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authorPeter Zijlstra <peterz@infradead.org>2013-10-17 15:32:10 +0200
committerIngo Molnar <mingo@kernel.org>2013-10-29 12:01:20 +0100
commite8a923cc1fff6e627f906655ad52ee694ef2f6d7 (patch)
tree10e56e0bd4463cfbc91a82132f75ecfebef2d474 /arch/x86/kernel/cpu
parentbf378d341e4873ed928dc3c636252e6895a21f50 (diff)
perf/x86: Fix NMI measurements
OK, so what I'm actually seeing on my WSM is that sched/clock.c is 'broken' for the purpose we're using it for. What triggered it is that my WSM-EP is broken :-( [ 0.001000] tsc: Fast TSC calibration using PIT [ 0.002000] tsc: Detected 2533.715 MHz processor [ 0.500180] TSC synchronization [CPU#0 -> CPU#6]: [ 0.505197] Measured 3 cycles TSC warp between CPUs, turning off TSC clock. [ 0.004000] tsc: Marking TSC unstable due to check_tsc_sync_source failed For some reason it consistently detects TSC skew, even though NHM+ should have a single clock domain for 'reasonable' systems. This marks sched_clock_stable=0, which means that we do fancy stuff to try and get a 'sane' clock. Part of this fancy stuff relies on the tick, clearly that's gone when NOHZ=y. So for idle cpus time gets stuck, until it either wakes up or gets kicked by another cpu. While this is perfectly fine for the scheduler -- it only cares about actually running stuff, and when we're running stuff we're obviously not idle. This does somewhat break down for perf which can trigger events just fine on an otherwise idle cpu. So I've got NMIs get get 'measured' as taking ~1ms, which actually don't last nearly that long: <idle>-0 [013] d.h. 886.311970: rcu_nmi_enter <-do_nmi ... <idle>-0 [013] d.h. 886.311997: perf_sample_event_took: HERE!!! : 1040990 So ftrace (which uses sched_clock(), not the fancy bits) only sees ~27us, but we measure ~1ms !! Now since all this measurement stuff lives in x86 code, we can actually fix it. Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: mingo@kernel.org Cc: dave.hansen@linux.intel.com Cc: eranian@google.com Cc: Don Zickus <dzickus@redhat.com> Cc: jmario@redhat.com Cc: acme@infradead.org Link: http://lkml.kernel.org/r/20131017133350.GG3364@laptop.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86/kernel/cpu')
-rw-r--r--arch/x86/kernel/cpu/perf_event.c6
1 files changed, 3 insertions, 3 deletions
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 9d8449158cf..8a87a322412 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1276,16 +1276,16 @@ void perf_events_lapic_init(void)
static int __kprobes
perf_event_nmi_handler(unsigned int cmd, struct pt_regs *regs)
{
- int ret;
u64 start_clock;
u64 finish_clock;
+ int ret;
if (!atomic_read(&active_events))
return NMI_DONE;
- start_clock = local_clock();
+ start_clock = sched_clock();
ret = x86_pmu.handle_irq(regs);
- finish_clock = local_clock();
+ finish_clock = sched_clock();
perf_sample_event_took(finish_clock - start_clock);