Messages in this thread | | | Date | Mon, 12 Dec 2011 14:53:46 -0500 | From | Don Zickus <> | Subject | Re: [PATCH 2/2] watchdog: Softlockup has regular windows where it is not armed |
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On Mon, Dec 05, 2011 at 09:28:22PM +1100, Anton Blanchard wrote: > > Hi Don, > > > > There might be a reason for this two stage sync but I haven't been > > > able to find it yet. Perhaps the unsynced versions of cpu_clock() > > > and sched_clock_tick() are not safe to call from all contexts? > > > > According to commit 8c2238eaaf0f774ca0f8d9daad7a616429bbb7f1 that was > > the case, cpu_clock wasn't NMI-safe. Now it is, thanks to Peter. > > Thanks, that makes sense now. > > > I have a couple of concerns about the patch. I am wondering about the > > overhead of getting the timestamp more often now as opposed to just > > setting a boolean for later. It makes sense to stamp it at the time > > of the call, don't know what the cost is. > > I had a similar concern since we do execute this quite a lot. The > overhead of cpu_clock is quite low on powerpc, but not sure about the > other architectures.
It seems like half of the users of touch_softlockup_watchdog is a slow path (ie they are purposely spinning a long time). The cpu_clock overhead for those paths, we probably don't need to care about.
The other half seems to deal with long idle/suspend/kgdb paths, which may not be that interesting in their own right, except for the fact they are called all the time for short delays and long delays. :-/
Perhaps I can move the touch_softlockup_watchdog() calls closer to the long path conditionals, minimize the calls a little bit.
> > > I am also concern about how this affects suspend/resume and kgdb. I > > cc'd Jason above for kgdb. I'll have to run some tests locally to > > see what long periods of delay look like. Oh and virt guests too. > > You don't have any test results from that setup do you? > > I haven't tested suspend resume, kgdb or virtual guests yet.
I'll try to setup a box and play with these paths to see what they look like.
Cheers, Don
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