linux-kernel - Re: [PATCH] usb/c67x00: Replace tasklet with work

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Date:   Mon, 11 Jan 2021 11:22:40 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Hillf Danton <hdanton@...a.com>
Cc:     jacmet@...site.dk, gregkh@...uxfoundation.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] usb/c67x00: Replace tasklet with work

On Mon, 11 Jan 2021, Hillf Danton wrote:

>On Sun, 10 Jan 2021 20:40:50 -0800 Davidlohr Bueso wrote:
>> Tasklets have long been deprecated as being too heavy on the system
>> by running in irq context - and this is not a performance critical
>> path. If a higher priority process wants to run, it must wait for
>> the tasklet to finish before doing so.
>>
>> c67x00_do_work() will now run in process context and have further
>> concurrency (tasklets being serialized among themselves), but this
>> is done holding the c67x00->lock, so it should be fine. Furthermore,
>> this patch fixes the usage of the lock in the callback as otherwise
>> it would need to be irq-safe.
>
>Can you add a couple of words about the need to be irq-safe because
>no lock is taken for scheduling either tasklet or work?

I was refering to the locking in the c67x00_do_work() tasklet callback.
Because it is currently under irq it should be disabling irq (or at least
BH) but after this patch that is no longer the case.

>>
>> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
>> ---
>>  drivers/usb/c67x00/c67x00-hcd.h   |  2 +-
>>  drivers/usb/c67x00/c67x00-sched.c | 12 +++++++-----
>>  2 files changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/usb/c67x00/c67x00-hcd.h b/drivers/usb/c67x00/c67x00-hcd.h
>> index 6b6b04a3fe0f..6332a6b5dce6 100644
>> --- a/drivers/usb/c67x00/c67x00-hcd.h
>> +++ b/drivers/usb/c67x00/c67x00-hcd.h
>> @@ -76,7 +76,7 @@ struct c67x00_hcd {
>>	u16 next_td_addr;
>>	u16 next_buf_addr;
>>
>> -	struct tasklet_struct tasklet;
>> +	struct work_struct work;
>>
>>	struct completion endpoint_disable;
>>
>> diff --git a/drivers/usb/c67x00/c67x00-sched.c b/drivers/usb/c67x00/c67x00-sched.c
>> index e65f1a0ae80b..af60f4fdd340 100644
>> --- a/drivers/usb/c67x00/c67x00-sched.c
>> +++ b/drivers/usb/c67x00/c67x00-sched.c
>> @@ -1123,24 +1123,26 @@ static void c67x00_do_work(struct c67x00_hcd *c67x00)
>>
>>  /* -------------------------------------------------------------------------- */
>>
>> -static void c67x00_sched_tasklet(struct tasklet_struct *t)
>> +static void c67x00_sched_work(struct work_struct *work)
>>  {
>> -	struct c67x00_hcd *c67x00 = from_tasklet(c67x00, t, tasklet);
>> +	struct c67x00_hcd *c67x00;
>> +
>> +	c67x00 = container_of(work, struct c67x00_hcd, work);
>>	c67x00_do_work(c67x00);
>>  }
>>
>>  void c67x00_sched_kick(struct c67x00_hcd *c67x00)
>>  {
>> -	tasklet_hi_schedule(&c67x00->tasklet);
>> +        queue_work(system_highpri_wq, &c67x00->work);
>
>Better if one line comment is added for highpri, given this is not a
>performance critical path.

I'm not sure the value here, considering the highprio is not being
changed here. There are a few drivers who use highpri workqueue and
care about latencies but they are still not performance critical (to
the overall system that is, which is what I meant by that).

Thanks,
Davidlohr