usb: don't offload isochronous urb completions to ksoftirq

[Steven Rostedt <rostedt@goodmis.org>]

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

 https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/4.16/patch-4.16.12-rt5.patch.xz

-- Steve
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