Messages in this thread Patch in this message | | | Subject | Re: clocksource changes in 2.6.31 - possible regression | From | john stultz <> | Date | Mon, 17 Aug 2009 14:10:58 -0700 |
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On Mon, 2009-08-17 at 11:27 -0700, Stephen Hemminger wrote: > On Mon, 17 Aug 2009 11:15:54 -0700 > john stultz <johnstul@us.ibm.com> wrote: > > > On Mon, 2009-08-17 at 11:01 -0700, Stephen Hemminger wrote: > > > On Mon, 17 Aug 2009 10:48:57 -0700 > > > john stultz <johnstul@us.ibm.com> wrote: > > > > > > > On Mon, 2009-08-17 at 09:03 -0700, Stephen Hemminger wrote: > > > > > The following commit causes a change for kernels built with HRT but > > > > > not actually using HRT. I typically use the generic kernel we ship > > > > > on test machines, and that kernel has NOHZ and HRT (for power savings/virt > > > > > and HRT for QoS), but I want to be able to enable TSC as a clock source > > > > > when doing performance tests with pktgen. > > > > > > > > > > The machine in question is a several year old Opteron box, that > > > > > normally reports clocksources: acpi_pm jiffies tsc > > > > > but now with 2.6.31-rc6, it only has acpi_pm. > > > > > > > > I might need to review the patch again, but I believe we just don't > > > > allow you to switch to non HRT compatible clocksources (like jiffies) if > > > > we're already in HRT mode (and thus would hang when switched). > > > > > > > > > > > > The behavior you describe where you can't switch to the TSC, may be due > > > > to the TSC disqualification code marking it as non HRT compatible > > > > (again, I need to double check). While I'm not sure that's really > > > > correct, as the TSC is fine for HRT, in this case on your box, the TSC > > > > has been marked as unstable (likely due to being unsynced on old AMD SMP > > > > systems). There is a real chance that the timekeeping code on your > > > > system could see the TSC go backwards, calculate a negative time > > > > interval, and then end up hanging. > > > > > > > > > > TSC was alway stable on this box, and worked fine. There was no > > > message in log about TSC instability. The change was bisected > > > down to that one commit. > > > > But just to clarify, the TSC was never selected as the default > > clocksource on the box either, right? > > correct. > > I am okay with turning it off on boot command line for my tests, > but it might be an issue for other users.
So looking at the code in question: /* * Don't show non-HRES clocksource if the tick code is * in one shot mode (highres=on or nohz=on) */ if (!tick_oneshot_mode_active() || (src->flags & CLOCK_SOURCE_VALID_FOR_HRES))
So we require the clock to be valid for hres if we're in hres mode.
Then looking at where that flag is manipulated: $ git grep -n CLOCK_SOURCE_VALID_FOR_HRES include/linux/clocksource.h:214:#define CLOCK_SOURCE_VALID_FOR_HRES kernel/time/clocksource.c:168: cs->flags &= ~(CLOCK_SOURCE_VALID_FOR_HRES | CLO kernel/time/clocksource.c:200: cs->flags |= CLOCK_SOURC kernel/time/clocksource.c:257: cs->flags |= CLOCK_SOURCE_VALID_ kernel/time/clocksource.c:285: cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES kernel/time/clocksource.c:517: !(ovr->flags & CLOCK_SOURCE_VALID_FOR_HRES)) kernel/time/clocksource.c:557: (src->flags & CLOCK_SOURCE_VALID_FOR kernel/time/timekeeping.c:273: ret = clock->flags & CLOCK_SOURCE_VALID_
We can see clocksource.c:168 is the only line that disables the flag and that's in clocksource_ratewd() after we've found an actual inconsistency from the watchdog.
So unless I'm missing a more subtle bug in the watchdog assignment of the CLOCK_SOURCE_VALID_FOR_HRES bit, I'm a little hesitant that its really as stable as you feel it is.
Mind running with the following patch and sending me the dmesg?
thanks -john
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 7466cb8..08ff940 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -198,6 +198,7 @@ static void clocksource_watchdog(unsigned long data) if ((cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) && (watchdog->flags & CLOCK_SOURCE_IS_CONTINUOUS)) { cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES; + printk("Marked %s valid for HRT\n", cs->name); /* * We just marked the clocksource as * highres-capable, notify the rest of the @@ -253,9 +254,10 @@ static void clocksource_check_watchdog(struct clocksource *cs) cpumask_first(cpu_online_mask)); } } else { - if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) + if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) { cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES; - + printk("Marked %s valid for HRT\n", cs->name); + } if (!watchdog || cs->rating > watchdog->rating) { if (watchdog) del_timer(&watchdog_timer); @@ -281,8 +283,10 @@ static void clocksource_check_watchdog(struct clocksource *cs) #else static void clocksource_check_watchdog(struct clocksource *cs) { - if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) + if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS){ cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES; + printk("Marked %s valid for HRT\n", cs->name); + } } static inline void clocksource_resume_watchdog(void) { }
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