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. https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/4.16/patch-4.16.12-rt5.patch.xz -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
