Re: [Xen-devel] [PATCH 0/8] xen: don't let keyhandlers block indefinitely on locks

[Jürgen Groß <jgross@xxxxxxxx>]

On 13.02.20 19:38, Andrew Cooper wrote:
> On 13/02/2020 12:54, Juergen Gross wrote:
> > Keyhandlers dumping hypervisor information to the console often need
> > to take locks while accessing data. In order to not block in case of
> > system inconsistencies it is convenient to use trylock variants when
> > obtaining the locks. On the other hand a busy system might easily
> > encounter held locks, so this patch series is adding special trylock
> > variants with a timeout used by keyhandlers.
> This is a backwards step.
>
> Keyhandlers are for debugging purposes.  When debugging it is far more 
> important to get the requested data, than almost anything else.
Right.

> The system will cope with a multi-second outage occurring approximately 
> never.  A person debugging who can't get the data has no chance of 
> fixing whatever problem they are looking for.
Right.

> This series seems to be breaking the one critical usecase for 
> keyhandlers, to fix what - not let debugging get in the way of the 
> smooth running of the system?  A system in need of debugging in the 
> first place has bigger problems than needing to run smoothly.
Okay, this warrants a longer default timeout.

A keyhandler blocking on a lock will produce exactly no further data,
and it will probably block other keyhandlers, too, due to hogging at
least one cpu completely.

With a longer lock timeout (1 second?) there is a much higher chance
that the keyhandler will finish its job producing more data than
without any timeout.

BTW, during development of my core scheduling series I was hit by that
problem multiple times. With the lock timeout I'd have spared dozens of
reboots.

> The only thing which should happen to improve system stability is for 
> keyhandlers to disable the system watchdog while they are running, in 
> case they happen to run for seconds of wallclock time. This is an issue 
> which isn't addressed by the series, because once a keyhandler does get 
> a lock, it keeps it until it is done.
Right, will add disabling the watchdog during keyhandler action.


Juergen

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