Commit e8a923cc authored Oct 17, 2013 by Peter Zijlstra Committed by Ingo Molnar Oct 29, 2013

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>

parent bf378d34

arch/x86/kernel/cpu/perf_event.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -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);

arch/x86/kernel/nmi.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -113,10 +113,10 @@ static int __kprobes nmi_handle(unsigned int type, struct pt_regs *regs, bool b2
		u64 before, delta, whole_msecs;
		int remainder_ns, decimal_msecs, thishandled;

		before = local_clock();
		before = sched_clock();
		thishandled = a->handler(type, regs);
		handled += thishandled;
		delta = local_clock() - before;
		delta = sched_clock() - before;
		trace_nmi_handler(a->handler, (int)delta, thishandled);

		if (delta < nmi_longest_ns)

Admin message