Re: [RFC] perf: need to expose sched_clock to correlate user sampleswith kernel samples
From: Thomas Gleixner
Date: Wed Feb 20 2013 - 05:07:18 EST
On Tue, 19 Feb 2013, John Stultz wrote:
> On 02/19/2013 01:50 PM, Thomas Gleixner wrote:
> > 2) Doing #1 will allow to observe the described time going backwards
> > scenario in kernel as well.
> > The reason why we did not get complaints about that scenario at all
> > (yet) is that the window and the probability to hit it are small
> > enough. Nevertheless it's a real issue for virtualized systems.
> > Now you came up with the great idea, that the timekeeping core is
> > able to calculate what the approximate safe value is for the
> > clocksource readout to be in a state where wreckage relative to the
> > last update of the clocksource is not observable, not matter how
> > long the scheduled out delay is and in which direction the NTP
> > update is going.
> So the other bit of caution here, is I realize my idea of "valid cycle ranges"
> has the potential for deadlock.
> While it should be fine for use with vdso, we have to be careful if we use
> this in-kernel, because if we're in the update path, the valid interval check
> could trigger the ktime_get() in hrtimer_interrupt() to spin forever. So we
> need to be sure we don't use this method anywhere in the code paths that
> trigger the update_wall_time() code.
> So some additional thinking may be necessary here. Though it may be as simple
> as making sure we don't loop on the cpu that does the timekeeping update.
Either that or make sure to use ktime_get_nocheck() in those code pathes.
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