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

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Message-ID: <20180222161949.3prjzbvkss4hjj2c@linutronix.de>
Date:   Thu, 22 Feb 2018 17:19:50 +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 16:46:41 [-0500], Alan Stern wrote:
> > 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.
> 
> Maybe.  It would be nice to see some real measurements.

I had an usb3 flash stick behind the EHCI controller which was passed
through from the host to a kvm guest. The performance numbers in the
guest were equal (some noise was there) with and without the patch.
The numbers with the patch were worse if lockdep was enabled which isn't
much of a surprise.
If you have anything specific requirements for a measurement then please
let me know and I see what I can do.

> Alan Stern

Sebastian