period_elapsed() to
> the softirq context. Using the CLOCK_MONOTONIC_SOFT base invokes the timer
> callback in softirq context as well which renders the tasklet useless.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Anna-Maria Gleixner <anna-maria@...utronix.de>
> Cc: Jaroslav Kysela <perex@...ex.cz>
> Cc: Takashi Iwai <tiwai@...e.com>
> Cc: Takashi Sakamoto <o-takashi@...amocchi.jp>
> Cc: alsa-devel@...a-project.org
> [o-takashi: avoid stall due to a call of hrtimer_cancel() on a callback
>             of hrtimer]
> Signed-off-by: Takashi Sakamoto <o-takashi@...amocchi.jp>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> On 2017-09-02 10:19:45 [+0900], Takashi Sakamoto wrote:
> > Yep. I request the authors to include this 
> Thank you for providing a fix.
> 
> v1…v2: merged Takashi Sakamoto fixup of the original patch into v2.
> 
> So this patch now is okay?

Note that you can try it by yourself easily, as it's a dummy driver
that doesn't need anything special.  Just run aplay for that device
(e.g. aplay -Dplughw:2 for card#2) can reproduce the original
problem.

> @@ -394,7 +386,12 @@ static enum hrtimer_restart dummy_hrtimer_callback(struct hrtimer *timer)
>  	dpcm = container_of(timer, struct dummy_hrtimer_pcm, timer);
>  	if (!atomic_read(&dpcm->running))
>  		return HRTIMER_NORESTART;
> -	tasklet_schedule(&dpcm->tasklet);
> +
> +	/* In a case of XRUN, this calls .trigger to stop PCM substream. */

As mentioned, the stop happens not only with XRUN but also in a normal
situation by draining.

Other than that, looks OK to me (but not tested it).


thanks,

Takashi


> +	snd_pcm_period_elapsed(dpcm->substream);
> +	if (!atomic_read(&dpcm->running))
> +		return HRTIMER_NORESTART;
> +
>  	hrtimer_forward_now(timer, dpcm->period_time);
>  	return HRTIMER_RESTART;
>  }
> @@ -414,14 +411,14 @@ static int dummy_hrtimer_stop(struct snd_pcm_substream *substream)
>  	struct dummy_hrtimer_pcm *dpcm = substream->runtime->private_data;
>  
>  	atomic_set(&dpcm->running, 0);
> -	hrtimer_cancel(&dpcm->timer);
> +	if (!hrtimer_callback_running(&dpcm->timer))
> +		hrtimer_cancel(&dpcm->timer);
>  	return 0;
>  }
>  
>  static inline void dummy_hrtimer_sync(struct dummy_hrtimer_pcm *dpcm)
>  {
>  	hrtimer_cancel(&dpcm->timer);
> -	tasklet_kill(&dpcm->tasklet);
>  }
>  
>  static snd_pcm_uframes_t
> @@ -466,12 +463,10 @@ static int dummy_hrtimer_create(struct snd_pcm_substream *substream)
>  	if (!dpcm)
>  		return -ENOMEM;
>  	substream->runtime->private_data = dpcm;
> -	hrtimer_init(&dpcm->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> +	hrtimer_init(&dpcm->timer, CLOCK_MONOTONIC_SOFT, HRTIMER_MODE_REL);
>  	dpcm->timer.function = dummy_hrtimer_callback;
>  	dpcm->substream = substream;
>  	atomic_set(&dpcm->running, 0);
> -	tasklet_init(&dpcm->tasklet, dummy_hrtimer_pcm_elapsed,
> -		     (unsigned long)dpcm);
>  	return 0;
>  }
>  
> -- 
> 2.14.1
>