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

[Mikulas Patocka]

On Tue, 12 Jun 2018, Alan Stern wrote:

> On Tue, 12 Jun 2018, Mikulas Patocka wrote:
> 
> > I have a single-core machine with usb2 soundcard. When I increase mplayer
> > priority (to real-time or high non-realtime priority), the sound is
> > stuttering. The reason for the stuttering is that mplayer at high priority
> > preempts the softirq thread, preventing URBs from being completed. It was
> > caused by the patch 428aac8a81058 that offloads URB completion to softirq.
> > 
> > This patch prevents offloading isochronous URBs to softirq to fix the
> > stuttering.
> 
> How about just not running mplayer at such a high priority?

I need to run mplayer at a high priority so that other work doesn't 
preempt mplayer and cause stuttering.

> Or raising the priority of the softirq thread?

Do you want to coordinate that with the softirq maintainers? I don't know 
if they would be happy to add an extra real-time softirq thread.

> Alan Stern

Mikulas

> > Fixes: c04ee4b1136e ("Revert "Revert "USB: EHCI: support running URB giveback in tasklet context""")
> > Cc: stable@xxxxxxxxxxxxxxx
> > 
> > ---
> >  drivers/usb/core/hcd.c    |   10 ++++++++++
> >  drivers/usb/host/ehci-q.c |   11 ++++++++++-
> >  include/linux/usb/hcd.h   |    2 ++
> >  3 files changed, 22 insertions(+), 1 deletion(-)
> > 
> > Index: linux-4.17/drivers/usb/core/hcd.c
> > ===================================================================
> > --- linux-4.17.orig/drivers/usb/core/hcd.c	2018-06-12 16:06:23.000000000 +0200
> > +++ linux-4.17/drivers/usb/core/hcd.c	2018-06-12 16:07:51.000000000 +0200
> > @@ -1858,6 +1858,16 @@ void usb_hcd_giveback_urb(struct usb_hcd
> >  }
> >  EXPORT_SYMBOL_GPL(usb_hcd_giveback_urb);
> >  
> > +void _usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb, int status)
> > +{
> > +	/* pass status to tasklet via unlinked */
> > +	if (likely(!urb->unlinked))
> > +		urb->unlinked = status;
> > +
> > +	__usb_hcd_giveback_urb(urb);
> > +}
> > +EXPORT_SYMBOL_GPL(_usb_hcd_giveback_urb);
> > +
> >  /*-------------------------------------------------------------------------*/
> >  
> >  /* Cancel all URBs pending on this endpoint and wait for the endpoint's
> > Index: linux-4.17/drivers/usb/host/ehci-q.c
> > ===================================================================
> > --- linux-4.17.orig/drivers/usb/host/ehci-q.c	2018-06-12 16:06:23.000000000 +0200
> > +++ linux-4.17/drivers/usb/host/ehci-q.c	2018-06-12 16:09:28.000000000 +0200
> > @@ -238,6 +238,8 @@ static int qtd_copy_status (
> >  
> >  static void
> >  ehci_urb_done(struct ehci_hcd *ehci, struct urb *urb, int status)
> > +__releases(ehci->lock)
> > +__acquires(ehci->lock)
> >  {
> >  	if (usb_pipetype(urb->pipe) == PIPE_INTERRUPT) {
> >  		/* ... update hc-wide periodic stats */
> > @@ -264,7 +266,14 @@ ehci_urb_done(struct ehci_hcd *ehci, str
> >  #endif
> >  
> >  	usb_hcd_unlink_urb_from_ep(ehci_to_hcd(ehci), urb);
> > -	usb_hcd_giveback_urb(ehci_to_hcd(ehci), urb, status);
> > +	if (usb_pipeisoc(urb->pipe)) {
> > +		/* complete() can reenter this HCD */
> > +		spin_unlock(&ehci->lock);
> > +		_usb_hcd_giveback_urb(ehci_to_hcd(ehci), urb, status);
> > +		spin_lock(&ehci->lock);
> > +	} else {
> > +		usb_hcd_giveback_urb(ehci_to_hcd(ehci), urb, status);
> > +	}
> >  }
> >  
> >  static int qh_schedule (struct ehci_hcd *ehci, struct ehci_qh *qh);
> > Index: linux-4.17/include/linux/usb/hcd.h
> > ===================================================================
> > --- linux-4.17.orig/include/linux/usb/hcd.h	2018-06-05 21:07:27.000000000 +0200
> > +++ linux-4.17/include/linux/usb/hcd.h	2018-06-12 16:07:11.000000000 +0200
> > @@ -428,6 +428,8 @@ extern int usb_hcd_submit_urb(struct urb
> >  extern int usb_hcd_unlink_urb(struct urb *urb, int status);
> >  extern void usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb,
> >  		int status);
> > +extern void _usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb,
> > +		int status);
> >  extern int usb_hcd_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
> >  		gfp_t mem_flags);
> >  extern void usb_hcd_unmap_urb_setup_for_dma(struct usb_hcd *, struct urb *);
> > --
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> > 
> > 
>