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authorPeter Zijlstra <peterz@infradead.org>2013-10-17 09:32:10 -0400
committerIngo Molnar <mingo@kernel.org>2013-10-29 07:01:20 -0400
commite8a923cc1fff6e627f906655ad52ee694ef2f6d7 (patch)
tree10e56e0bd4463cfbc91a82132f75ecfebef2d474 /arch/x86/kernel/nmi.c
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/nmi.c')
-rw-r--r--arch/x86/kernel/nmi.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index ba77ebc2c353..6fcb49ce50a1 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -113,10 +113,10 @@ static int __kprobes nmi_handle(unsigned int type, struct pt_regs *regs, bool b2
113 u64 before, delta, whole_msecs; 113 u64 before, delta, whole_msecs;
114 int remainder_ns, decimal_msecs, thishandled; 114 int remainder_ns, decimal_msecs, thishandled;
115 115
116 before = local_clock(); 116 before = sched_clock();
117 thishandled = a->handler(type, regs); 117 thishandled = a->handler(type, regs);
118 handled += thishandled; 118 handled += thishandled;
119 delta = local_clock() - before; 119 delta = sched_clock() - before;
120 trace_nmi_handler(a->handler, (int)delta, thishandled); 120 trace_nmi_handler(a->handler, (int)delta, thishandled);
121 121
122 if (delta < nmi_longest_ns) 122 if (delta < nmi_longest_ns)