freedesktop bug id: 100548, bisected to sched/clock commit

freedesktop bug id: 100548, bisected to sched/clock commit

[Lofstedt, Marta]

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Hi,

We have this "old" Lenovo Cantiga laptop(Intel Core 2 Duo L9400), hocked up to our i915 pre-merge CI system, that has started to give unstable results after commit:

commit 7b09cc5a9debc86c903c2eff8f8a1fdef773c649
Author: Pavel Tatashin <pasha.tatashin@oracle.com>
Date:   Wed Mar 22 16:24:17 2017 -0400

    sched/clock: Fix broken stable to unstable transfer

The issue is describe more in detail here:
https://bugs.freedesktop.org/show_bug.cgi?id=100548

We have reverted above patch and then issue is no longer reproducible.
Also, note that this issue has not been reproduced on any of our other machines,
https://intel-gfx-ci.01.org/CI/

So, why is this only affecting the Core 2 Duo?

BR,
Marta






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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Wed, Apr 12, 2017 at 12:04:00PM +0000, Lofstedt, Marta wrote:
> Hi,
> 
> We have this "old" Lenovo Cantiga laptop(Intel Core 2 Duo L9400), hocked up to our i915 pre-merge CI system, that has started to give unstable results after commit:
> 
> commit 7b09cc5a9debc86c903c2eff8f8a1fdef773c649
> Author: Pavel Tatashin <pasha.tatashin@oracle.com>
> Date:   Wed Mar 22 16:24:17 2017 -0400
> 
>     sched/clock: Fix broken stable to unstable transfer
> 
> The issue is describe more in detail here:
> https://bugs.freedesktop.org/show_bug.cgi?id=100548

I don't click links.

> We have reverted above patch and then issue is no longer reproducible.
> Also, note that this issue has not been reproduced on any of our other machines,
> https://intel-gfx-ci.01.org/CI/
> 
> So, why is this only affecting the Core 2 Duo?

Core2 doesn't have a usable TSC and would revert to the slow path. I'll
have another look at that patch.

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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Jani Nikula]

On Wed, 12 Apr 2017, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Apr 12, 2017 at 12:04:00PM +0000, Lofstedt, Marta wrote:
>> Hi,
>> 
>> We have this "old" Lenovo Cantiga laptop(Intel Core 2 Duo L9400), hocked up to our i915 pre-merge CI system, that has started to give unstable results after commit:
>> 
>> commit 7b09cc5a9debc86c903c2eff8f8a1fdef773c649
>> Author: Pavel Tatashin <pasha.tatashin@oracle.com>
>> Date:   Wed Mar 22 16:24:17 2017 -0400
>> 
>>     sched/clock: Fix broken stable to unstable transfer
>> 
>> The issue is describe more in detail here:
>> https://bugs.freedesktop.org/show_bug.cgi?id=100548
>
> I don't click links.

Frankly, I find that more than a little offensive response to a
regression report.

BR,
Jani.

>
>> We have reverted above patch and then issue is no longer reproducible.
>> Also, note that this issue has not been reproduced on any of our other machines,
>> https://intel-gfx-ci.01.org/CI/
>> 
>> So, why is this only affecting the Core 2 Duo?
>
> Core2 doesn't have a usable TSC and would revert to the slow path. I'll
> have another look at that patch.
>
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> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Jani Nikula, Intel Open Source Technology Center
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Wed, Apr 12, 2017 at 04:30:32PM +0300, Jani Nikula wrote:
> On Wed, 12 Apr 2017, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Wed, Apr 12, 2017 at 12:04:00PM +0000, Lofstedt, Marta wrote:

> >> The issue is describe more in detail here:
> >> https://bugs.freedesktop.org/show_bug.cgi?id=100548
> >
> > I don't click links.
> 
> Frankly, I find that more than a little offensive response to a
> regression report.

*shrug*, its more than you would've gotten if you'd filed a kernel
bugzilla. Those go straight to /dev/null.
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Martin Peres]

On 12/04/17 15:31, Peter Zijlstra wrote:
> On Wed, Apr 12, 2017 at 12:04:00PM +0000, Lofstedt, Marta wrote:
>> Hi,
>>
>> We have this "old" Lenovo Cantiga laptop(Intel Core 2 Duo L9400), hocked up to our i915 pre-merge CI system, that has started to give unstable results after commit:
>>
>> commit 7b09cc5a9debc86c903c2eff8f8a1fdef773c649
>> Author: Pavel Tatashin <pasha.tatashin@oracle.com>
>> Date:   Wed Mar 22 16:24:17 2017 -0400
>>
>>     sched/clock: Fix broken stable to unstable transfer
>>
>> The issue is describe more in detail here:
>> https://bugs.freedesktop.org/show_bug.cgi?id=100548
>
> I don't click links.
>
>> We have reverted above patch and then issue is no longer reproducible.
>> Also, note that this issue has not been reproduced on any of our other machines,
>> https://intel-gfx-ci.01.org/CI/
>>
>> So, why is this only affecting the Core 2 Duo?
>
> Core2 doesn't have a usable TSC and would revert to the slow path. I'll
> have another look at that patch.
>

So, by default, it is using the hpet clock source. FYI, I tried the only 
other available clock source (acpi_pm) and got the same result.
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Wed, Apr 12, 2017 at 04:40:11PM +0300, Martin Peres wrote:

> > > So, why is this only affecting the Core 2 Duo?
> > 
> > Core2 doesn't have a usable TSC and would revert to the slow path. I'll
> > have another look at that patch.
> > 
> 
> So, by default, it is using the hpet clock source. FYI, I tried the only
> other available clock source (acpi_pm) and got the same result.

So because HPET is unbearably slow we've cobbled together something that
takes the HPET (or rather get-time-of-day CLOCK_MONOTONIC) value at tick
time and uses TSC to add per-cpu increments to that. Using windowing to
keep the TSC maddness at bay.

The patch in question affects the windowing.. clearly something went
amiss.
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Martin Peres]

On 12/04/17 17:32, Peter Zijlstra wrote:
> On Wed, Apr 12, 2017 at 04:40:11PM +0300, Martin Peres wrote:
>
>>>> So, why is this only affecting the Core 2 Duo?
>>>
>>> Core2 doesn't have a usable TSC and would revert to the slow path. I'll
>>> have another look at that patch.
>>>
>>
>> So, by default, it is using the hpet clock source. FYI, I tried the only
>> other available clock source (acpi_pm) and got the same result.
>
> So because HPET is unbearably slow we've cobbled together something that
> takes the HPET (or rather get-time-of-day CLOCK_MONOTONIC) value at tick
> time and uses TSC to add per-cpu increments to that. Using windowing to
> keep the TSC maddness at bay.
>
> The patch in question affects the windowing.. clearly something went
> amiss.
>

Good to know. Is there a way to disable this behaviour, as a workaround 
for our CI system until a proper fix lands? We already pushed locally 
the revert for this patch, but that may affect other platforms which do 
not exhibit the problem.
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Wed, Apr 12, 2017 at 05:49:53PM +0300, Martin Peres wrote:
> 
> 
> On 12/04/17 17:32, Peter Zijlstra wrote:
> > On Wed, Apr 12, 2017 at 04:40:11PM +0300, Martin Peres wrote:
> > 
> > > > > So, why is this only affecting the Core 2 Duo?
> > > > 
> > > > Core2 doesn't have a usable TSC and would revert to the slow path. I'll
> > > > have another look at that patch.
> > > > 
> > > 
> > > So, by default, it is using the hpet clock source. FYI, I tried the only
> > > other available clock source (acpi_pm) and got the same result.
> > 
> > So because HPET is unbearably slow we've cobbled together something that
> > takes the HPET (or rather get-time-of-day CLOCK_MONOTONIC) value at tick
> > time and uses TSC to add per-cpu increments to that. Using windowing to
> > keep the TSC maddness at bay.
> > 
> > The patch in question affects the windowing.. clearly something went
> > amiss.
> > 
> 
> Good to know. Is there a way to disable this behaviour, as a workaround for
> our CI system until a proper fix lands? We already pushed locally the revert
> for this patch, but that may affect other platforms which do not exhibit the
> problem.

No, the only thing you can do is revert for now.
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Wed, Apr 12, 2017 at 05:49:53PM +0300, Martin Peres wrote:

> Good to know. Is there a way to disable this behaviour, as a workaround for
> our CI system until a proper fix lands? We already pushed locally the revert
> for this patch, but that may affect other platforms which do not exhibit the
> problem.

Blergh, so the patch is correct, but the __gtod_offset calculation is
fed with absolute crap numbers due to 'circumstances' and then using it
ends up being worse than not using it.

Something like the below could be a work-around, but let me see if I
can't fix things better.

---
 arch/x86/kernel/tsc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 714dfba..8ab883a 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -374,6 +374,8 @@ static int __init tsc_setup(char *str)
 		tsc_clocksource_reliable = 1;
 	if (!strncmp(str, "noirqtime", 9))
 		no_sched_irq_time = 1;
+	if (!strcmp(str, "unstable"))
+		mark_tsc_unstable("boot parameter");
 	return 1;
 }
 
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Martin Peres]

On 13/04/17 14:48, Peter Zijlstra wrote:
> On Wed, Apr 12, 2017 at 05:49:53PM +0300, Martin Peres wrote:
>
>> Good to know. Is there a way to disable this behaviour, as a workaround for
>> our CI system until a proper fix lands? We already pushed locally the revert
>> for this patch, but that may affect other platforms which do not exhibit the
>> problem.
>
> Blergh, so the patch is correct, but the __gtod_offset calculation is
> fed with absolute crap numbers due to 'circumstances' and then using it
> ends up being worse than not using it.

Thanks for taking this bug seriously!

>
> Something like the below could be a work-around, but let me see if I
> can't fix things better.

Yeah, this should be a last-resort.

Thanks,
Martin
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Thu, Apr 13, 2017 at 03:30:25PM +0300, Martin Peres wrote:
> On 13/04/17 14:48, Peter Zijlstra wrote:
> > On Wed, Apr 12, 2017 at 05:49:53PM +0300, Martin Peres wrote:
> > 
> > > Good to know. Is there a way to disable this behaviour, as a workaround for
> > > our CI system until a proper fix lands? We already pushed locally the revert
> > > for this patch, but that may affect other platforms which do not exhibit the
> > > problem.
> > 
> > Blergh, so the patch is correct, but the __gtod_offset calculation is
> > fed with absolute crap numbers due to 'circumstances' and then using it
> > ends up being worse than not using it.
> 
> Thanks for taking this bug seriously!

So I've not actually dug out a Core2 machine, so have only tested this
by poking random values into the TSC MSR on an otherwise 'good' machine.

Could you give it a go to see if it works for you?

Thomas, how much hate?

---
Subject: sched/clock,x86/tsc: Improve clock continuity for stable->unstable transition
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu Apr 13 14:56:44 CEST 2017

Marta reported that commit:

  7b09cc5a9deb ("sched/clock: Fix broken stable to unstable transfer")

Appeared to have broken things on a Core2Duo machine. While that patch
is in fact correct, it exposes a problem with commit:

  5680d8094ffa ("sched/clock: Provide better clock continuity")

Where we hoped that TSC would not make big jumps after SMP bringup. Of
course, TSC needs to prove us wrong. Because Core2 comes up with a
semi-stable TSC and only goes funny once we probe the idle drivers,
because Core2 stops TSC on idle.

Now we could of course delay the final switch to stable longer, but it
would be better to entirely remove the assumption that TSC doesn't
make big jumps and improve things all-round.

So instead we have the clocksource watchdog call a special function
when it finds the TSC is still good (there's a race, it could've
gotten bad between us determining it's still good and calling our
function, do we care?).

This function then updates the __gtod_offset using sane values, which
is the value needed for clock continuity when being marked unstable.

Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Martin Peres <martin.peres@linux.intel.com>
Reported-by: "Lofstedt, Marta" <marta.lofstedt@intel.com>
Fixes: 5680d8094ffa ("sched/clock: Provide better clock continuity")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/tsc.c       |   12 ++++++++++
 include/linux/clocksource.h |    1 
 include/linux/sched/clock.h |    2 -
 kernel/sched/clock.c        |   50 ++++++++++++++++++++++++--------------------
 kernel/time/clocksource.c   |    3 ++
 5 files changed, 45 insertions(+), 23 deletions(-)

--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -374,6 +374,8 @@ static int __init tsc_setup(char *str)
 		tsc_clocksource_reliable = 1;
 	if (!strncmp(str, "noirqtime", 9))
 		no_sched_irq_time = 1;
+	if (!strcmp(str, "unstable"))
+		mark_tsc_unstable("boot parameter");
 	return 1;
 }
 
@@ -1127,6 +1129,15 @@ static void tsc_cs_mark_unstable(struct
 	pr_info("Marking TSC unstable due to clocksource watchdog\n");
 }
 
+static void tsc_cs_tick_stable(struct clocksource *cs)
+{
+	if (tsc_unstable)
+		return;
+
+	if (using_native_sched_clock())
+		sched_clock_tick_stable();
+}
+
 /*
  * .mask MUST be CLOCKSOURCE_MASK(64). See comment above read_tsc()
  */
@@ -1140,6 +1151,7 @@ static struct clocksource clocksource_ts
 	.archdata               = { .vclock_mode = VCLOCK_TSC },
 	.resume			= tsc_resume,
 	.mark_unstable		= tsc_cs_mark_unstable,
+	.tick_stable		= tsc_cs_tick_stable,
 };
 
 void mark_tsc_unstable(char *reason)
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -96,6 +96,7 @@ struct clocksource {
 	void (*suspend)(struct clocksource *cs);
 	void (*resume)(struct clocksource *cs);
 	void (*mark_unstable)(struct clocksource *cs);
+	void (*tick_stable)(struct clocksource *cs);
 
 	/* private: */
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
--- a/include/linux/sched/clock.h
+++ b/include/linux/sched/clock.h
@@ -63,8 +63,8 @@ extern void clear_sched_clock_stable(voi
  */
 extern u64 __sched_clock_offset;
 
-
 extern void sched_clock_tick(void);
+extern void sched_clock_tick_stable(void);
 extern void sched_clock_idle_sleep_event(void);
 extern void sched_clock_idle_wakeup_event(u64 delta_ns);
 
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -152,25 +152,15 @@ static void __clear_sched_clock_stable(v
 {
 	struct sched_clock_data *scd = this_scd();
 
-	/*
-	 * Attempt to make the stable->unstable transition continuous.
-	 *
-	 * Trouble is, this is typically called from the TSC watchdog
-	 * timer, which is late per definition. This means the tick
-	 * values can already be screwy.
-	 *
-	 * Still do what we can.
-	 */
-	__gtod_offset = (scd->tick_raw + __sched_clock_offset) - (scd->tick_gtod);
+	if (!sched_clock_stable())
+		return;
 
 	printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n",
 			scd->tick_gtod, __gtod_offset,
 			scd->tick_raw,  __sched_clock_offset);
 
 	tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
-
-	if (sched_clock_stable())
-		schedule_work(&sched_clock_work);
+	schedule_work(&sched_clock_work);
 }
 
 void clear_sched_clock_stable(void)
@@ -347,21 +337,37 @@ void sched_clock_tick(void)
 {
 	struct sched_clock_data *scd;
 
+	if (sched_clock_stable())
+		return;
+
+	if (unlikely(!sched_clock_running))
+		return;
+
 	WARN_ON_ONCE(!irqs_disabled());
 
-	/*
-	 * Update these values even if sched_clock_stable(), because it can
-	 * become unstable at any point in time at which point we need some
-	 * values to fall back on.
-	 *
-	 * XXX arguably we can skip this if we expose tsc_clocksource_reliable
-	 */
 	scd = this_scd();
 	scd->tick_raw  = sched_clock();
 	scd->tick_gtod = ktime_get_ns();
+	sched_clock_local(scd);
+}
 
-	if (!sched_clock_stable() && likely(sched_clock_running))
-		sched_clock_local(scd);
+void sched_clock_tick_stable(void)
+{
+	u64 gtod, clock;
+
+	if (!sched_clock_stable())
+		return;
+
+	/*
+	 * Called under watchdog_lock.
+	 *
+	 * The watchdog just found this TSC to (still) be stable, so now is a
+	 * good moment to update our __gtod_offset. Because once we find the
+	 * TSC to be unstable, any computation will be computing crap.
+	 */
+	gtod = ktime_get_ns();
+	clock = sched_clock();
+	__gtod_offset = (clock + __sched_clock_offset) - gtod;
 }
 
 /*
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -233,6 +233,9 @@ static void clocksource_watchdog(unsigne
 			continue;
 		}
 
+		if (cs == curr_clocksource && cs->tick_stable)
+			cs->tick_stable(cs);
+
 		if (!(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) &&
 		    (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) &&
 		    (watchdog->flags & CLOCK_SOURCE_IS_CONTINUOUS)) {
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Thomas Gleixner]

On Thu, 13 Apr 2017, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 03:30:25PM +0300, Martin Peres wrote:
> > On 13/04/17 14:48, Peter Zijlstra wrote:
> > > On Wed, Apr 12, 2017 at 05:49:53PM +0300, Martin Peres wrote:
> > > 
> > > > Good to know. Is there a way to disable this behaviour, as a workaround for
> > > > our CI system until a proper fix lands? We already pushed locally the revert
> > > > for this patch, but that may affect other platforms which do not exhibit the
> > > > problem.
> > > 
> > > Blergh, so the patch is correct, but the __gtod_offset calculation is
> > > fed with absolute crap numbers due to 'circumstances' and then using it
> > > ends up being worse than not using it.
> > 
> > Thanks for taking this bug seriously!
> 
> So I've not actually dug out a Core2 machine, so have only tested this
> by poking random values into the TSC MSR on an otherwise 'good' machine.
> 
> Could you give it a go to see if it works for you?
> 
> Thomas, how much hate?

Well you know, how much I love TSC and its completely non-sensical
behaviour. If that's necessary to cure it, go for it.

Thanks,

	tglx


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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Lofstedt, Marta]

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@infradead.org]
> Sent: Thursday, April 13, 2017 4:24 PM
> To: Martin Peres <martin.peres@linux.intel.com>
> Cc: Lofstedt, Marta <marta.lofstedt@intel.com>;
> pasha.tatashin@oracle.com; intel-gfx@lists.freedesktop.org; Thomas
> Gleixner <tglx@linutronix.de>
> Subject: Re: freedesktop bug id: 100548, bisected to sched/clock commit
> 
> On Thu, Apr 13, 2017 at 03:30:25PM +0300, Martin Peres wrote:
> > On 13/04/17 14:48, Peter Zijlstra wrote:
> > > On Wed, Apr 12, 2017 at 05:49:53PM +0300, Martin Peres wrote:
> > >
> > > > Good to know. Is there a way to disable this behaviour, as a
> > > > workaround for our CI system until a proper fix lands? We already
> > > > pushed locally the revert for this patch, but that may affect
> > > > other platforms which do not exhibit the problem.
> > >
> > > Blergh, so the patch is correct, but the __gtod_offset calculation
> > > is fed with absolute crap numbers due to 'circumstances' and then
> > > using it ends up being worse than not using it.
> >
> > Thanks for taking this bug seriously!
> 
> So I've not actually dug out a Core2 machine, so have only tested this by
> poking random values into the TSC MSR on an otherwise 'good' machine.
> 
> Could you give it a go to see if it works for you?

Sorry Peter, I still see regression on the Core2 machine, with your patch.

/Marta

> 
> Thomas, how much hate?
> 
> ---
> Subject: sched/clock,x86/tsc: Improve clock continuity for stable->unstable
> transition
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Thu Apr 13 14:56:44 CEST 2017
> 
> Marta reported that commit:
> 
>   7b09cc5a9deb ("sched/clock: Fix broken stable to unstable transfer")
> 
> Appeared to have broken things on a Core2Duo machine. While that patch is
> in fact correct, it exposes a problem with commit:
> 
>   5680d8094ffa ("sched/clock: Provide better clock continuity")
> 
> Where we hoped that TSC would not make big jumps after SMP bringup. Of
> course, TSC needs to prove us wrong. Because Core2 comes up with a semi-
> stable TSC and only goes funny once we probe the idle drivers, because
> Core2 stops TSC on idle.
> 
> Now we could of course delay the final switch to stable longer, but it would
> be better to entirely remove the assumption that TSC doesn't make big
> jumps and improve things all-round.
> 
> So instead we have the clocksource watchdog call a special function when it
> finds the TSC is still good (there's a race, it could've gotten bad between us
> determining it's still good and calling our function, do we care?).
> 
> This function then updates the __gtod_offset using sane values, which is the
> value needed for clock continuity when being marked unstable.
> 
> Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
> Cc: Martin Peres <martin.peres@linux.intel.com>
> Reported-by: "Lofstedt, Marta" <marta.lofstedt@intel.com>
> Fixes: 5680d8094ffa ("sched/clock: Provide better clock continuity")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/kernel/tsc.c       |   12 ++++++++++
>  include/linux/clocksource.h |    1
>  include/linux/sched/clock.h |    2 -
>  kernel/sched/clock.c        |   50 ++++++++++++++++++++++++------------------
> --
>  kernel/time/clocksource.c   |    3 ++
>  5 files changed, 45 insertions(+), 23 deletions(-)
> 
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -374,6 +374,8 @@ static int __init tsc_setup(char *str)
>  		tsc_clocksource_reliable = 1;
>  	if (!strncmp(str, "noirqtime", 9))
>  		no_sched_irq_time = 1;
> +	if (!strcmp(str, "unstable"))
> +		mark_tsc_unstable("boot parameter");
>  	return 1;
>  }
> 
> @@ -1127,6 +1129,15 @@ static void tsc_cs_mark_unstable(struct
>  	pr_info("Marking TSC unstable due to clocksource
> watchdog\n");  }
> 
> +static void tsc_cs_tick_stable(struct clocksource *cs) {
> +	if (tsc_unstable)
> +		return;
> +
> +	if (using_native_sched_clock())
> +		sched_clock_tick_stable();
> +}
> +
>  /*
>   * .mask MUST be CLOCKSOURCE_MASK(64). See comment above
> read_tsc()
>   */
> @@ -1140,6 +1151,7 @@ static struct clocksource clocksource_ts
>  	.archdata               = { .vclock_mode = VCLOCK_TSC },
>  	.resume			= tsc_resume,
>  	.mark_unstable		=
> tsc_cs_mark_unstable,
> +	.tick_stable		= tsc_cs_tick_stable,
>  };
> 
>  void mark_tsc_unstable(char *reason)
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -96,6 +96,7 @@ struct clocksource {
>  	void (*suspend)(struct clocksource *cs);
>  	void (*resume)(struct clocksource *cs);
>  	void (*mark_unstable)(struct clocksource *cs);
> +	void (*tick_stable)(struct clocksource *cs);
> 
>  	/* private: */
>  #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
> --- a/include/linux/sched/clock.h
> +++ b/include/linux/sched/clock.h
> @@ -63,8 +63,8 @@ extern void clear_sched_clock_stable(voi
>   */
>  extern u64 __sched_clock_offset;
> 
> -
>  extern void sched_clock_tick(void);
> +extern void sched_clock_tick_stable(void);
>  extern void sched_clock_idle_sleep_event(void);
>  extern void sched_clock_idle_wakeup_event(u64 delta_ns);
> 
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -152,25 +152,15 @@ static void __clear_sched_clock_stable(v  {
>  	struct sched_clock_data *scd = this_scd();
> 
> -	/*
> -	 * Attempt to make the stable->unstable transition
> continuous.
> -	 *
> -	 * Trouble is, this is typically called from the TSC watchdog
> -	 * timer, which is late per definition. This means the tick
> -	 * values can already be screwy.
> -	 *
> -	 * Still do what we can.
> -	 */
> -	__gtod_offset = (scd->tick_raw + __sched_clock_offset) -
> (scd->tick_gtod);
> +	if (!sched_clock_stable())
> +		return;
> 
>  	printk(KERN_INFO "sched_clock: Marking unstable (%lld,
> %lld)<-(%lld, %lld)\n",
>  			scd->tick_gtod, __gtod_offset,
>  			scd->tick_raw,
> __sched_clock_offset);
> 
>  	tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
> -
> -	if (sched_clock_stable())
> -		schedule_work(&sched_clock_work);
> +	schedule_work(&sched_clock_work);
>  }
> 
>  void clear_sched_clock_stable(void)
> @@ -347,21 +337,37 @@ void sched_clock_tick(void)  {
>  	struct sched_clock_data *scd;
> 
> +	if (sched_clock_stable())
> +		return;
> +
> +	if (unlikely(!sched_clock_running))
> +		return;
> +
>  	WARN_ON_ONCE(!irqs_disabled());
> 
> -	/*
> -	 * Update these values even if sched_clock_stable(), because
> it can
> -	 * become unstable at any point in time at which point we
> need some
> -	 * values to fall back on.
> -	 *
> -	 * XXX arguably we can skip this if we expose
> tsc_clocksource_reliable
> -	 */
>  	scd = this_scd();
>  	scd->tick_raw  = sched_clock();
>  	scd->tick_gtod = ktime_get_ns();
> +	sched_clock_local(scd);
> +}
> 
> -	if (!sched_clock_stable() && likely(sched_clock_running))
> -		sched_clock_local(scd);
> +void sched_clock_tick_stable(void)
> +{
> +	u64 gtod, clock;
> +
> +	if (!sched_clock_stable())
> +		return;
> +
> +	/*
> +	 * Called under watchdog_lock.
> +	 *
> +	 * The watchdog just found this TSC to (still) be stable, so now
> is a
> +	 * good moment to update our __gtod_offset. Because once
> we find the
> +	 * TSC to be unstable, any computation will be computing crap.
> +	 */
> +	gtod = ktime_get_ns();
> +	clock = sched_clock();
> +	__gtod_offset = (clock + __sched_clock_offset) - gtod;
>  }
> 
>  /*
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -233,6 +233,9 @@ static void clocksource_watchdog(unsigne
>  			continue;
>  		}
> 
> +		if (cs == curr_clocksource && cs->tick_stable)
> +			cs->tick_stable(cs);
> +
>  		if (!(cs->flags &
> CLOCK_SOURCE_VALID_FOR_HRES) &&
>  		    (cs->flags &
> CLOCK_SOURCE_IS_CONTINUOUS) &&
>  		    (watchdog->flags &
> CLOCK_SOURCE_IS_CONTINUOUS)) {
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Tue, Apr 18, 2017 at 02:10:07PM +0000, Lofstedt, Marta wrote:
> Sorry Peter, I still see regression on the Core2 machine, with your patch.
> 

Blergh, ok. I'll see if I can dig out an actual Core2 machine somewhere.
I should have enough parts about.
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Tue, Apr 18, 2017 at 05:53:56PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 18, 2017 at 02:10:07PM +0000, Lofstedt, Marta wrote:
> > Sorry Peter, I still see regression on the Core2 machine, with your patch.
> > 
> 
> Blergh, ok. I'll see if I can dig out an actual Core2 machine somewhere.
> I should have enough parts about.

Argh!

My Core2 seems to work as expected _without_ this patch (time is
continuous at the stable->unstable switch), but is broken (time jumps
backwards by almost .2s) with this patch -- because by the time ACPI
Processor marks TSC as busted, we haven't ran the clocksource watchdog
yet.


Just for my sanity, could you confirm "tsc=unstable" (which requires the
patch) actually works for you?

I'll go prod at things now that I have an actual Core2 running; although
sadly I don't see an obvious problem without this patch.
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Re: freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Thu, Apr 20, 2017 at 07:19:50PM +0200, Peter Zijlstra wrote:

> Just for my sanity, could you confirm "tsc=unstable" (which requires the
> patch) actually works for you?


Also, could you get me the dmesg of a 'broken' boot?
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