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

From: Steven Rostedt
Date: Wed Jun 13 2018 - 18:31:54 EST

On Wed, 13 Jun 2018 15:30:31 -0400 (EDT)
Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:

> 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?

What we do for softirqs in the RT patch is to have whoever raised the
softirq run the softirq. If local_bh_disabled() is active (bh is
disabled) then a bit is set in the current task struct, where when
local_bh_enable() is called, it will then execute the softirqs that it
raised while bh was disabled.

Perhaps try out the PREEMPT_RT patch and see if the problem goes away.
Hopefully this softirq work may make it into the kernel soon. We could
even enabled it without full PREEMPT_RT.

-- Steve