From: Peter Zijlstra <pet...@infradead.org>
As reported by Leo; the existing implementation is broken when the
clock and counter don't intersect at 0.
Use the sched_clock's struct clock_read_data information to correctly
implement cap_user_time and cap_user_time_zero.
Note that the ARM64 counter is architecturally only guaranteed to be
56bit wide (implementations are allowed to be wider) and the existing
perf ABI cannot deal with wrap-around.
This implementation should also be faster than the old; seeing how we
don't need to recompute mult and shift all the time.
[leoyan: Use quot/rem to convert cyc to ns to avoid overflow]
Reported-by: Leo Yan <leo....@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
arch/arm64/kernel/perf_event.c | 40 ++++++++++++++++++++++++++--------
1 file changed, 31 insertions(+), 9 deletions(-)
diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index 4d7879484cec..35c2c737d4af 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -19,6 +19,7 @@
#include <linux/of.h>
#include <linux/perf/arm_pmu.h>
#include <linux/platform_device.h>
+#include <linux/sched_clock.h>
#include <linux/smp.h>
/* ARMv8 Cortex-A53 specific event types. */
@@ -1165,28 +1166,49 @@ device_initcall(armv8_pmu_driver_init)
void arch_perf_update_userpage(struct perf_event *event,
struct perf_event_mmap_page *userpg, u64 now)
{
- u32 freq;
- u32 shift;
+ struct clock_read_data *rd;
+ unsigned int seq;
+ u64 quot, rem, ns;
/*
* Internal timekeeping for enabled/running/stopped times
* is always computed with the sched_clock.
*/
- freq = arch_timer_get_rate();
userpg->cap_user_time = 1;
+ userpg->cap_user_time_zero = 1;
+
+ do {
+ rd = sched_clock_read_begin(&seq);
+
+ userpg->time_mult = rd->mult;
+ userpg->time_shift = rd->shift;
+ userpg->time_zero = rd->epoch_ns;
+
+ /*
+ * This isn't strictly correct, the ARM64 counter can be
+ * 'short' and then we get funnies when it wraps. The correct
+ * thing would be to extend the perf ABI with a cycle and mask
+ * value, but because wrapping on ARM64 is very rare in
+ * practise this 'works'.
+ */
+ quot = rd->epoch_cyc >> rd->shift;
+ rem = rd->epoch_cyc & (((u64)1 << rd->shift) - 1);
+ ns = quot * rd->mult + ((rem * rd->mult) >> rd->shift);
+ userpg->time_zero -= ns;
+
+ } while (sched_clock_read_retry(seq));
+
+ userpg->time_offset = userpg->time_zero - now;
- clocks_calc_mult_shift(&userpg->time_mult, &shift, freq,
- NSEC_PER_SEC, 0);
/*
* time_shift is not expected to be greater than 31 due to
* the original published conversion algorithm shifting a
* 32-bit value (now specifies a 64-bit value) - refer
* perf_event_mmap_page documentation in perf_event.h.
*/
- if (shift == 32) {
- shift = 31;
+ if (userpg->time_shift == 32) {
+ userpg->time_shift = 31;
userpg->time_mult >>= 1;
}
- userpg->time_shift = (u16)shift;
- userpg->time_offset = -now;
+
}
--
2.17.1
