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>
Subject: [PATCH v4] perf: Align user space counter reading with code
Date: Wed, 17 Aug 2022 10:49:09 -0700 [thread overview]
Message-ID: <20220817174909.877139-1-irogers@google.com> (raw)
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>
Reviewed-by: Rob Herring <robh@kernel.org>
v4. Is a rebase.
---
include/uapi/linux/perf_event.h | 35 +++++++++++++++++----------
tools/include/uapi/linux/perf_event.h | 35 +++++++++++++++++----------
2 files changed, 44 insertions(+), 26 deletions(-)
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 03b370062741..f35c10aa71b9 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -541,9 +541,13 @@ 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;
@@ -551,6 +555,9 @@ 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();
@@ -593,25 +600,27 @@ 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:
+ * seqcount loop described above. mul_u64_u32_shr will compute:
+ *
+ * (u64)(((unsigned __int128)cyc * time_mult) >> time_shift)
+ *
+ * This delta can then be added to enabled and possible running (if
+ * index) to improve the scaling. Due to event multiplexing, running
+ * may be zero and so care is needed to avoid division by zero.
*
* enabled += delta;
* if (index)
* 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 581ed4bdc062..9c93e521a6b0 100644
--- a/tools/include/uapi/linux/perf_event.h
+++ b/tools/include/uapi/linux/perf_event.h
@@ -541,9 +541,13 @@ 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;
@@ -551,6 +555,9 @@ 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();
@@ -593,25 +600,27 @@ 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:
+ * seqcount loop described above. mul_u64_u32_shr will compute:
+ *
+ * (u64)(((unsigned __int128)cyc * time_mult) >> time_shift)
+ *
+ * This delta can then be added to enabled and possible running (if
+ * index) to improve the scaling. Due to event multiplexing, running
+ * may be zero and so care is needed to avoid division by zero.
*
* enabled += delta;
* if (index)
* 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;
--
2.37.1.595.g718a3a8f04-goog
reply other threads:[~2022-08-17 17:49 UTC|newest]
Thread overview: [no followups] expand[flat|nested] mbox.gz Atom feed
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20220817174909.877139-1-irogers@google.com \
--to=irogers@google.com \
--cc=acme@kernel.org \
--cc=adrian.hunter@intel.com \
--cc=ak@linux.intel.com \
--cc=alexander.shishkin@linux.intel.com \
--cc=anshuman.khandual@arm.com \
--cc=eranian@google.com \
--cc=jolsa@kernel.org \
--cc=kjain@linux.ibm.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-perf-users@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mingo@redhat.com \
--cc=namhyung@kernel.org \
--cc=peterz@infradead.org \
--cc=robh@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox
all inboxes | Powered by JetHome®