[PATCH v2 0/4] perf tools: Backward ring buffer support

[PATCH v2 0/4] perf tools: Backward ring buffer support

[Wang Nan]

Commit 9ecda41acb97 ("perf/core: Add ::write_backward attribute to
perf event") introduces backward ring buffer. This 4 patches add basic
support for reading from it, and add a new test case for it.

v1 -> v2:
  Patch 1/5 in v1 has been collected by perf/core, so remove it;
  Change function names in patch 2/5 in v1 (1/4 in v2):
      __perf_evlist__mmap_read -> perf_mmap__read

Wang Nan (4):
  perf tools: Extract perf_mmap__read()
  perf tools: Rename variable in perf_mmap__read()
  perf tools: Support reading from backward ring buffer
  perf tests: Add test to check backward ring buffer

 tools/perf/tests/Build                  |   1 +
 tools/perf/tests/backward-ring-buffer.c | 151 ++++++++++++++++++++++++++++++++
 tools/perf/tests/builtin-test.c         |   4 +
 tools/perf/tests/tests.h                |   1 +
 tools/perf/util/evlist.c                |  95 +++++++++++++++-----
 tools/perf/util/evlist.h                |   4 +
 6 files changed, 233 insertions(+), 23 deletions(-)
 create mode 100644 tools/perf/tests/backward-ring-buffer.c

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
-- 
1.8.3.4

[PATCH v2 1/4] perf tools: Extract perf_mmap__read()

[Wang Nan]

Extract event reader from perf_evlist__mmap_read() to perf__mmap_read().
Future commit will feed it with manually computed 'head' and 'old'
pointers.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
---
 tools/perf/util/evlist.c | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 85271e5..96c7191 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -679,24 +679,15 @@ static struct perf_evsel *perf_evlist__event2evsel(struct perf_evlist *evlist,
 	return NULL;
 }
 
-union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
+static union perf_event *
+perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
+		u64 old, u64 *prev)
 {
-	struct perf_mmap *md = &evlist->mmap[idx];
-	u64 head;
-	u64 old = md->prev;
-	int diff;
 	unsigned char *data = md->base + page_size;
 	union perf_event *event = NULL;
+	int diff = head - old;
 
-	/*
-	 * Check if event was unmapped due to a POLLHUP/POLLERR.
-	 */
-	if (!atomic_read(&md->refcnt))
-		return NULL;
-
-	head = perf_mmap__read_head(md);
-	diff = head - old;
-	if (evlist->overwrite) {
+	if (overwrite) {
 		/*
 		 * If we're further behind than half the buffer, there's a chance
 		 * the writer will bite our tail and mess up the samples under us.
@@ -751,11 +742,29 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
 	}
 
 broken_event:
-	md->prev = old;
+	if (prev)
+		*prev = old;
 
 	return event;
 }
 
+union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
+{
+	struct perf_mmap *md = &evlist->mmap[idx];
+	u64 head;
+	u64 old = md->prev;
+
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!atomic_read(&md->refcnt))
+		return NULL;
+
+	head = perf_mmap__read_head(md);
+
+	return perf_mmap__read(md, evlist->overwrite, head, old, &md->prev);
+}
+
 static bool perf_mmap__empty(struct perf_mmap *md)
 {
 	return perf_mmap__read_head(md) == md->prev && !md->auxtrace_mmap.base;
-- 
1.8.3.4

[PATCH v2 2/4] perf tools: Rename variable in perf_mmap__read()

[Wang Nan]

In perf_mmap__read(), give better names to pointers. Origianl
name 'old' and 'head' directly related to pointers in ring buffer
control page. For backward ring buffer, the meaning of 'head' point
is not 'the first byte of free space', but 'the first byte of the last
record'. To reduce confusion, rename 'old' to 'start', 'head' to 'end'.
'start' -> 'end' is the records should be read from.

Change parameter order.

Change 'overwrite' to 'check_messup'. When reading from 'head', no
need to check messup for for backward ring buffer.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
---
 tools/perf/util/evlist.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 96c7191..17cd014 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -679,30 +679,31 @@ static struct perf_evsel *perf_evlist__event2evsel(struct perf_evlist *evlist,
 	return NULL;
 }
 
+/* When check_messup is true, 'end' must points to a good entry */
 static union perf_event *
-perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
-		u64 old, u64 *prev)
+perf_mmap__read(struct perf_mmap *md, bool check_messup, u64 start,
+		u64 end, u64 *prev)
 {
 	unsigned char *data = md->base + page_size;
 	union perf_event *event = NULL;
-	int diff = head - old;
+	int diff = end - start;
 
-	if (overwrite) {
+	if (check_messup) {
 		/*
 		 * If we're further behind than half the buffer, there's a chance
 		 * the writer will bite our tail and mess up the samples under us.
 		 *
-		 * If we somehow ended up ahead of the head, we got messed up.
+		 * If we somehow ended up ahead of the 'end', we got messed up.
 		 *
-		 * In either case, truncate and restart at head.
+		 * In either case, truncate and restart at 'end'.
 		 */
 		if (diff > md->mask / 2 || diff < 0) {
 			fprintf(stderr, "WARNING: failed to keep up with mmap data.\n");
 
 			/*
-			 * head points to a known good entry, start there.
+			 * 'end' points to a known good entry, start there.
 			 */
-			old = head;
+			start = end;
 			diff = 0;
 		}
 	}
@@ -710,7 +711,7 @@ perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
 	if (diff >= (int)sizeof(event->header)) {
 		size_t size;
 
-		event = (union perf_event *)&data[old & md->mask];
+		event = (union perf_event *)&data[start & md->mask];
 		size = event->header.size;
 
 		if (size < sizeof(event->header) || diff < (int)size) {
@@ -722,8 +723,8 @@ perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
 		 * Event straddles the mmap boundary -- header should always
 		 * be inside due to u64 alignment of output.
 		 */
-		if ((old & md->mask) + size != ((old + size) & md->mask)) {
-			unsigned int offset = old;
+		if ((start & md->mask) + size != ((start + size) & md->mask)) {
+			unsigned int offset = start;
 			unsigned int len = min(sizeof(*event), size), cpy;
 			void *dst = md->event_copy;
 
@@ -738,12 +739,12 @@ perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
 			event = (union perf_event *) md->event_copy;
 		}
 
-		old += size;
+		start += size;
 	}
 
 broken_event:
 	if (prev)
-		*prev = old;
+		*prev = start;
 
 	return event;
 }
@@ -762,7 +763,7 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
 
 	head = perf_mmap__read_head(md);
 
-	return perf_mmap__read(md, evlist->overwrite, head, old, &md->prev);
+	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);
 }
 
 static bool perf_mmap__empty(struct perf_mmap *md)
-- 
1.8.3.4

[PATCH v2 3/4] perf tools: Support reading from backward ring buffer

[Wang Nan]

perf_evlist__mmap_read_backward() is introduced for reading backward
ring buffer. Different from reading forward, before reading, caller
needs to call perf_evlist__mmap_read_catchup() first.

Backward ring buffer should be read from 'head' pointer, not '0'.
perf_evlist__mmap_read_catchup() saves 'head' to 'md->prev', then
make it remember the start position after each reading.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
---
 tools/perf/util/evlist.c | 39 +++++++++++++++++++++++++++++++++++++++
 tools/perf/util/evlist.h |  4 ++++
 2 files changed, 43 insertions(+)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 17cd014..2e0b7b0 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -766,6 +766,45 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
 	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);
 }
 
+union perf_event *
+perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
+{
+	struct perf_mmap *md = &evlist->mmap[idx];
+	u64 head, end;
+	u64 start = md->prev;
+
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!atomic_read(&md->refcnt))
+		return NULL;
+
+	/* NOTE: head is negative in this case */
+	head = perf_mmap__read_head(md);
+
+	if (!head)
+		return NULL;
+
+	end = head + md->mask + 1;
+
+	if ((end - head) > -head)
+		end = 0;
+
+	return perf_mmap__read(md, false, start, end, &md->prev);
+}
+
+void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx)
+{
+	struct perf_mmap *md = &evlist->mmap[idx];
+	u64 head;
+
+	if (!atomic_read(&md->refcnt))
+		return;
+
+	head = perf_mmap__read_head(md);
+	md->prev = head;
+}
+
 static bool perf_mmap__empty(struct perf_mmap *md)
 {
 	return perf_mmap__read_head(md) == md->prev && !md->auxtrace_mmap.base;
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index 208897a..85d1b59 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -129,6 +129,10 @@ struct perf_sample_id *perf_evlist__id2sid(struct perf_evlist *evlist, u64 id);
 
 union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx);
 
+union perf_event *perf_evlist__mmap_read_backward(struct perf_evlist *evlist,
+						  int idx);
+void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx);
+
 void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx);
 
 int perf_evlist__open(struct perf_evlist *evlist);
-- 
1.8.3.4

[PATCH v2 4/4] perf tests: Add test to check backward ring buffer

[Wang Nan]

This test checks reading from backward ring buffer.

Test result:

 # ~/perf test 'ring buffer'
 45: Test backward reading from ring buffer                   : Ok

Test case is a while loop which calls prctl(PR_SET_NAME) multiple
times. Each prctl should issue 2 events: one PERF_RECORD_SAMPLE,
one PERF_RECORD_COMM.

The first round creates a relative large ring buffer (256 pages). It
can afford all events. Read from it and check the count of each type of
events.

The second round creates a small ring buffer (1 page) and makes it
overwritable. Check the correctness of the buffer.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
---
 tools/perf/tests/Build                  |   1 +
 tools/perf/tests/backward-ring-buffer.c | 151 ++++++++++++++++++++++++++++++++
 tools/perf/tests/builtin-test.c         |   4 +
 tools/perf/tests/tests.h                |   1 +
 4 files changed, 157 insertions(+)
 create mode 100644 tools/perf/tests/backward-ring-buffer.c

diff --git a/tools/perf/tests/Build b/tools/perf/tests/Build
index 449fe97..66a2898 100644
--- a/tools/perf/tests/Build
+++ b/tools/perf/tests/Build
@@ -38,6 +38,7 @@ perf-y += cpumap.o
 perf-y += stat.o
 perf-y += event_update.o
 perf-y += event-times.o
+perf-y += backward-ring-buffer.o
 
 $(OUTPUT)tests/llvm-src-base.c: tests/bpf-script-example.c tests/Build
 	$(call rule_mkdir)
diff --git a/tools/perf/tests/backward-ring-buffer.c b/tools/perf/tests/backward-ring-buffer.c
new file mode 100644
index 0000000..d9ba991
--- /dev/null
+++ b/tools/perf/tests/backward-ring-buffer.c
@@ -0,0 +1,151 @@
+/*
+ * Test backward bit in event attribute, read ring buffer from end to
+ * beginning
+ */
+
+#include <perf.h>
+#include <evlist.h>
+#include <sys/prctl.h>
+#include "tests.h"
+#include "debug.h"
+
+#define NR_ITERS 111
+
+static void testcase(void)
+{
+	int i;
+
+	for (i = 0; i < NR_ITERS; i++) {
+		char proc_name[10];
+
+		snprintf(proc_name, sizeof(proc_name), "p:%d\n", i);
+		prctl(PR_SET_NAME, proc_name);
+	}
+}
+
+static int count_samples(struct perf_evlist *evlist, int *sample_count,
+			 int *comm_count)
+{
+	int i;
+
+	for (i = 0; i < evlist->nr_mmaps; i++) {
+		union perf_event *event;
+
+		perf_evlist__mmap_read_catchup(evlist, i);
+		while ((event = perf_evlist__mmap_read_backward(evlist, i)) != NULL) {
+			const u32 type = event->header.type;
+
+			switch (type) {
+			case PERF_RECORD_SAMPLE:
+				(*sample_count)++;
+				break;
+			case PERF_RECORD_COMM:
+				(*comm_count)++;
+				break;
+			default:
+				pr_err("Unexpected record of type %d\n", type);
+				return TEST_FAIL;
+			}
+		}
+	}
+	return TEST_OK;
+}
+
+static int do_test(struct perf_evlist *evlist, int mmap_pages,
+		   int *sample_count, int *comm_count)
+{
+	int err;
+	char sbuf[STRERR_BUFSIZE];
+
+	err = perf_evlist__mmap(evlist, mmap_pages, true);
+	if (err < 0) {
+		pr_debug("perf_evlist__mmap: %s\n",
+			 strerror_r(errno, sbuf, sizeof(sbuf)));
+		return TEST_FAIL;
+	}
+
+	perf_evlist__enable(evlist);
+	testcase();
+	perf_evlist__disable(evlist);
+
+	err = count_samples(evlist, sample_count, comm_count);
+	perf_evlist__munmap(evlist);
+	return err;
+}
+
+
+int test__backward_ring_buffer(int subtest __maybe_unused)
+{
+	int ret = TEST_SKIP, err, sample_count = 0, comm_count = 0;
+	char pid[16], sbuf[STRERR_BUFSIZE];
+	struct perf_evlist *evlist;
+	struct perf_evsel *evsel __maybe_unused;
+	struct parse_events_error parse_error;
+	struct record_opts opts = {
+		.target = {
+			.uid = UINT_MAX,
+			.uses_mmap = true,
+		},
+		.freq	      = 0,
+		.mmap_pages   = 256,
+		.default_interval = 1,
+	};
+
+	snprintf(pid, sizeof(pid), "%d", getpid());
+	pid[sizeof(pid) - 1] = '\0';
+	opts.target.tid = opts.target.pid = pid;
+
+	evlist = perf_evlist__new();
+	if (!evlist) {
+		pr_debug("No ehough memory to create evlist\n");
+		return TEST_FAIL;
+	}
+
+	err = perf_evlist__create_maps(evlist, &opts.target);
+	if (err < 0) {
+		pr_debug("Not enough memory to create thread/cpu maps\n");
+		goto out_delete_evlist;
+	}
+
+	bzero(&parse_error, sizeof(parse_error));
+	err = parse_events(evlist, "syscalls:sys_enter_prctl", &parse_error);
+	if (err) {
+		pr_debug("Failed to parse tracepoint event, try use root\n");
+		ret = TEST_SKIP;
+		goto out_delete_evlist;
+	}
+
+	perf_evlist__config(evlist, &opts, NULL);
+
+	/* Set backward bit, ring buffer should be writing from end */
+	evlist__for_each(evlist, evsel)
+		evsel->attr.write_backward = 1;
+
+	err = perf_evlist__open(evlist);
+	if (err < 0) {
+		pr_debug("perf_evlist__open: %s\n",
+			 strerror_r(errno, sbuf, sizeof(sbuf)));
+		goto out_delete_evlist;
+	}
+
+	ret = TEST_FAIL;
+	err = do_test(evlist, opts.mmap_pages, &sample_count,
+		      &comm_count);
+	if (err != TEST_OK)
+		goto out_delete_evlist;
+
+	if ((sample_count != NR_ITERS) || (comm_count != NR_ITERS)) {
+		pr_err("Unexpected counter: sample_count=%d, comm_count=%d\n",
+		       sample_count, comm_count);
+		goto out_delete_evlist;
+	}
+
+	err = do_test(evlist, 1, &sample_count, &comm_count);
+	if (err != TEST_OK)
+		goto out_delete_evlist;
+
+	ret = TEST_OK;
+out_delete_evlist:
+	perf_evlist__delete(evlist);
+	return ret;
+}
diff --git a/tools/perf/tests/builtin-test.c b/tools/perf/tests/builtin-test.c
index 93c4670..0e95c20 100644
--- a/tools/perf/tests/builtin-test.c
+++ b/tools/perf/tests/builtin-test.c
@@ -208,6 +208,10 @@ static struct test generic_tests[] = {
 		.func = test__event_times,
 	},
 	{
+		.desc = "Test backward reading from ring buffer",
+		.func = test__backward_ring_buffer,
+	},
+	{
 		.func = NULL,
 	},
 };
diff --git a/tools/perf/tests/tests.h b/tools/perf/tests/tests.h
index 0fc9469..c57e72c 100644
--- a/tools/perf/tests/tests.h
+++ b/tools/perf/tests/tests.h
@@ -86,6 +86,7 @@ int test__synthesize_stat(int subtest);
 int test__synthesize_stat_round(int subtest);
 int test__event_update(int subtest);
 int test__event_times(int subtest);
+int test__backward_ring_buffer(int subtest);
 
 #if defined(__arm__) || defined(__aarch64__)
 #ifdef HAVE_DWARF_UNWIND_SUPPORT
-- 
1.8.3.4

Re: [PATCH v2 3/4] perf tools: Support reading from backward ring buffer

[Arnaldo Carvalho de Melo]

Em Wed, Apr 27, 2016 at 02:19:22AM +0000, Wang Nan escreveu:
> perf_evlist__mmap_read_backward() is introduced for reading backward
> ring buffer. Different from reading forward, before reading, caller
> needs to call perf_evlist__mmap_read_catchup() first.
> 
> Backward ring buffer should be read from 'head' pointer, not '0'.
> perf_evlist__mmap_read_catchup() saves 'head' to 'md->prev', then
> make it remember the start position after each reading.
> 
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Zefan Li <lizefan@huawei.com>
> Cc: pi3orama@163.com
> ---
>  tools/perf/util/evlist.c | 39 +++++++++++++++++++++++++++++++++++++++
>  tools/perf/util/evlist.h |  4 ++++
>  2 files changed, 43 insertions(+)
> 
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 17cd014..2e0b7b0 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -766,6 +766,45 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
>  	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);
>  }
>  
> +union perf_event *
> +perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
> +{
> +	struct perf_mmap *md = &evlist->mmap[idx];
> +	u64 head, end;
> +	u64 start = md->prev;
> +
> +	/*
> +	 * Check if event was unmapped due to a POLLHUP/POLLERR.
> +	 */
> +	if (!atomic_read(&md->refcnt))
> +		return NULL;
> +
> +	/* NOTE: head is negative in this case */

What is this comment for, can you ellaborate? Are you double sure this
arithmetic with u64, negative values, that -head below are all ok?

I've applied the first two patches in this series.

I also need to check why we now need that catchup thing :-\

> +	head = perf_mmap__read_head(md);
> +
> +	if (!head)
> +		return NULL;
> +
> +	end = head + md->mask + 1;
> +
> +	if ((end - head) > -head)
> +		end = 0;
> +
> +	return perf_mmap__read(md, false, start, end, &md->prev);
> +}
> +
> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx)
> +{
> +	struct perf_mmap *md = &evlist->mmap[idx];
> +	u64 head;
> +
> +	if (!atomic_read(&md->refcnt))
> +		return;
> +
> +	head = perf_mmap__read_head(md);
> +	md->prev = head;
> +}
> +
>  static bool perf_mmap__empty(struct perf_mmap *md)
>  {
>  	return perf_mmap__read_head(md) == md->prev && !md->auxtrace_mmap.base;
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index 208897a..85d1b59 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -129,6 +129,10 @@ struct perf_sample_id *perf_evlist__id2sid(struct perf_evlist *evlist, u64 id);
>  
>  union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx);
>  
> +union perf_event *perf_evlist__mmap_read_backward(struct perf_evlist *evlist,
> +						  int idx);
> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx);
> +
>  void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx);
>  
>  int perf_evlist__open(struct perf_evlist *evlist);
> -- 
> 1.8.3.4

[tip:perf/core] perf evlist: Extract perf_mmap__read()

[tip-bot for Wang Nan]

Commit-ID:  0f4ccd11813f59d766039dfdd13aa98245a67294
Gitweb:     http://git.kernel.org/tip/0f4ccd11813f59d766039dfdd13aa98245a67294
Author:     Wang Nan <wangnan0@huawei.com>
AuthorDate: Wed, 27 Apr 2016 02:19:20 +0000
Committer:  Arnaldo Carvalho de Melo <acme@redhat.com>
CommitDate: Thu, 5 May 2016 21:04:03 -0300

perf evlist: Extract perf_mmap__read()

Extract event reader from perf_evlist__mmap_read() to perf__mmap_read().
Future commit will feed it with manually computed 'head' and 'old'
pointers.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1461723563-67451-2-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/util/evlist.c | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 85271e5..96c7191 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -679,24 +679,15 @@ static struct perf_evsel *perf_evlist__event2evsel(struct perf_evlist *evlist,
 	return NULL;
 }
 
-union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
+static union perf_event *
+perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
+		u64 old, u64 *prev)
 {
-	struct perf_mmap *md = &evlist->mmap[idx];
-	u64 head;
-	u64 old = md->prev;
-	int diff;
 	unsigned char *data = md->base + page_size;
 	union perf_event *event = NULL;
+	int diff = head - old;
 
-	/*
-	 * Check if event was unmapped due to a POLLHUP/POLLERR.
-	 */
-	if (!atomic_read(&md->refcnt))
-		return NULL;
-
-	head = perf_mmap__read_head(md);
-	diff = head - old;
-	if (evlist->overwrite) {
+	if (overwrite) {
 		/*
 		 * If we're further behind than half the buffer, there's a chance
 		 * the writer will bite our tail and mess up the samples under us.
@@ -751,11 +742,29 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
 	}
 
 broken_event:
-	md->prev = old;
+	if (prev)
+		*prev = old;
 
 	return event;
 }
 
+union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
+{
+	struct perf_mmap *md = &evlist->mmap[idx];
+	u64 head;
+	u64 old = md->prev;
+
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!atomic_read(&md->refcnt))
+		return NULL;
+
+	head = perf_mmap__read_head(md);
+
+	return perf_mmap__read(md, evlist->overwrite, head, old, &md->prev);
+}
+
 static bool perf_mmap__empty(struct perf_mmap *md)
 {
 	return perf_mmap__read_head(md) == md->prev && !md->auxtrace_mmap.base;

[tip:perf/core] perf evlist: Rename variable in perf_mmap__read()

[tip-bot for Wang Nan]

Commit-ID:  b6b85dad30ad7e7394990e2317a780577974a4e6
Gitweb:     http://git.kernel.org/tip/b6b85dad30ad7e7394990e2317a780577974a4e6
Author:     Wang Nan <wangnan0@huawei.com>
AuthorDate: Wed, 27 Apr 2016 02:19:21 +0000
Committer:  Arnaldo Carvalho de Melo <acme@redhat.com>
CommitDate: Thu, 5 May 2016 21:04:04 -0300

perf evlist: Rename variable in perf_mmap__read()

In perf_mmap__read(), give better names to pointers. Original name 'old'
and 'head' directly related to pointers in ring buffer control page. For
backward ring buffer, the meaning of 'head' point is not 'the first byte
of free space', but 'the first byte of the last record'. To reduce
confusion, rename 'old' to 'start', 'head' to 'end'.  'start' -> 'end'
is the direction the records should be read from.

Change parameter order.

Change 'overwrite' to 'check_messup'. When reading from 'head', no need
to check messup for for backward ring buffer.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1461723563-67451-3-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/util/evlist.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 96c7191..17cd014 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -679,30 +679,31 @@ static struct perf_evsel *perf_evlist__event2evsel(struct perf_evlist *evlist,
 	return NULL;
 }
 
+/* When check_messup is true, 'end' must points to a good entry */
 static union perf_event *
-perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
-		u64 old, u64 *prev)
+perf_mmap__read(struct perf_mmap *md, bool check_messup, u64 start,
+		u64 end, u64 *prev)
 {
 	unsigned char *data = md->base + page_size;
 	union perf_event *event = NULL;
-	int diff = head - old;
+	int diff = end - start;
 
-	if (overwrite) {
+	if (check_messup) {
 		/*
 		 * If we're further behind than half the buffer, there's a chance
 		 * the writer will bite our tail and mess up the samples under us.
 		 *
-		 * If we somehow ended up ahead of the head, we got messed up.
+		 * If we somehow ended up ahead of the 'end', we got messed up.
 		 *
-		 * In either case, truncate and restart at head.
+		 * In either case, truncate and restart at 'end'.
 		 */
 		if (diff > md->mask / 2 || diff < 0) {
 			fprintf(stderr, "WARNING: failed to keep up with mmap data.\n");
 
 			/*
-			 * head points to a known good entry, start there.
+			 * 'end' points to a known good entry, start there.
 			 */
-			old = head;
+			start = end;
 			diff = 0;
 		}
 	}
@@ -710,7 +711,7 @@ perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
 	if (diff >= (int)sizeof(event->header)) {
 		size_t size;
 
-		event = (union perf_event *)&data[old & md->mask];
+		event = (union perf_event *)&data[start & md->mask];
 		size = event->header.size;
 
 		if (size < sizeof(event->header) || diff < (int)size) {
@@ -722,8 +723,8 @@ perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
 		 * Event straddles the mmap boundary -- header should always
 		 * be inside due to u64 alignment of output.
 		 */
-		if ((old & md->mask) + size != ((old + size) & md->mask)) {
-			unsigned int offset = old;
+		if ((start & md->mask) + size != ((start + size) & md->mask)) {
+			unsigned int offset = start;
 			unsigned int len = min(sizeof(*event), size), cpy;
 			void *dst = md->event_copy;
 
@@ -738,12 +739,12 @@ perf_mmap__read(struct perf_mmap *md, bool overwrite, u64 head,
 			event = (union perf_event *) md->event_copy;
 		}
 
-		old += size;
+		start += size;
 	}
 
 broken_event:
 	if (prev)
-		*prev = old;
+		*prev = start;
 
 	return event;
 }
@@ -762,7 +763,7 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
 
 	head = perf_mmap__read_head(md);
 
-	return perf_mmap__read(md, evlist->overwrite, head, old, &md->prev);
+	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);
 }
 
 static bool perf_mmap__empty(struct perf_mmap *md)

Re: [PATCH v2 3/4] perf tools: Support reading from backward ring buffer

[Wangnan (F)]

On 2016/5/6 4:07, Arnaldo Carvalho de Melo wrote:
> Em Wed, Apr 27, 2016 at 02:19:22AM +0000, Wang Nan escreveu:
>> perf_evlist__mmap_read_backward() is introduced for reading backward
>> ring buffer. Different from reading forward, before reading, caller
>> needs to call perf_evlist__mmap_read_catchup() first.
>>
>> Backward ring buffer should be read from 'head' pointer, not '0'.
>> perf_evlist__mmap_read_catchup() saves 'head' to 'md->prev', then
>> make it remember the start position after each reading.
>>
>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Zefan Li <lizefan@huawei.com>
>> Cc: pi3orama@163.com
>> ---
>>   tools/perf/util/evlist.c | 39 +++++++++++++++++++++++++++++++++++++++
>>   tools/perf/util/evlist.h |  4 ++++
>>   2 files changed, 43 insertions(+)
>>
>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
>> index 17cd014..2e0b7b0 100644
>> --- a/tools/perf/util/evlist.c
>> +++ b/tools/perf/util/evlist.c
>> @@ -766,6 +766,45 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
>>   	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);
>>   }
>>   
>> +union perf_event *
>> +perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
>> +{
>> +	struct perf_mmap *md = &evlist->mmap[idx];
>> +	u64 head, end;
>> +	u64 start = md->prev;
>> +
>> +	/*
>> +	 * Check if event was unmapped due to a POLLHUP/POLLERR.
>> +	 */
>> +	if (!atomic_read(&md->refcnt))
>> +		return NULL;
>> +
>> +	/* NOTE: head is negative in this case */
> What is this comment for, can you ellaborate? Are you double sure this
> arithmetic with u64, negative values, that -head below are all ok?

Yes. In backward ring buffer, kernel write data from '0'. Each
time it write a record, it minus sizeof(record) from 'head'
pointer. So '-head' means number of bytes already written.

> I've applied the first two patches in this series.
>
> I also need to check why we now need that catchup thing :-\

I think catchup is necessary. I tried to get rid of it by setting
md->prev to -1 and initialize it to 0 when reading forwardly and
to head when reading backwardly, but it require too much code
adjustments.

>> +	head = perf_mmap__read_head(md);
>> +
>> +	if (!head)
>> +		return NULL;
>> +
>> +	end = head + md->mask + 1;
>> +
>> +	if ((end - head) > -head)
>> +		end = 0;
>> +

This '-head' is used to detect wrapping. Never use positive pointer.

>> +	return perf_mmap__read(md, false, start, end, &md->prev);
>> +}
>> +
>> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx)
>> +{
>> +	struct perf_mmap *md = &evlist->mmap[idx];
>> +	u64 head;
>> +
>> +	if (!atomic_read(&md->refcnt))
>> +		return;
>> +
>> +	head = perf_mmap__read_head(md);
>> +	md->prev = head;
>> +}
>> +
>>   static bool perf_mmap__empty(struct perf_mmap *md)
>>   {
>>   	return perf_mmap__read_head(md) == md->prev && !md->auxtrace_mmap.base;
>> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
>> index 208897a..85d1b59 100644
>> --- a/tools/perf/util/evlist.h
>> +++ b/tools/perf/util/evlist.h
>> @@ -129,6 +129,10 @@ struct perf_sample_id *perf_evlist__id2sid(struct perf_evlist *evlist, u64 id);
>>   
>>   union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx);
>>   
>> +union perf_event *perf_evlist__mmap_read_backward(struct perf_evlist *evlist,
>> +						  int idx);
>> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx);
>> +
>>   void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx);
>>   
>>   int perf_evlist__open(struct perf_evlist *evlist);
>> -- 
>> 1.8.3.4

Re: [PATCH v2 3/4] perf tools: Support reading from backward ring buffer

[Wangnan (F)]

On 2016/5/6 21:40, Wangnan (F) wrote:
>
>
> On 2016/5/6 4:07, Arnaldo Carvalho de Melo wrote:
>> Em Wed, Apr 27, 2016 at 02:19:22AM +0000, Wang Nan escreveu:
>>> perf_evlist__mmap_read_backward() is introduced for reading backward
>>> ring buffer. Different from reading forward, before reading, caller
>>> needs to call perf_evlist__mmap_read_catchup() first.
>>>
>>> Backward ring buffer should be read from 'head' pointer, not '0'.
>>> perf_evlist__mmap_read_catchup() saves 'head' to 'md->prev', then
>>> make it remember the start position after each reading.
>>>
>>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>>> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
>>> Cc: Peter Zijlstra <peterz@infradead.org>
>>> Cc: Zefan Li <lizefan@huawei.com>
>>> Cc: pi3orama@163.com
>>> ---
>>>   tools/perf/util/evlist.c | 39 +++++++++++++++++++++++++++++++++++++++
>>>   tools/perf/util/evlist.h |  4 ++++
>>>   2 files changed, 43 insertions(+)
>>>
>>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
>>> index 17cd014..2e0b7b0 100644
>>> --- a/tools/perf/util/evlist.c
>>> +++ b/tools/perf/util/evlist.c
>>> @@ -766,6 +766,45 @@ union perf_event *perf_evlist__mmap_read(struct 
>>> perf_evlist *evlist, int idx)
>>>       return perf_mmap__read(md, evlist->overwrite, old, head, 
>>> &md->prev);
>>>   }
>>>   +union perf_event *
>>> +perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
>>> +{
>>> +    struct perf_mmap *md = &evlist->mmap[idx];
>>> +    u64 head, end;
>>> +    u64 start = md->prev;
>>> +
>>> +    /*
>>> +     * Check if event was unmapped due to a POLLHUP/POLLERR.
>>> +     */
>>> +    if (!atomic_read(&md->refcnt))
>>> +        return NULL;
>>> +
>>> +    /* NOTE: head is negative in this case */
>> What is this comment for, can you ellaborate? Are you double sure this
>> arithmetic with u64, negative values, that -head below are all ok?
>
> Yes. In backward ring buffer, kernel write data from '0'. Each
> time it write a record, it minus sizeof(record) from 'head'
> pointer. So '-head' means number of bytes already written.
>
>> I've applied the first two patches in this series.
>>
>> I also need to check why we now need that catchup thing :-\
>
> I think catchup is necessary. I tried to get rid of it by setting
> md->prev to -1 and initialize it to 0 when reading forwardly and
> to head when reading backwardly, but it require too much code
> adjustments.
>

In addtion, reader of backward ring buffer tends to fetch the most recent
record. Since each reading gets earlier record, a catchup function is always
required.

Thank you.
>>> +    head = perf_mmap__read_head(md);
>>> +
>>> +    if (!head)
>>> +        return NULL;
>>> +
>>> +    end = head + md->mask + 1;
>>> +
>>> +    if ((end - head) > -head)
>>> +        end = 0;
>>> +
>
> This '-head' is used to detect wrapping. Never use positive pointer.
>
>>> +    return perf_mmap__read(md, false, start, end, &md->prev);
>>> +}
>>> +
>>> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int 
>>> idx)
>>> +{
>>> +    struct perf_mmap *md = &evlist->mmap[idx];
>>> +    u64 head;
>>> +
>>> +    if (!atomic_read(&md->refcnt))
>>> +        return;
>>> +
>>> +    head = perf_mmap__read_head(md);
>>> +    md->prev = head;
>>> +}
>>> +
>>>   static bool perf_mmap__empty(struct perf_mmap *md)
>>>   {
>>>       return perf_mmap__read_head(md) == md->prev && 
>>> !md->auxtrace_mmap.base;
>>> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
>>> index 208897a..85d1b59 100644
>>> --- a/tools/perf/util/evlist.h
>>> +++ b/tools/perf/util/evlist.h
>>> @@ -129,6 +129,10 @@ struct perf_sample_id 
>>> *perf_evlist__id2sid(struct perf_evlist *evlist, u64 id);
>>>     union perf_event *perf_evlist__mmap_read(struct perf_evlist 
>>> *evlist, int idx);
>>>   +union perf_event *perf_evlist__mmap_read_backward(struct 
>>> perf_evlist *evlist,
>>> +                          int idx);
>>> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int 
>>> idx);
>>> +
>>>   void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx);
>>>     int perf_evlist__open(struct perf_evlist *evlist);
>>> -- 
>>> 1.8.3.4
>