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

From: Peter Zijlstra
Date: Thu Jan 19 2017 - 08:37:05 EST

On Thu, Jan 19, 2017 at 11:19:24AM +0100, Peter Zijlstra wrote:

> I'll go have a prod. Thanks!

This seems to cure it for me.

---
Subject: sched/clock: Fix hotplug issue

Mike reported that he could trigger the WARN_ON_ONCE() in
set_sched_clock_stable() using hotplug.

This exposed a fundamental problem with the interface, we should never
mark the TSC stable if we ever find it to be unstable. Therefore
set_sched_clock_stable() is a broken interface.

The reason it existed is that not having it is a pain, it means all
relevant architecture code needs to call clear_sched_clock_stable()
where appropriate.

Of the three architectures that select HAVE_UNSTABLE_SCHED_CLOCK ia64
and parisc are trivial in that they never called
set_sched_clock_stable(), so add an unconditional call to
clear_sched_clock_stable() to them.

For x86 the story is a lot more involved, and what this patch tries to
do is ensure we preserve the status quo. So even is Cyrix or Transmeta
have usable TSC they never called set_sched_clock_stable() so they now
get an explicit mark unstable.

XXX: what about Xen ?

Fixes: 9881b024b7d7 ("sched/clock: Delay switching sched_clock to stable")
Reported-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/ia64/kernel/setup.c | 2 ++
arch/parisc/kernel/setup.c | 2 ++
arch/x86/kernel/cpu/amd.c | 6 ++++--
arch/x86/kernel/cpu/centaur.c | 2 ++
arch/x86/kernel/cpu/common.c | 3 +++
arch/x86/kernel/cpu/cyrix.c | 2 ++
arch/x86/kernel/cpu/intel.c | 6 ++++--
arch/x86/kernel/cpu/transmeta.c | 3 +++
arch/x86/kernel/kvmclock.c | 2 +-
include/linux/sched.h | 1 -
kernel/sched/clock.c | 29 ++++++++---------------------
11 files changed, 31 insertions(+), 27 deletions(-)

diff --git a/arch/ia64/kernel/setup.c b/arch/ia64/kernel/setup.c
index 7ec7acc..c483ece 100644
--- a/arch/ia64/kernel/setup.c
+++ b/arch/ia64/kernel/setup.c
@@ -619,6 +619,8 @@ setup_arch (char **cmdline_p)
check_sal_cache_flush();
#endif
paging_init();
+
+ clear_sched_clock_stable();
}

/*
diff --git a/arch/parisc/kernel/setup.c b/arch/parisc/kernel/setup.c
index 2e66a88..068ed36 100644
--- a/arch/parisc/kernel/setup.c
+++ b/arch/parisc/kernel/setup.c
@@ -36,6 +36,7 @@
#undef PCI_DEBUG
#include <linux/proc_fs.h>
#include <linux/export.h>
+#include <linux/sched.h>

#include <asm/processor.h>
#include <asm/sections.h>
@@ -176,6 +177,7 @@ void __init setup_arch(char **cmdline_p)
conswitchp = &dummy_con; /* we use do_take_over_console() later ! */
#endif

+ clear_sched_clock_stable();
}

/*
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 1d31672..80e657e 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -541,8 +541,10 @@ 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())
- set_sched_clock_stable();
+ if (check_tsc_unstable())
+ clear_sched_clock_stable();
+ } else {
+ clear_sched_clock_stable();
}

/* 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 1661d8e..c4bab8c 100644
--- a/arch/x86/kernel/cpu/centaur.c
+++ b/arch/x86/kernel/cpu/centaur.c
@@ -104,6 +104,8 @@ static void early_init_centaur(struct cpuinfo_x86 *c)
#ifdef CONFIG_X86_64
set_cpu_cap(c, X86_FEATURE_SYSENTER32);
#endif
+
+ clear_sched_clock_stable();
}

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 9bab7a8..0bdb1ab 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -83,6 +83,7 @@ static void default_init(struct cpuinfo_x86 *c)
strcpy(c->x86_model_id, "386");
}
#endif
+ clear_sched_clock_stable();
}

static const struct cpu_dev default_cpu = {
@@ -1055,6 +1056,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
*/
if (this_cpu->c_init)
this_cpu->c_init(c);
+ else
+ clear_sched_clock_stable();

/* 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 bd9dcd6..47416f9 100644
--- a/arch/x86/kernel/cpu/cyrix.c
+++ b/arch/x86/kernel/cpu/cyrix.c
@@ -9,6 +9,7 @@
#include <asm/pci-direct.h>
#include <asm/tsc.h>
#include <asm/cpufeature.h>
+#include <linux/sched.h>

#include "cpu.h"

@@ -183,6 +184,7 @@ static void early_init_cyrix(struct cpuinfo_x86 *c)
set_cpu_cap(c, X86_FEATURE_CYRIX_ARR);
break;
}
+ clear_sched_clock_stable();
}

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 203f860..026c728 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -119,8 +119,10 @@ 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())
- set_sched_clock_stable();
+ if (check_tsc_unstable())
+ clear_sched_clock_stable();
+ } else {
+ clear_sched_clock_stable();
}

/* 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 3417856..c1ea5b9 100644
--- a/arch/x86/kernel/cpu/transmeta.c
+++ b/arch/x86/kernel/cpu/transmeta.c
@@ -1,4 +1,5 @@
#include <linux/kernel.h>
+#include <linux/sched.h>
#include <linux/mm.h>
#include <asm/cpufeature.h>
#include <asm/msr.h>
@@ -14,6 +15,8 @@ static void early_init_transmeta(struct cpuinfo_x86 *c)
if (xlvl >= 0x80860001)
c->x86_capability[CPUID_8086_0001_EDX] = cpuid_edx(0x80860001);
}
+
+ clear_sched_clock_stable();
}

static void init_transmeta(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 2a5cafd..542710b 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -107,12 +107,12 @@ static inline void kvm_sched_clock_init(bool stable)
{
if (!stable) {
pv_time_ops.sched_clock = kvm_clock_read;
+ clear_sched_clock_stable();
return;
}

kvm_sched_clock_offset = kvm_clock_read();
pv_time_ops.sched_clock = kvm_sched_clock_read;
- set_sched_clock_stable();

printk(KERN_INFO "kvm-clock: using sched offset of %llu cycles\n",
kvm_sched_clock_offset);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 68b35dd..8a9bbb6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2522,7 +2522,6 @@ extern void sched_clock_init_late(void);
* is reliable after all:
*/
extern int sched_clock_stable(void);
-extern void set_sched_clock_stable(void);
extern void clear_sched_clock_stable(void);

extern void sched_clock_tick(void);
diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 7713b2b..ad64efe 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -83,8 +83,15 @@ void sched_clock_init(void)
}

#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+/*
+ * We must start with !__sched_clock_stable because the unstable -> stable
+ * transition is accurate, while the stable -> unstable transition is not.
+ *
+ * Similarly we start with __sched_clock_stable_early, thereby assuming we
+ * will become stable, such that there's only a single 1 -> 0 transition.
+ */
static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
-static int __sched_clock_stable_early;
+static int __sched_clock_stable_early = 1;

/*
* We want: ktime_get_ns() + gtod_offset == sched_clock() + raw_offset
@@ -132,24 +139,6 @@ static void __set_sched_clock_stable(void)
tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
}

-void set_sched_clock_stable(void)
-{
- __sched_clock_stable_early = 1;
-
- smp_mb(); /* matches sched_clock_init_late() */
-
- /*
- * This really should only be called early (before
- * sched_clock_init_late()) when guestimating our sched_clock() is
- * solid.
- *
- * After that we test stability and we can negate our guess using
- * clear_sched_clock_stable, possibly from a watchdog.
- */
- if (WARN_ON_ONCE(sched_clock_running == 2))
- __set_sched_clock_stable();
-}
-
static void __clear_sched_clock_stable(struct work_struct *work)
{
struct sched_clock_data *scd = this_scd();
@@ -199,8 +188,6 @@ void sched_clock_init_late(void)

if (__sched_clock_stable_early)
__set_sched_clock_stable();
- else
- __clear_sched_clock_stable(NULL);
}

/*