Messages in this thread | | | Subject | Re: [PATCH] x86, tsc: Skip refined tsc calibration on systems with reliable TSC. | From | john stultz <> | Date | Tue, 06 Mar 2012 19:26:30 -0800 |
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On Tue, 2012-03-06 at 18:05 -0800, Alok Kataria wrote: > Hi John, > > On Tue, 2012-03-06 at 17:32 -0800, john stultz wrote: > > On Tue, 2012-02-21 at 18:19 -0800, Alok Kataria wrote: > > > [Oops forgot to copy LKML, now it is, sorry for the duplicates] > > > > > > While running the latest Linux as guest under VMware in highly > > > over-committed situations, we have seen cases when the refined TSC > > > algorithm fails to get a valid tsc_start value in > > > tsc_refine_calibration_work from multiple attempts. As a result the > > > kernel keeps on scheduling the tsc_irqwork task for later. Subsequently > > > after several attempts when it gets a valid start value it goes through > > > the refined calibration and either bails out or uses the new results. > > > Given that the kernel originally read the TSC frequency from the > > > platform, which is the best it can get, I don't think there is much > > > value in refining it. > > > > > > So IMO, for systems which get the TSC frequency from the platform we > > > should skip the refined tsc algorithm. > > > > > > We can use the TSC_RELIABLE cpu cap flag to detect this, right now it is > > > set only on VMware and for Moorestown Penwell both of which have there > > > own TSC calibration methods. > > > > So this looks ok to me, only one nit below... > > > > > > > > Index: linux-2.6/arch/x86/kernel/tsc.c > > > =================================================================== > > > --- linux-2.6.orig/arch/x86/kernel/tsc.c 2012-02-21 17:31:01.000000000 -0800 > > > +++ linux-2.6/arch/x86/kernel/tsc.c 2012-02-21 17:39:05.000000000 -0800 > > > @@ -874,6 +874,13 @@ static void tsc_refine_calibration_work( > > > goto out; > > > > > > /* > > > + * Trust the results of the earlier calibration on systems > > > + * exporting a reliable TSC. > > > + */ > > > + if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) > > > + goto out; > > > + > > > + /* > > > > Instead of dropping out in the function called by the work-queue, why > > not just avoid scheduling the work-queue to begin with? > > > > The FEATURE_TSC_RELIABLE isn't something that is set late, and needs the > > delay, right? > > Right, but the reason I did it as part of work-queue was because on the > "out" path it still registered the tsc clocksource for us, the patch you > suggested doesn't do that. Please find below a patch on similar lines, > which registers the clocksource on RELIABLE_TSC systems, instead of > relying on irq_work to do that.
Ah, yes. Thanks for pointing that out!
I'll go ahead and queue your revision.
thanks -john
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