Re: kworker/0:3+pm hogging CPU

Alan Stern
2020-08-29T16:01:16Z

On Sat, Aug 29, 2020 at 11:50:03AM +0200, Salvatore Bonaccorso wrote:
> Hi Alan,
>
> I'm following up on this thread because a user in Debian (Dirk, Cc'ed)
> as well encountered the same/similar issue:
>
> On Tue, Jul 21, 2020 at 10:33:25AM -0400, Alan Stern wrote:
> > On Tue, Jul 21, 2020 at 07:59:17AM +0200, Michal Hocko wrote:
> > > > Sorry, my mistake. The module name needs to be "xhci_hcd" with an '_'
> > > > character, not a '-' character -- the same as what shows up in the lsmod
> > > > output.
> > >
> > >
> > > [14766.973734] xhci_hcd 0000:00:14.0: Get port status 2-1 read: 0xe000088, return 0x88
> > > [14766.973738] xhci_hcd 0000:00:14.0: Get port status 2-2 read: 0xe000088, return 0x88
> > > [14766.973742] xhci_hcd 0000:00:14.0: Get port status 2-3 read: 0xe0002a0, return 0x2a0
> > > [14766.973746] xhci_hcd 0000:00:14.0: Get port status 2-4 read: 0xe0002a0, return 0x2a0
> > > [14766.973750] xhci_hcd 0000:00:14.0: Get port status 2-5 read: 0xe0002a0, return 0x2a0
> > > [14766.973754] xhci_hcd 0000:00:14.0: Get port status 2-6 read: 0xe0002a0, return 0x2a0
> > > [14766.973759] xhci_hcd 0000:00:14.0: Get port status 2-1 read: 0xe000088, return 0x88
> > > [14766.973763] xhci_hcd 0000:00:14.0: Get port status 2-2 read: 0xe000088, return 0x88
> >
> > According to the xHCI specification, those 02a0 values are normal and
> > the 0088 values indicate the port is disabled and has an over-current
> > condition. I don't know about the e000 bits in the upper part of the
> > word; according to my copy of the spec those bits should be 0.
> >
> > If your machine has only two physical SuperSpeed (USB-3) ports then
> > perhaps the other four ports are internally wired in a way that creates
> > a permanent over-current indication.
> >
> > > [14766.973771] xhci_hcd 0000:00:14.0: set port remote wake mask, actual port 0 status = 0xe000088
> > > [14766.973780] xhci_hcd 0000:00:14.0: set port remote wake mask, actual port 1 status = 0xe000088
> > > [14766.973789] xhci_hcd 0000:00:14.0: set port remote wake mask, actual port 2 status = 0xe0002a0
> > > [14766.973798] xhci_hcd 0000:00:14.0: set port remote wake mask, actual port 3 status = 0xe0002a0
> > > [14766.973807] xhci_hcd 0000:00:14.0: set port remote wake mask, actual port 4 status = 0xe0002a0
> > > [14766.973816] xhci_hcd 0000:00:14.0: set port remote wake mask, actual port 5 status = 0xe0002a0
> > > [14766.973830] xhci_hcd 0000:00:14.0: Bus suspend bailout, port over-current detected
> > >
> > > Repeating again and again. The last message suggests a HW problem? But
> > > why does the kernel try the same thing over and over?
> >
> > Because over-current is supposed to be a transient condition that goes
> > away quickly. It means there's a short circuit or something similar.
>
> Dirk exprienced the same issue aand enabled dynamic debugging showed
> similar pattern. His dmesg excerpt is attached.
>
> The Debian report is at https://bugs.debian.org/966703
>
> What could be tracked down is that the issue is uncovered since
> e9fb08d617bf ("xhci: prevent bus suspend if a roothub port detected a
> over-current condition") which was applied in 5.7-rc3 and backported
> to several stable releases (v5.6.8, v5.4.36 and v4.19.119).
>
> Dirk found additionally:
>
> > I just found out, that if none of the two USB ports is connected, there
> > are two kworker processes with permanently high CPU load, if one USB
> > port is connected and the other not, there is one such kworker process,
> > and if both USB ports are connected, there is no kworker process with
> > high CPU load.
> > I think, this supports your suspicion that these kworker processes are
> > connected with the overcurrent condition for both USB ports that I also
> > see in the dmesg output.
>
> Reverting the above commit covers the problem again. But I'm not
> exprienced enough here to claim if this is a HW issue or if the Kernel
> should handle the situation otherwise. Is there anything else Dirk can
> provide?

It is undoubtedly a hardware issue. The dmesg extract shows that ports
1-10, 1-11, and 2-5 (which is probably the same port as one of the
others) have overcurrent conditions; I'm guessing that these are the
ports which have external connections.

What were the devices Dirk plugged in that got rid of the kworker
processes? In particular, were they USB-2 or USB-3? (The dmesg log for
when the devices were first attached can answer these questions.)

As far as I know, there is no way for the kernel to work around this
problem. Preventing the controller from going into runtime suspend is
probably the best solution.

Perhaps Mathias (the xhci-hcd maintainer) will have more suggestions.

Alan Stern