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Date:   Wed,  8 Jun 2022 22:23:53 -0700
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Subject: [PATCH v2 2/4] perf: Align user space counter reading with code
From:   Ian Rogers <irogers@google.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Kajol Jain <kjain@linux.ibm.com>,
        Andi Kleen <ak@linux.intel.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        Rob Herring <robh@kernel.org>
Cc:     Stephane Eranian <eranian@google.com>,
        Ian Rogers <irogers@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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