linux-kernel - Re: [RFC PATCH] usb: hcd: complete URBs in threaded-IRQ context instead of tasklet

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Date:   Fri, 16 Feb 2018 21:38:08 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, linux-usb@...r.kernel.org
Subject: Re: [RFC PATCH] usb: hcd: complete URBs in threaded-IRQ context
 instead of tasklet

On 2018-02-16 13:29:01 [-0500], Alan Stern wrote:
> We originally used tasklets because we didn't want to incur the delays 
> associated with running in a process context.  It seems odd to be 
> reversing that decision now.

The theaded interrupt runs SCHED_FIFO priority 50 by default. The only
thing that can interrupt it are interrupts, a softirq (not ksoftirqd)
and other tasks with a higher priority than 50.
There should be no downside performance wise.

> > The URBs from the root-hub never create an interrupt so I currently
> > process them in a workqueue (I'm not sure if an URB-enqueue in the
> > completion handler would break something).
> 
> It worked okay before we changed over to using tasklets.

Ah okay. I've seen that HCDs were no longer dropping their internal lock
and I wasn't sure if such a change was also applied in RH-code.

> Alan Stern

Sebastian