Re: Runtime PM workqueue killing system performance with USB

Will Deacon
2014-05-22T17:39:57Z

Hi Alan,

On Thu, May 22, 2014 at 04:02:06PM +0100, Alan Stern wrote:
> On Thu, 22 May 2014, Will Deacon wrote:
> > Consequently, I see a kworker thread on each CPU consuming a significant
> > amount of the system resources. Worse, if I enable something like kmemleak
> > (which adds more work to the failed suspend operation), I end up failing
> > to boot entirely (NFS bombs out).
> >
> > Reverting db7c7c0aeef5 ("usb: Always return 0 or -EBUSY to the runtime
> > PM core.") fixes this for me, but the commit log suggests that will break
> > lsusb. That patch has also been in for three and a half years...
> >
> > Any ideas on how to fix this properly? In what ways does the PM core react
> > badly to -ENOENT?
>
> Okay, this is a bad problem.
>
> The PM core takes any error response other than -EBUSY or -EAGAIN as an
> indication that the device is in a runtime-PM error state. While that
> is appropriate for a USB device, perhaps it's not so appropriate for a
> USB host controller.
>
> Anyway, there are two possible ways of handling this. One is to avoid
> changing the error code to -EBUSY when the device in question is a root
> hub. Just let it go into a runtime-PM error state; it won't matter
> since the controller doesn't support runtime PM anyway. You can test
> this by changing the "if (status != 0)" line in usb_runtime_suspend to
>
> if (status != 0 && udev->parent)

I'd tried something like this already, but I prefer your patch below. Plus,
this hack results in a failure being logged to dmesg on the initial suspend
attempt.

> The other approach is to disable runtime PM for the root hub when the
> host controller driver doesn't have a bus_suspend or bus_resume method.
> This seems like a cleaner approach; the patch below implements it.

Thanks for this! I can confirm that your patch below fixes the issue for me,
so:

Reported-by: Will Deacon <will.deacon@arm.com>
Tested-by: Will Deacon <will.deacon@arm.com>

Cheers,

Will

> Index: usb-3.15/drivers/usb/core/hub.c
> ===================================================================
> --- usb-3.15.orig/drivers/usb/core/hub.c
> +++ usb-3.15/drivers/usb/core/hub.c
> @@ -1703,8 +1703,19 @@ static int hub_probe(struct usb_interfac
> */
> pm_runtime_set_autosuspend_delay(&hdev->dev, 0);
>
> - /* Hubs have proper suspend/resume support. */
> - usb_enable_autosuspend(hdev);
> + /*
> + * Hubs have proper suspend/resume support, except for root hubs
> + * where the controller driver doesn't have bus_suspend and
> + * bus_resume methods.
> + */
> + if (hdev->parent) { /* normal device */
> + usb_enable_autosuspend(hdev);
> + } else { /* root hub */
> + const struct hc_driver *drv = bus_to_hcd(hdev->bus)->driver;
> +
> + if (drv->bus_suspend && drv->bus_resume)
> + usb_enable_autosuspend(hdev);
> + }
>
> if (hdev->level == MAX_TOPO_LEVEL) {
> dev_err(&intf->dev,
>
>