linux-kernel - sysrq-w and sysrq-t useless with cgroups and systemd enabled?

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Date:	Tue, 15 Nov 2011 11:55:48 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	linux-kernel@...r.kernel.org
Subject: sysrq-w and sysrq-t useless with cgroups and systemd enabled?

For several kernel and Fedora versions now, I've gotten only garbage
out of sysrq-w and sysrq-t.  The output looks like this:

[432904.642680] rt_rq[0]:/system/home.mount
[432904.642681]   .rt_nr_running                 : 0
[432904.642682]   .rt_throttled                  : 0
[432904.642682]   .rt_time                       : 0.000000
[432904.642683]   .rt_runtime                    : 0.000000
[432904.642685]
[432904.642685] rt_rq[0]:/system/boot-efi.mount
[432904.642686]   .rt_nr_running                 : 0
[432904.642687]   .rt_throttled                  : 0
[432904.642688]   .rt_time                       : 0.000000
[432904.642689]   .rt_runtime                    : 0.000000
[432904.642690]
[432904.642690] rt_rq[0]:/system/boot.mount
[432904.642691]   .rt_nr_running                 : 0
[432904.642692]   .rt_throttled                  : 0
[432904.642693]   .rt_time                       : 0.000000
[432904.642694]   .rt_runtime                    : 0.000000
[432904.642695]
[432904.642695] rt_rq[0]:/system/fsck@...rvice
[432904.642696]   .rt_nr_running                 : 0
[432904.642697]   .rt_throttled                  : 0
[432904.642698]   .rt_time                       : 0.000000
[432904.642699]   .rt_runtime                    : 0.000000
[432904.642700]
[432904.642700] rt_rq[0]:/system/cryptsetup@...rvice
[432904.642701]   .rt_nr_running                 : 0
[432904.642702]   .rt_throttled                  : 0
[432904.642703]   .rt_time                       : 0.000000
[432904.642704]   .rt_runtime                    : 0.000000
[432904.642705]
[432904.642706] rt_rq[0]:/system/dev-mapper-vg_midnight_ssd\x2dswap.swap
[432904.642707]   .rt_nr_running                 : 0
[432904.642708]   .rt_throttled                  : 0
[432904.642708]   .rt_time                       : 0.000000
[432904.642709]   .rt_runtime                    : 0.000000

except that it goes on for pages and pages.  I can't find any
information about actual hung tasks in the mess (even by searching the
log -- I think the interesting information is either not there or is
so swamped by log spew that it's actually getting lost by my syslog
daemon).

Is this some kind of bad interaction between scheduling groups and the
task dumper that's getting exacerbated by the huge number of cgroups
that systemd creates?

--Andy
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