linux-kernel - Re: [PATCH] usb: don't offload isochronous urb completions to ksoftirq

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Date:   Wed, 13 Jun 2018 15:30:31 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     Alan Stern <stern@...land.harvard.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ming Lei <ming.lei@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: don't offload isochronous urb completions to
 ksoftirq

On Wed, 13 Jun 2018, Alan Stern wrote:

> [Steve: Sorry for dumping you into the middle of this discussion.  
> Please see especially the last two paragraphs below.  Mikulas is
> getting dropouts with USB audio because part of the processing uses a
> tasklet.]

The problem is this:

I have a single core machine with a usb2 sound card. When I increase the 
priority of a music player, the audio starts skipping.

The reason for the skipping is that the ehci usb driver is offloading urb 
callbacks using tasklet_hi_schedule, the callbacks end up being offloaded 
to the ksoftirqd thread (that has priority 0), the music player with 
elevated priority preempts ksoftirqd and causes delays in the urb 
callbacks.

Is this some deficiency in the softirq subsystem? (should we perhaps treat 
tasklet_hi specially and not offload it as much as the others?) Or should 
the ehci driver be fixed not to use tasklets?

Mikulas