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Date:   Sat, 29 Aug 2020 11:50:03 +0200
From:   Salvatore Bonaccorso <carnil@...ian.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Dirk Kostrewa <dirk.kostrewa@...lbox.org>
Subject: Re: kworker/0:3+pm hogging CPU

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?

Regards,
Salvatore

Download attachment "dmesg.txt.gz" of type "application/gzip" (28485 bytes)

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