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

[Alan Stern]

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?  Or raising 
the priority of the softirq thread?

Alan Stern

> 
> 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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