Re: tip.today - scheduler bam boom crash (cpu hotplug)

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 27, 2017 at 05:11:34PM +0100, Paolo Bonzini wrote:
> On 27/02/2017 16:59, Peter Zijlstra wrote:

> > Now, I've not yet figured out the ordering between when we set
> > pv_time_ops.sched_clock and when we do the 'normal' TSC init stuff.
> 
> I think the ordering is fine:
> 
> - pv_time_ops.sched_clock is set here:
> 
> 	start_kernel (init/main.c line 509)
> 	  setup_arch
> 	    kvmclock_init
> 	      kvm_sched_clock_init
> 
> - TSC can be declared unstable only after this:
> 
> 	start_kernel (init/main.c line 628)
> 	  late_time_init
> 	    tsc_init
> 
> So by the time the tsc_cs_mark_unstable or mark_tsc_unstable can call
> clear_sched_clock_stable, pv_time_ops.sched_clock has been set.
> 
> > But it appears to me, we should not be calling
> > clear_sched_clock_stable() on TSC bits when we don't end up using
> > native_sched_clock().
> 
> Yes, this makes sense.

Something like the below then (completely untested for now) has a chance
of working. It does however change behaviour a little.

I'm trying to debug something else; after that I'll give this a little
more consideration.

---
 arch/x86/kernel/cpu/amd.c       |  4 +---
 arch/x86/kernel/cpu/centaur.c   |  2 +-
 arch/x86/kernel/cpu/common.c    |  4 ++--
 arch/x86/kernel/cpu/cyrix.c     |  2 +-
 arch/x86/kernel/cpu/intel.c     |  4 +---
 arch/x86/kernel/cpu/transmeta.c |  2 +-
 arch/x86/kernel/tsc.c           | 33 +++++++++++++++++++++------------
 7 files changed, 28 insertions(+), 23 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 4e95b2e0d95f..bc3bbb6a8ab0 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -555,10 +555,8 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (check_tsc_unstable())
-			clear_sched_clock_stable();
 	} else {
-		clear_sched_clock_stable();
+		mark_tsc_unstable("not invariant");
 	}
 
 	/* Bit 12 of 8000_0007 edx is accumulated power mechanism. */
diff --git a/arch/x86/kernel/cpu/centaur.c b/arch/x86/kernel/cpu/centaur.c
index 2c234a6d94c4..0fdff183aa30 100644
--- a/arch/x86/kernel/cpu/centaur.c
+++ b/arch/x86/kernel/cpu/centaur.c
@@ -105,7 +105,7 @@ static void early_init_centaur(struct cpuinfo_x86 *c)
 	set_cpu_cap(c, X86_FEATURE_SYSENTER32);
 #endif
 
-	clear_sched_clock_stable();
+	mark_tsc_unstable("not invariant");
 }
 
 static void init_centaur(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f07005e6f461..2b7ff648ea25 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -86,7 +86,7 @@ static void default_init(struct cpuinfo_x86 *c)
 			strcpy(c->x86_model_id, "386");
 	}
 #endif
-	clear_sched_clock_stable();
+	mark_tsc_unstable("not invariant");
 }
 
 static const struct cpu_dev default_cpu = {
@@ -1076,7 +1076,7 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	if (this_cpu->c_init)
 		this_cpu->c_init(c);
 	else
-		clear_sched_clock_stable();
+		mark_tsc_unstable("not invariant");
 
 	/* Disable the PN if appropriate */
 	squash_the_stupid_serial_number(c);
diff --git a/arch/x86/kernel/cpu/cyrix.c b/arch/x86/kernel/cpu/cyrix.c
index 47416f959a48..35057d67e864 100644
--- a/arch/x86/kernel/cpu/cyrix.c
+++ b/arch/x86/kernel/cpu/cyrix.c
@@ -184,7 +184,7 @@ static void early_init_cyrix(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_CYRIX_ARR);
 		break;
 	}
-	clear_sched_clock_stable();
+	mark_tsc_unstable("not invariant");
 }
 
 static void init_cyrix(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 017ecd3bb553..e0e192e43a4c 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -161,10 +161,8 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (check_tsc_unstable())
-			clear_sched_clock_stable();
 	} else {
-		clear_sched_clock_stable();
+		mark_tsc_unstable("not invariant");
 	}
 
 	/* Penwell and Cloverview have the TSC which doesn't sleep on S3 */
diff --git a/arch/x86/kernel/cpu/transmeta.c b/arch/x86/kernel/cpu/transmeta.c
index c1ea5b999839..fa243ffd1c84 100644
--- a/arch/x86/kernel/cpu/transmeta.c
+++ b/arch/x86/kernel/cpu/transmeta.c
@@ -16,7 +16,7 @@ static void early_init_transmeta(struct cpuinfo_x86 *c)
 			c->x86_capability[CPUID_8086_0001_EDX] = cpuid_edx(0x80860001);
 	}
 
-	clear_sched_clock_stable();
+	mark_tsc_unstable("not invariant");
 }
 
 static void init_transmeta(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 2724dc82f992..bf6627aff54d 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -326,9 +326,16 @@ unsigned long long sched_clock(void)
 {
 	return paravirt_sched_clock();
 }
+
+static inline bool using_native_sched_clock(void)
+{
+	return pv_time_ops.sched_clock == native_sched_clock;
+}
 #else
 unsigned long long
 sched_clock(void) __attribute__((alias("native_sched_clock")));
+
+static inline bool using_native_sched_clock(void) { return true; }
 #endif
 
 int check_tsc_unstable(void)
@@ -1112,7 +1119,8 @@ static void tsc_cs_mark_unstable(struct clocksource *cs)
 	if (tsc_unstable)
 		return;
 	tsc_unstable = 1;
-	clear_sched_clock_stable();
+	if (using_native_sched_clock())
+		clear_sched_clock_stable();
 	disable_sched_clock_irqtime();
 	pr_info("Marking TSC unstable due to clocksource watchdog\n");
 }
@@ -1134,18 +1142,19 @@ static struct clocksource clocksource_tsc = {
 
 void mark_tsc_unstable(char *reason)
 {
-	if (!tsc_unstable) {
-		tsc_unstable = 1;
+	if (tsc_unstable)
+		return;
+	tsc_unstable = 1;
+	if (using_native_sched_clock())
 		clear_sched_clock_stable();
-		disable_sched_clock_irqtime();
-		pr_info("Marking TSC unstable due to %s\n", reason);
-		/* Change only the rating, when not registered */
-		if (clocksource_tsc.mult)
-			clocksource_mark_unstable(&clocksource_tsc);
-		else {
-			clocksource_tsc.flags |= CLOCK_SOURCE_UNSTABLE;
-			clocksource_tsc.rating = 0;
-		}
+	disable_sched_clock_irqtime();
+	pr_info("Marking TSC unstable due to %s\n", reason);
+	/* Change only the rating, when not registered */
+	if (clocksource_tsc.mult)
+		clocksource_mark_unstable(&clocksource_tsc);
+	else {
+		clocksource_tsc.flags |= CLOCK_SOURCE_UNSTABLE;
+		clocksource_tsc.rating = 0;
 	}
 }