netdev - Re: [PATCH 9/9] mmc: Convert from tasklet to BH workqueue

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Message-ID: <CAPDyKFp5KET0HR+8MwO4cf0O6W2kyFqHoKcVf5jbgBuLuQUcFA@mail.gmail.com>
Date: Tue, 2 Apr 2024 12:15:45 +0200
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Tejun Heo <tj@...nel.org>
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Subject: Re: [PATCH 9/9] mmc: Convert from tasklet to BH workqueue

On Thu, 28 Mar 2024 at 17:21, Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Thu, Mar 28, 2024 at 01:53:25PM +0100, Ulf Hansson wrote:
> > At this point we have suggested to drivers to switch to use threaded
> > irq handlers (and regular work queues if needed too). That said,
> > what's the benefit of using the BH work queue?
>
> BH workqueues should behave about the same as tasklets which have more
> limited interface and is subtly broken in an expensive-to-fix way (around
> freeing in-flight work item), so the plan is to replace tasklets with BH
> workqueues and remove tasklets from the kernel.

Seems like a good approach!

>
> The [dis]advantages of BH workqueues over threaded IRQs or regular threaded
> workqueues are the same as when you compare them to tasklets. No thread
> switching overhead, so latencies will be a bit tighter. Wheteher that
> actually matters really depends on the use case. Here, the biggest advantage
> is that it's mostly interchangeable with tasklets and can thus be swapped
> easily.

Right, thanks for clarifying!

However, the main question is then - if/when it makes sense to use the
BH workqueue for an mmc host driver. Unless there are some HW
limitations, a threaded irq handler should be sufficient, I think.

That said, moving to threaded irq handlers is a different topic and
doesn't prevent us from moving to BH workqueues as it seems like a
step in the right direction.

Kind regards
Uffe