perf/x86/intel, watchdog: Switch NMI watchdog to ref cycles on x86

The NMI watchdog uses either the fixed cycles or a generic cycles
counter. This causes a lot of conflicts with users of the PMU who want
to run a full group including the cycles fixed counter, for example
the --topdown support recently added to perf stat. The code needs to
fall back to not use groups, which can cause measurement inaccuracy
due to multiplexing errors.

This patch switches the NMI watchdog to use reference cycles
on Intel systems.  This is actually more accurate than cycles,
because cycles can tick faster than the measured CPU Frequency
due to Turbo mode.

The ref cycles always tick at their frequency, or slower when
the system is idling. That means the NMI watchdog can never
expire too early, unlike with cycles.

The reference cycles tick roughly at the frequency of the TSC,
so the same period computation can be used.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: acme@kernel.org
Cc: jolsa@kernel.org
Link: http://lkml.kernel.org/r/1465478079-19993-1-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index 7788ce643bf4577d443b1007a1e4d37c8d28aaff..016f4263fad473962f61359ac641be41db7a808a 100644 (file)
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -18,8 +18,16 @@
 #include <linux/nmi.h>
 #include <linux/module.h>
 #include <linux/delay.h>
+#include <linux/perf_event.h>
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
+int hw_nmi_get_event(void)
+{
+       if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL)
+               return PERF_COUNT_HW_REF_CPU_CYCLES;
+       return PERF_COUNT_HW_CPU_CYCLES;
+}
+
 u64 hw_nmi_get_sample_period(int watchdog_thresh)
 {
        return (u64)(cpu_khz) * 1000 * watchdog_thresh;

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 4630eeae18e08df43be6567ca5fbcb2027643c65..79858af272090a881bef7fc66ddbd1330d03f0c2 100644 (file)
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -66,6 +66,7 @@ static inline bool trigger_allbutself_cpu_backtrace(void)
 
 #ifdef CONFIG_LOCKUP_DETECTOR
 u64 hw_nmi_get_sample_period(int watchdog_thresh);
+int hw_nmi_get_event(void);
 extern int nmi_watchdog_enabled;
 extern int soft_watchdog_enabled;
 extern int watchdog_user_enabled;

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 9acb29f280ec95813612a913307e4dcaec8b7e03..8dd30fcd91be967dcca8e3ddf7050dde23b37227 100644 (file)
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -315,6 +315,12 @@ static int is_softlockup(unsigned long touch_ts)
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 
+/* Can be overriden by architecture */
+__weak int hw_nmi_get_event(void)
+{
+       return PERF_COUNT_HW_CPU_CYCLES;
+}
+
 static struct perf_event_attr wd_hw_attr = {
        .type           = PERF_TYPE_HARDWARE,
        .config         = PERF_COUNT_HW_CPU_CYCLES,
@@ -604,6 +610,7 @@ static int watchdog_nmi_enable(unsigned int cpu)
 
        wd_attr = &wd_hw_attr;
        wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
+       wd_attr->config = hw_nmi_get_event();
 
        /* Try to register using hardware perf events */
        event = perf_event_create_kernel_counter(wd_attr, cpu, NULL, watchdog_overflow_callback, NULL);