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Subject[PATCH 2/4] perf: Align user space counter reading with code
From
Align the user space counter reading documentation with the code in
perf_mmap__read_self. Previously the documentation was based on the perf
rdpmc test, but now general purpose code is provided by libperf.

Signed-off-by: Ian Rogers <irogers@google.com>
---
include/uapi/linux/perf_event.h | 32 ++++++++++++++++-----------
tools/include/uapi/linux/perf_event.h | 32 ++++++++++++++++-----------
2 files changed, 38 insertions(+), 26 deletions(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index d37629dbad72..3b84e0ad0723 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -538,9 +538,13 @@ struct perf_event_mmap_page {
*
* if (pc->cap_usr_time && enabled != running) {
* cyc = rdtsc();
- * time_offset = pc->time_offset;
* time_mult = pc->time_mult;
* time_shift = pc->time_shift;
+ * time_offset = pc->time_offset;
+ * if (pc->cap_user_time_short) {
+ * time_cycles = pc->time_cycles;
+ * time_mask = pc->time_mask;
+ * }
* }
*
* index = pc->index;
@@ -548,6 +552,9 @@ struct perf_event_mmap_page {
* if (pc->cap_user_rdpmc && index) {
* width = pc->pmc_width;
* pmc = rdpmc(index - 1);
+ * pmc <<= 64 - width;
+ * pmc >>= 64 - width;
+ * count += pmc;
* }
*
* barrier();
@@ -590,25 +597,24 @@ struct perf_event_mmap_page {
* If cap_usr_time the below fields can be used to compute the time
* delta since time_enabled (in ns) using rdtsc or similar.
*
- * u64 quot, rem;
- * u64 delta;
- *
- * quot = (cyc >> time_shift);
- * rem = cyc & (((u64)1 << time_shift) - 1);
- * delta = time_offset + quot * time_mult +
- * ((rem * time_mult) >> time_shift);
+ * cyc = time_cycles + ((cyc - time_cycles) & time_mask);
+ * delta = time_offset + mul_u64_u32_shr(cyc, time_mult, time_shift);
*
* Where time_offset,time_mult,time_shift and cyc are read in the
* seqcount loop described above. This delta can then be added to
- * enabled and possible running (if index), improving the scaling:
+ * enabled and possible running (if index) to improve the scaling. Due
+ * to event multiplexing, running maybe zero and so care is needed to
+ * avoid division by zero.
*
* enabled += delta;
- * if (index)
+ * if (idx)
* running += delta;
*
- * quot = count / running;
- * rem = count % running;
- * count = quot * enabled + (rem * enabled) / running;
+ * if (running != 0) {
+ * quot = count / running;
+ * rem = count % running;
+ * count = quot * enabled + (rem * enabled) / running;
+ * }
*/
__u16 time_shift;
__u32 time_mult;
diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
index d37629dbad72..3b84e0ad0723 100644
--- a/tools/include/uapi/linux/perf_event.h
+++ b/tools/include/uapi/linux/perf_event.h
@@ -538,9 +538,13 @@ struct perf_event_mmap_page {
*
* if (pc->cap_usr_time && enabled != running) {
* cyc = rdtsc();
- * time_offset = pc->time_offset;
* time_mult = pc->time_mult;
* time_shift = pc->time_shift;
+ * time_offset = pc->time_offset;
+ * if (pc->cap_user_time_short) {
+ * time_cycles = pc->time_cycles;
+ * time_mask = pc->time_mask;
+ * }
* }
*
* index = pc->index;
@@ -548,6 +552,9 @@ struct perf_event_mmap_page {
* if (pc->cap_user_rdpmc && index) {
* width = pc->pmc_width;
* pmc = rdpmc(index - 1);
+ * pmc <<= 64 - width;
+ * pmc >>= 64 - width;
+ * count += pmc;
* }
*
* barrier();
@@ -590,25 +597,24 @@ struct perf_event_mmap_page {
* If cap_usr_time the below fields can be used to compute the time
* delta since time_enabled (in ns) using rdtsc or similar.
*
- * u64 quot, rem;
- * u64 delta;
- *
- * quot = (cyc >> time_shift);
- * rem = cyc & (((u64)1 << time_shift) - 1);
- * delta = time_offset + quot * time_mult +
- * ((rem * time_mult) >> time_shift);
+ * cyc = time_cycles + ((cyc - time_cycles) & time_mask);
+ * delta = time_offset + mul_u64_u32_shr(cyc, time_mult, time_shift);
*
* Where time_offset,time_mult,time_shift and cyc are read in the
* seqcount loop described above. This delta can then be added to
- * enabled and possible running (if index), improving the scaling:
+ * enabled and possible running (if index) to improve the scaling. Due
+ * to event multiplexing, running maybe zero and so care is needed to
+ * avoid division by zero.
*
* enabled += delta;
- * if (index)
+ * if (idx)
* running += delta;
*
- * quot = count / running;
- * rem = count % running;
- * count = quot * enabled + (rem * enabled) / running;
+ * if (running != 0) {
+ * quot = count / running;
+ * rem = count % running;
+ * count = quot * enabled + (rem * enabled) / running;
+ * }
*/
__u16 time_shift;
__u32 time_mult;
--
2.36.1.255.ge46751e96f-goog
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