Messages in this thread | | | Subject | Re: [PATCH] x86, tsc: Skip refined tsc calibration on systems with reliable TSC. | From | Alok Kataria <> | Date | Tue, 28 Feb 2012 11:09:16 -0800 |
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Ping...any comments on this one.
Thanks, Alok
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. > > Thanks, > Alok > > -- > > From: Alok N Kataria <akataria@vmware.com> > > For systems which get the TSC frequency directly from the platform > and don't go through the native TSC calibration algorithm, we should > trust those values and not try to refine those. > > This patch is applicable for kernel from v2.6.38 to current mainline. > > Signed-off-by: Alok N Kataria <akataria@vmware.com> > Cc: John Stultz <johnstul@us.ibm.com> > Cc: Dirk Brandewie <dirk.brandewie@gmail.com> > Cc: Alan Cox <alan@linux.intel.com> > Cc: stable@kernel.org > > > 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; > + > + /* > * Since the work is started early in boot, we may be > * delayed the first time we expire. So set the workqueue > * again once we know timers are working. >
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