panel unhide slow after virtual desktop change

[David L]

On Wed, Dec 28, 2016 at 12:01 PM, David L <idht4n at gmail.com> wrote:
> My 16.10 kubuntu system is currently in state where hidden panels
> unhide slowly (>5 seconds and possibly proportional to the number of
> desktops I change through... I saw a 30 second delay once) after I
> change virtual desktops.  It's currently reproducible but I suspect a
> logout and login would fix it because I'm pretty sure I would have
> noticed this behavior before this if it happened all of the time.  The
> CPU load of Xorg and plasmashell are highish during this period.
>
> 3252 root      20   0  289212 164096  93696 S  75.8  1.0  41:42.34 Xorg
> 3629 dgl       20   0 6576264 873620 269468 R  66.2  5.3  15:13.88
> plasmashell
>
> The desktops that I switched from/to were relatively empty.
>
> There were some errors in .xsession-errors that may or may not be
> related:
> QXcbConnection: XCB error: 2 (BadValue), sequence: 15463, resource id:
> 138412037, major code: 142 (Unknown), minor code: 3
> QXcbConnection: XCB error: 2 (BadValue), sequence: 15464, resource id:
> 140509189, major code: 142 (Unknown), minor code: 3
> QXcbConnection: XCB error: 2 (BadValue), sequence: 15465, resource id:
> 83886085, major code: 142 (Unknown), minor code: 3
> QXcbConnection: XCB error: 2 (BadValue), sequence: 15466, resource id:
> 12582917, major code: 142 (Unknown), minor code: 3
> QXcbConnection: XCB error: 2 (BadValue), sequence: 15467, resource id:
> 109051909, major code: 142 (Unknown), minor code: 3
> QXcbConnection: XCB error: 3 (BadWindow), sequence: 48602, resource
> id: 29802249, major code: 3 (GetWindowAttributes), minor code: 0
> QXcbConnection: XCB error: 3 (BadWindow), sequence: 49890, resource
> id: 29802465, major code: 3 (GetWindowAttributes), minor code: 0
>
> Any ideas why it is in this state or how to get out of it without
> restarting this session?

I straced plasmashell during the time that it was hogging CPU and the
panel was unresponsive and saw a lot of blocks like this:

writev(5, [{"\24\0\6\0i\0\300\4:\1\0\0:\1\0\0\0\0\0\0\1\0\0\0\24\0\6\0i\0\300\4"...,
216}], 1) = 216
futex(0x7ffe37ca5294, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAIT_PRIVATE, 2, NULL) = -1 EAGAIN
(Resource temporarily unavailable)
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\16\0\2\0i\0\300\4(\0\4\0i\0\300\4\254\2\0\0\0\0\0\0",
24}], 1) = 24
futex(0x7ffe37ca5844, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0i\0\300\4\337\1\0\0\37\0\0\0\0\0\0\0\240\206\1\0",
24}], 1) = 24
futex(0x7ffe37ca53a4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0i\0\300\4Z\1\0\0\6\0\0\0\0\0\0\0\377\377\377\377\24\0\6\0i\0\300\4"...,
96}], 1) = 96
futex(0x7ffe37ca52f4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0
\3:\1\0\0:\1\0\0\0\0\0\0\1\0\0\0\24\0\6\0\5\0 \3"..., 216}], 1) = 216
futex(0x7ffe37ca5294, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAIT_PRIVATE, 2, NULL) = -1 EAGAIN
(Resource temporarily unavailable)
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\16\0\2\0\5\0 \3(\0\4\0\5\0 \3\254\2\0\0\0\0\0\0", 24}], 1) = 24
futex(0x7ffe37ca5844, FUTEX_WAIT_PRIVATE, 1, NULL) = -1 EAGAIN
(Resource temporarily unavailable)
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0 \3\337\1\0\0\37\0\0\0\0\0\0\0\240\206\1\0",
24}], 1) = 24
futex(0x7ffe37ca53a4, FUTEX_WAIT_PRIVATE, 1, NULL) = -1 EAGAIN
(Resource temporarily unavailable)
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0
\3Z\1\0\0\6\0\0\0\0\0\0\0\377\377\377\377\24\0\6\0\5\0 \3"..., 96}],
1) = 96
futex(0x7ffe37ca52f4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0\200\3:\1\0\0:\1\0\0\0\0\0\0\1\0\0\0\24\0\6\0\5\0\200\3"...,
216}], 1) = 216
futex(0x7ffe37ca5294, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 1
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\16\0\2\0\5\0\200\3(\0\4\0\5\0\200\3\254\2\0\0\0\0\0\0",
24}], 1) = 24
futex(0x7ffe37ca5844, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0\200\3\337\1\0\0\37\0\0\0\0\0\0\0\240\206\1\0",
24}], 1) = 24
futex(0x7ffe37ca53a4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0\200\3Z\1\0\0\6\0\0\0\0\0\0\0\377\377\377\377\24\0\6\0\5\0\200\3"...,
96}], 1) = 96
futex(0x7ffe37ca52f4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0\0\5:\1\0\0:\1\0\0\0\0\0\0\1\0\0\0\24\0\6\0\5\0\0\5"...,
216}], 1) = 216
futex(0x7ffe37ca5294, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 1
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\16\0\2\0\5\0\0\5(\0\4\0\5\0\0\5\254\2\0\0\0\0\0\0", 24}], 1) = 24
futex(0x7ffe37ca5844, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0\0\5\337\1\0\0\37\0\0\0\0\0\0\0\240\206\1\0",
24}], 1) = 24
futex(0x7ffe37ca53a4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\5\0\0\5Z\1\0\0\6\0\0\0\0\0\0\0\377\377\377\377\24\0\6\0\5\0\0\5"...,
96}], 1) = 96
futex(0x7ffe37ca52f4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\4\0\200\10:\1\0\0:\1\0\0\0\0\0\0\1\0\0\0\24\0\6\0\4\0\200\10"...,
216}], 1) = 216
futex(0x7ffe37ca5294, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\16\0\2\0\4\0\200\10(\0\4\0\4\0\200\10\254\2\0\0\0\0\0\0",
24}], 1) = 24
futex(0x7ffe37ca5844, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\4\0\200\10\337\1\0\0\37\0\0\0\0\0\0\0\240\206\1\0",
24}], 1) = 24
futex(0x7ffe37ca53a4, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x556bb8628558, FUTEX_WAKE_PRIVATE, 1) = 0
poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=5, revents=POLLOUT}])
writev(5, [{"\24\0\6\0\4\0\200\10Z\1\0\0\6\0\0\0\0\0\0\0\377\377\377\377\24\0\6\0\4\0\200\10"...,
96}], 1) = 96

Does that give any clues?