RE: [PATCH V2 2/8] perf tools: rewrite perf mmap read for overwrite

[Liang, Kan]

> > -union perf_event *perf_mmap__read_backward(struct perf_mmap *map)
> > +union perf_event *perf_mmap__read_backward(struct perf_mmap *map,
> > +					   u64 *start, u64 end)
> >   {
> > -	u64 head, end;
> > -	u64 start = map->prev;
> > -
> > -	/*
> > -	 * Check if event was unmapped due to a POLLHUP/POLLERR.
> > -	 */
> > -	if (!refcount_read(&map->refcnt))
> > -		return NULL;
> > -
> 
> Is removing this checking safe?

It was duplicate as the one in perf_mmap__read_catchup.
Once planned to remove one. But it looks I removed both accidently.
Will keep one in V3.

> 
> > -	head = perf_mmap__read_head(map);
> > -	if (!head)
> > -		return NULL;
> > +	union perf_event *event = NULL;
> >
> > -	/*
> > -	 * 'head' pointer starts from 0. Kernel minus sizeof(record) form
> > -	 * it each time when kernel writes to it, so in fact 'head' is
> > -	 * negative. 'end' pointer is made manually by adding the size of
> > -	 * the ring buffer to 'head' pointer, means the validate data can
> > -	 * read is the whole ring buffer. If 'end' is positive, the ring
> > -	 * buffer has not fully filled, so we must adjust 'end' to 0.
> > -	 *
> > -	 * However, since both 'head' and 'end' is unsigned, we can't
> > -	 * simply compare 'end' against 0. Here we compare '-head' and
> > -	 * the size of the ring buffer, where -head is the number of bytes
> > -	 * kernel write to the ring buffer.
> > -	 */
> > -	if (-head < (u64)(map->mask + 1))
> > -		end = 0;
> > -	else
> > -		end = head + map->mask + 1;
> > +	event = perf_mmap__read(map, *start, end, &map->prev);
> > +	*start = map->prev;
> >
> > -	return perf_mmap__read(map, start, end, &map->prev);
> > +	return event;
> >   }
> 
> perf_mmap__read_backward() and perf_mmap__read_forward() become
> asymmetrical,
> but their names are symmetrical.
> 
> 
> >
> > -void perf_mmap__read_catchup(struct perf_mmap *map)
> > +static int overwrite_rb_find_range(void *buf, int mask, u64 head,
> > +				   u64 *start, u64 *end);
> > +
> > +int perf_mmap__read_catchup(struct perf_mmap *map,
> > +			    bool overwrite,
> > +			    u64 *start, u64 *end,
> > +			    unsigned long *size)
> >   {
> > -	u64 head;
> > +	u64 head = perf_mmap__read_head(map);
> > +	u64 old = map->prev;
> > +	unsigned char *data = map->base + page_size;
> >
> > -	if (!refcount_read(&map->refcnt))
> > -		return;
> > +	*start = overwrite ? head : old;
> > +	*end = overwrite ? old : head;
> >
> > -	head = perf_mmap__read_head(map);
> > -	map->prev = head;
> > +	if (*start == *end)
> > +		return 0;
> > +
> > +	*size = *end - *start;
> > +	if (*size > (unsigned long)(map->mask) + 1) {
> > +		if (!overwrite) {
> > +			WARN_ONCE(1, "failed to keep up with mmap data.
> (warn only once)\n");
> > +
> > +			map->prev = head;
> > +			perf_mmap__consume(map, overwrite);
> > +			return 0;
> > +		}
> > +
> > +		/*
> > +		 * Backward ring buffer is full. We still have a chance to read
> > +		 * most of data from it.
> > +		 */
> > +		if (overwrite_rb_find_range(data, map->mask, head, start,
> end))
> > +			return -1;
> > +	}
> > +
> > +	return 1;
> 
> Coding suggestion: this function returns 2 different value (1 and -1) in
> two fail path. Should return 0 for success and -1 for failure.
>

For perf top, -1 is enough for failure.
But for perf_mmap__push, it looks two fail paths are needed, aren't they?
 
> You totally redefine perf_mmap__read_catchup(). The original meaning of
> this function is adjust md->prev so following perf_mmap__read() can read
> events one by one safely.  Only backward ring buffer needs catchup:
> kernel's 'head' pointer keeps moving (backward), while md->prev keeps
> unchanged. For forward ring buffer, md->prev will catchup each time when
> reading from the ring buffer.
>
> Your patch totally changes its meaning. Now its meaning is finding the
> available data from a ring buffer (report start and end). At least we
> need a better name.

Sure, I will introduce a new function to do it in V3.

> In addition, if I understand your code correctly, we
> don't need to report 'size' to caller, because size is determined by
> start and end.

Sure, I will remove the 'size' in V3.

> 
> In addition, since the concept of backward and overwrite are now
> unified, we can avoid updating map->prev during one-by-one reading
> (because backward reading don't need consume operation). I think we can
> decouple the updating of map->prev and these two basic read functions.
> This can makes the operations on map->prev clearer.  Now the moving
> direction of map->prev is confusing for backward ring buffer: it moves
> forward during one by one reading, and moves backward when block
> reading
> (in perf record) 

For reading, I think both one by one reading and block reading move forward
for overwrite.
It starts from head and ends in map->prev. No?

For one-by-one reading, yes, it doesnât need to update map->prev for each read.
But both need to update the map->prev when the block is finished.

> and when syncing.
> 
> Then the core of two reading become uniform. Let's get rid of
> duplication.
> 
> Summarize:
> 
> 1. Compute both start and end before reading for both forward and
> backward (perf_mmap__read_catchup, but need a better name).

How about perf_mmap__read_init?

> 
> 2. Don't update map->prev in perf_mmap__read. Let perf_mmap__read()
> update start pointer, so it become stateless and suit for both backward
> and forward reading.
> 
> 3. Update md->prev for forward reading. Merge it into
> perf_mmap__consume?

It looks we have to pass the updated 'start' to perf_mmap__consume.
Move interfaces like perf_evlist__mmap_read need to be changed.
That would impacts other tools which only support non-overwrite mode.

How about update both 'md->prev' and 'start' in perf_mmap__read()
like the patch as below?

Introducing a new perf_mmap__read_event to get rid of
the perf_mmap__read_backward/forward.

perf_mmap__read_backward will be removed later.
Keep perf_mmap__read_forward since other tools still use it.

It has to update the 'end' for non-overwrite mode in each read since the
ringbuffer is not paused.

diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
index 484a394..74f33fd 100644
--- a/tools/perf/util/mmap.c
+++ b/tools/perf/util/mmap.c
@@ -22,16 +22,16 @@ size_t perf_mmap__mmap_len(struct perf_mmap *map)
 
 /* When check_messup is true, 'end' must points to a good entry */
 static union perf_event *perf_mmap__read(struct perf_mmap *map,
-					 u64 start, u64 end, u64 *prev)
+					 u64 *start, u64 end, u64 *prev)
 {
 	unsigned char *data = map->base + page_size;
 	union perf_event *event = NULL;
-	int diff = end - start;
+	int diff = end - *start;
 
 	if (diff >= (int)sizeof(event->header)) {
 		size_t size;
 
-		event = (union perf_event *)&data[start & map->mask];
+		event = (union perf_event *)&data[*start & map->mask];
 		size = event->header.size;
 
 		if (size < sizeof(event->header) || diff < (int)size) {
@@ -43,8 +43,8 @@ static union perf_event *perf_mmap__read(struct perf_mmap *map,
 		 * Event straddles the mmap boundary -- header should always
 		 * be inside due to u64 alignment of output.
 		 */
-		if ((start & map->mask) + size != ((start + size) & map->mask)) {
-			unsigned int offset = start;
+		if ((*start & map->mask) + size != ((*start + size) & map->mask)) {
+			unsigned int offset = *start;
 			unsigned int len = min(sizeof(*event), size), cpy;
 			void *dst = map->event_copy;
 
@@ -59,12 +59,12 @@ static union perf_event *perf_mmap__read(struct perf_mmap *map,
 			event = (union perf_event *)map->event_copy;
 		}
 
-		start += size;
+		*start += size;
 	}
 
 broken_event:
 	if (prev)
-		*prev = start;
+		*prev = *start;
 
 	return event;
 }
@@ -132,6 +107,42 @@ void perf_mmap__read_catchup(struct perf_mmap *map)
 	map->prev = head;
 }
+
+/*
+ * Read event from ring buffer. Return one event for each call.
+ * Support both overwirte and non-overwrite mode.
+ * The start and end are only available for overwirte mode, which
+ * pause the ringbuffer.
+ *
+ * Usage:
+ * perf_mmap__read_init
+ * while(event = perf_mmap__read_event) {
+ * 	//process the event
+ * 	perf_mmap__consume
+ * }
+ * perf_mmap__read_done
+ */
+union perf_event *perf_mmap__read_event(struct perf_mmap *map,
+					bool overwrite,
+					u64 *start, u64 end)
+{
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!refcount_read(&map->refcnt))
+		return NULL;
+
+	if (!overwrite)
+		end = perf_mmap__read_head(map);
+
+	return perf_mmap__read(map, start, end, &map->prev);
+}
+
 static bool perf_mmap__empty(struct perf_mmap *map)
 {
 	return perf_mmap__read_head(map) == map->prev && !map->auxtrace_mmap.base;


Thanks,
Kan