Messages in this thread Patch in this message | | | Subject | Re: [PATCH] x86, tsc: Skip refined tsc calibration on systems with reliable TSC. | From | Alok Kataria <> | Date | Tue, 06 Mar 2012 18:05:30 -0800 |
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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.
> Here's what I queued up, let me know if it looks ok to you and I'll push > it on to Thomas. > > thanks > -john > > From 50cd62f326fa3204763717c9808bdc29ba10512c Mon Sep 17 00:00:00 2001 > From: Alok Kataria <akataria@vmware.com> > Date: Tue, 21 Feb 2012 18:19:55 -0800 > Subject: [PATCH] x86, tsc: Skip refined tsc calibration on systems with reliable TSC. > > 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 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. > > 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 > [jstultz: Reworked to simply not schedule the refining work, > rather then scheduling the work and bombing out later] > Signed-off-by: John Stultz <john.stultz@linaro.org> > --- > arch/x86/kernel/tsc.c | 8 ++++++++ > 1 files changed, 8 insertions(+), 0 deletions(-) > > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c > index a62c201..b7d4d33 100644 > --- a/arch/x86/kernel/tsc.c > +++ b/arch/x86/kernel/tsc.c > @@ -932,6 +932,14 @@ static int __init init_tsc_clocksource(void) > clocksource_tsc.rating = 0; > clocksource_tsc.flags &= ~CLOCK_SOURCE_IS_CONTINUOUS; > } > + > + /* > + * Trust the results of the earlier calibration on systems > + * exporting a reliable TSC. > + */ > + if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) > + return 0; > + > schedule_delayed_work(&tsc_irqwork, 0); > return 0; > }
Index: linux-2.6/arch/x86/kernel/tsc.c =================================================================== --- linux-2.6.orig/arch/x86/kernel/tsc.c 2012-02-21 17:25:35.000000000 -0800 +++ linux-2.6/arch/x86/kernel/tsc.c 2012-03-06 17:56:00.000000000 -0800 @@ -938,6 +938,16 @@ static int __init init_tsc_clocksource(v clocksource_tsc.rating = 0; clocksource_tsc.flags &= ~CLOCK_SOURCE_IS_CONTINUOUS; } + + /* + * Trust the results of the earlier calibration on systems + * exporting a reliable TSC. + */ + if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) { + clocksource_register_khz(&clocksource_tsc, tsc_khz); + return 0; + } + schedule_delayed_work(&tsc_irqwork, 0); return 0; }
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