Re: [PATCH] POWER: perf_event: Skip updating kernel counters if register value shrinks
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: 2011-03-31 06:05:14
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On Wed, 2011-03-30 at 14:36 -0400, Eric B Munson wrote:
On Wed, 30 Mar 2011, Benjamin Herrenschmidt wrote:quoted
On Tue, 2011-03-29 at 10:25 -0400, Eric B Munson wrote:quoted
Here I made the assumption that the hardware would never remove more events in a speculative roll back than it had added. This is not a situation I encoutered in my limited testing, so I didn't think underflow was possible. I will send out a V2 using the signed 32 bit delta and remeber to CC stable this time.I'm not thinking about underflow but rollover... or that isn't possible with those counters ? IE. They don't wrap back to 0 after hitting ffffffff ?They do roll over to 0 after ffffffff, but I thought that case was already covered by the perf_event_interrupt. Are you concerned that we will reset a counter and speculative roll back will underflow that counter?
No, but take this part of the patch:
quoted hunk ↗ jump to hunk
--- a/arch/powerpc/kernel/perf_event.c +++ b/arch/powerpc/kernel/perf_event.c@@ -416,6 +416,15 @@ static void power_pmu_read(struct perf_event *event) prev = local64_read(&event->hw.prev_count); barrier(); val = read_pmc(event->hw.idx); + /* + * POWER7 can roll back counter values, if the new value is + * smaller than the previous value it will cause the delta + * and the counter to have bogus values. If this is the + * case skip updating anything until the counter grows again. + * This can lead to a small lack of precision in the counters. + */ + if (val < prev) + return; } while (local64_cmpxchg(&event->hw.prev_count, prev, val) != prev);
Doesn't that mean that power_pmu_read() can only ever increase the value of the perf_event and so will essentially -stop- once the counter rolls over ? Similar comments every where you do this type of comparison. Cheers, Ben.