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watchdog: add watchdog_cpumask sysctl to assist nohz

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Change the default behavior of watchdog so it only runs on the
housekeeping cores when nohz_full is enabled at build and boot time.
Allow modifying the set of cores the watchdog is currently running on
with a new kernel.watchdog_cpumask sysctl.

In the current system, the watchdog subsystem runs a periodic timer that
schedules the watchdog kthread to run.  However, nohz_full cores are
designed to allow userspace application code running on those cores to
have 100% access to the CPU.  So the watchdog system prevents the
nohz_full application code from being able to run the way it wants to,
thus the motivation to suppress the watchdog on nohz_full cores, which
this patchset provides by default.

However, if we disable the watchdog globally, then the housekeeping
cores can't benefit from the watchdog functionality.  So we allow
disabling it only on some cores.  See Documentation/lockup-watchdogs.txt
for more information.

[jhubbard@nvidia.com: fix a watchdog crash in some configurations]
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Chris Metcalf
2015-06-24 16:55:45 -07:00
committed by Linus Torvalds
parent b5242e98c1
commit fe4ba3c343
6 changed files with 112 additions and 5 deletions
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21
Documentation/sysctl/kernel.txt
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@@ -923,6 +923,27 @@ and nmi_watchdog.
==============================================================
watchdog_cpumask:
This value can be used to control on which cpus the watchdog may run.
The default cpumask is all possible cores, but if NO_HZ_FULL is
enabled in the kernel config, and cores are specified with the
nohz_full= boot argument, those cores are excluded by default.
Offline cores can be included in this mask, and if the core is later
brought online, the watchdog will be started based on the mask value.
Typically this value would only be touched in the nohz_full case
to re-enable cores that by default were not running the watchdog,
if a kernel lockup was suspected on those cores.
The argument value is the standard cpulist format for cpumasks,
so for example to enable the watchdog on cores 0, 2, 3, and 4 you
might say:
echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
==============================================================
watchdog_thresh:
This value can be used to control the frequency of hrtimer and NMI