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authorSteven Rostedt <srostedt@redhat.com>2010-03-31 13:21:56 -0400
committerSteven Rostedt <rostedt@goodmis.org>2010-03-31 22:57:04 -0400
commit66a8cb95ed04025664d1db4e952155ee1dccd048 (patch)
tree74417422a78bc8198de46b0e52e490175af866e0 /include
parenteb0c53771fb2f5f66b0edb3ebce33be4bbf1c285 (diff)
ring-buffer: Add place holder recording of dropped events
Currently, when the ring buffer drops events, it does not record the fact that it did so. It does inform the writer that the event was dropped by returning a NULL event, but it does not put in any place holder where the event was dropped. This is not a trivial thing to add because the ring buffer mostly runs in overwrite (flight recorder) mode. That is, when the ring buffer is full, new data will overwrite old data. In a produce/consumer mode, where new data is simply dropped when the ring buffer is full, it is trivial to add the placeholder for dropped events. When there's more room to write new data, then a special event can be added to notify the reader about the dropped events. But in overwrite mode, any new write can overwrite events. A place holder can not be inserted into the ring buffer since there never may be room. A reader could also come in at anytime and miss the placeholder. Luckily, the way the ring buffer works, the read side can find out if events were lost or not, and how many events. Everytime a write takes place, if it overwrites the header page (the next read) it updates a "overrun" variable that keeps track of the number of lost events. When a reader swaps out a page from the ring buffer, it can record this number, perfom the swap, and then check to see if the number changed, and take the diff if it has, which would be the number of events dropped. This can be stored by the reader and returned to callers of the reader. Since the reader page swap will fail if the writer moved the head page since the time the reader page set up the swap, this gives room to record the overruns without worrying about races. If the reader sets up the pages, records the overrun, than performs the swap, if the swap succeeds, then the overrun variable has not been updated since the setup before the swap. For binary readers of the ring buffer, a flag is set in the header of each sub page (sub buffer) of the ring buffer. This flag is embedded in the size field of the data on the sub buffer, in the 31st bit (the size can be 32 or 64 bits depending on the architecture), but only 27 bits needs to be used for the actual size (less actually). We could add a new field in the sub buffer header to also record the number of events dropped since the last read, but this will change the format of the binary ring buffer a bit too much. Perhaps this change can be made if the information on the number of events dropped is considered important enough. Note, the notification of dropped events is only used by consuming reads or peeking at the ring buffer. Iterating over the ring buffer does not keep this information because the necessary data is only available when a page swap is made, and the iterator does not swap out pages. Cc: Robert Richter <robert.richter@amd.com> Cc: Andi Kleen <andi@firstfloor.org> Cc: Li Zefan <lizf@cn.fujitsu.com> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: "Luis Claudio R. Goncalves" <lclaudio@uudg.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/ring_buffer.h6
1 files changed, 4 insertions, 2 deletions
diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 5fcc31ed5771..c8297761e414 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -120,9 +120,11 @@ int ring_buffer_write(struct ring_buffer *buffer,
120 unsigned long length, void *data); 120 unsigned long length, void *data);
121 121
122struct ring_buffer_event * 122struct ring_buffer_event *
123ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts); 123ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts,
124 unsigned long *lost_events);
124struct ring_buffer_event * 125struct ring_buffer_event *
125ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts); 126ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts,
127 unsigned long *lost_events);
126 128
127struct ring_buffer_iter * 129struct ring_buffer_iter *
128ring_buffer_read_start(struct ring_buffer *buffer, int cpu); 130ring_buffer_read_start(struct ring_buffer *buffer, int cpu);