Re: [PATCH] mailbox: bcm: Replace tasklet with threaded irq

[Rob Rice <rob.rice@broadcom.com>]

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Davidlohr,

I haven’t worked on this driver for 4 years. So my memory is a little vague. But I have notes that we found using the tasklet was 40% faster than threaded IRQ. We were trying to get performance up. So that seemed important at the time. I’m not prepared to defend that now. :-)

Rob

> On Jan 14, 2021, at 10:53 PM, Davidlohr Bueso <dave@stgolabs.net> wrote:
> 
> On Thu, 14 Jan 2021, Jassi Brar wrote:
> 
>> On Thu, Jan 14, 2021 at 6:21 PM Davidlohr Bueso <dave@stgolabs.net> 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.
>>> 
>>> Use a more suitable alternative such as threaded irqs and do the
>>> async work in process context.
>>> 
>>> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
>>> ---
>> Please cc the author and other contributors to this file, esp when
>> this is vendor specific code.
> 
> So looking at who to Cc I noticed:
> 
> commit 8aef00f090bcbe5237c5a6628e7c000890267efe
> Author: Rob Rice <rob.rice@broadcom.com>
> Date:   Mon Nov 14 13:26:01 2016 -0500
> 
>    mailbox: bcm-pdc: Convert from threaded IRQ to tasklet
> 
>    Previously used threaded IRQs in the PDC driver to defer
>    processing the rx DMA ring after getting an rx done interrupt.
>    Instead, use a tasklet at normal priority for deferred processing.
> 
> ... which is exactly the opposite of what modern Linux should be
> doing. Rob, could this not be done in process context?
> 
> Thanks,
> Davidlohr


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