On 24/06/2022, 08:31:55, Michael Ellerman wrote:

Laurent Dufour [off-list ref] writes:

quoted

In some cricunstances it may be interesting to reconfigure the watchdog
from inside the kernel.

On PowerPC, this may helpful before and after a LPAR migration (LPM) is
initiated, because it implies some latencies, watchdog, and especially NMI
watchdog is expected to be triggered during this operation. Reconfiguring
the watchdog, would prevent it to happen too frequently during LPM.

The watchdog_mutex is exported to allow some variable to be changed under
its protection and prevent any conflict.
The lockup_detector_reconfigure() function is exported and is expected to
be called under the protection of watchdog_mutex.

Signed-off-by: Laurent Dufour <redacted>
---
 include/linux/nmi.h | 3 +++
 kernel/watchdog.c   | 6 +++---
 2 files changed, 6 insertions(+), 3 deletions(-)

Is there a maintainer for kernel/watchdog.c ?

Nothing clearly identified AFAICT.

I'll add the commit signers reported by get_maintainer.pl.

There's Wim & Guenter at linux-watchdog@vger but I think that's only for
drivers/watchdog?

Maybe we should Cc that list anyway?

Yes, that's a good idea.

quoted

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 750c7f395ca9..84300fb0f90a 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h

@@ -122,6 +122,9 @@ int watchdog_nmi_probe(void);
 int watchdog_nmi_enable(unsigned int cpu);
 void watchdog_nmi_disable(unsigned int cpu);
 
+extern struct mutex watchdog_mutex;
+void lockup_detector_reconfigure(void);

It would be preferable if we didn't export the mutex.

I think you could arrange that by ...

Renaming lockup_detector_configure() to __lockup_detector_configure()
and then adding a new lockup_detector_configure() that is non-static and
takes the lock around __lockup_detector_configure().

Unfortunately, that will not be enough, because this mutex is also used to
protect wd_watchdog, to ensure it is not changed while another operation is
in progress.

I may try finding another way to protect that value, may be using
WRITE/READ_ONCE(). Indeed, the only requirement is to read a stable value
in watchdog_calc_timeouts().

Thanks,
Laurent.