diff options
author | Eric B Munson <emunson@mgebm.net> | 2011-04-15 08:12:30 +0000 |
---|---|---|
committer | Benjamin Herrenschmidt <benh@kernel.crashing.org> | 2011-04-18 13:08:23 +1000 |
commit | 86c74ab317c1ef4d37325e0d7ca8a01a796b0bd7 (patch) | |
tree | 49afb7aae9c12b0cd0361b996a9b9e630a25ea63 /arch/powerpc/kernel | |
parent | 09597cfe93d3cc2c6e064a3ead5956b882511560 (diff) |
powerpc/perf_event: Skip updating kernel counters if register value shrinks
Because of speculative event roll back, it is possible for some event coutners
to decrease between reads on POWER7. This causes a problem with the way that
counters are updated. Delta calues are calculated in a 64 bit value and the
top 32 bits are masked. If the register value has decreased, this leaves us
with a very large positive value added to the kernel counters. This patch
protects against this by skipping the update if the delta would be negative.
This can lead to a lack of precision in the coutner values, but from my testing
the value is typcially fewer than 10 samples at a time.
Signed-off-by: Eric B Munson <emunson@mgebm.net>
Cc: stable@kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Diffstat (limited to 'arch/powerpc/kernel')
-rw-r--r-- | arch/powerpc/kernel/perf_event.c | 37 |
1 files changed, 30 insertions, 7 deletions
diff --git a/arch/powerpc/kernel/perf_event.c b/arch/powerpc/kernel/perf_event.c index c4063b7f49a..822f63008ae 100644 --- a/arch/powerpc/kernel/perf_event.c +++ b/arch/powerpc/kernel/perf_event.c @@ -398,6 +398,25 @@ static int check_excludes(struct perf_event **ctrs, unsigned int cflags[], return 0; } +static u64 check_and_compute_delta(u64 prev, u64 val) +{ + u64 delta = (val - prev) & 0xfffffffful; + + /* + * 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 unless we rolled a counter over. If a coutner is + * rolled back, it will be smaller, but within 256, which is the maximum + * number of events to rollback at once. If we dectect a rollback + * return 0. This can lead to a small lack of precision in the + * counters. + */ + if (prev > val && (prev - val) < 256) + delta = 0; + + return delta; +} + static void power_pmu_read(struct perf_event *event) { s64 val, delta, prev; @@ -416,10 +435,11 @@ static void power_pmu_read(struct perf_event *event) prev = local64_read(&event->hw.prev_count); barrier(); val = read_pmc(event->hw.idx); + delta = check_and_compute_delta(prev, val); + if (!delta) + return; } while (local64_cmpxchg(&event->hw.prev_count, prev, val) != prev); - /* The counters are only 32 bits wide */ - delta = (val - prev) & 0xfffffffful; local64_add(delta, &event->count); local64_sub(delta, &event->hw.period_left); } @@ -449,8 +469,9 @@ static void freeze_limited_counters(struct cpu_hw_events *cpuhw, val = (event->hw.idx == 5) ? pmc5 : pmc6; prev = local64_read(&event->hw.prev_count); event->hw.idx = 0; - delta = (val - prev) & 0xfffffffful; - local64_add(delta, &event->count); + delta = check_and_compute_delta(prev, val); + if (delta) + local64_add(delta, &event->count); } } @@ -458,14 +479,16 @@ static void thaw_limited_counters(struct cpu_hw_events *cpuhw, unsigned long pmc5, unsigned long pmc6) { struct perf_event *event; - u64 val; + u64 val, prev; int i; for (i = 0; i < cpuhw->n_limited; ++i) { event = cpuhw->limited_counter[i]; event->hw.idx = cpuhw->limited_hwidx[i]; val = (event->hw.idx == 5) ? pmc5 : pmc6; - local64_set(&event->hw.prev_count, val); + prev = local64_read(&event->hw.prev_count); + if (check_and_compute_delta(prev, val)) + local64_set(&event->hw.prev_count, val); perf_event_update_userpage(event); } } @@ -1197,7 +1220,7 @@ static void record_and_restart(struct perf_event *event, unsigned long val, /* we don't have to worry about interrupts here */ prev = local64_read(&event->hw.prev_count); - delta = (val - prev) & 0xfffffffful; + delta = check_and_compute_delta(prev, val); local64_add(delta, &event->count); /* |