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authorStephane Eranian <eranian@gmail.com>2009-11-20 22:19:57 +0100
committerIngo Molnar <mingo@elte.hu>2009-11-21 14:11:41 +0100
commit8904b18046c2f050107f6449e887e7c1142b9ab9 (patch)
treec0a23af5059b60f3dd565e2a2a88c8737979588f /kernel
parent6f10581aeaa5543a3b7a8c7a87a064375ec357f8 (diff)
perf_events: Fix default watermark calculation
This patch fixes the default watermark value for the sampling buffer. With the existing calculation (watermark = max(PAGE_SIZE, max_size / 2)), no notification was ever received when the buffer was exactly 1 page. This was because you would never cross the threshold (there is no partial samples). In certain configuration, there was no possibilty detecting the problem because there was not enough space left to store the LOST record.In fact, there may be a more generic problem here. The kernel should ensure that there is alaways enough space to store one LOST record. This patch sets the default watermark to half the buffer size. With such limit, we are guaranteed to get a notification even with a single page buffer assuming no sample is bigger than a page. Signed-off-by: Stephane Eranian <eranian@gmail.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Paul Mackerras <paulus@samba.org> LKML-Reference: <20091120212509.344964101@chello.nl> Signed-off-by: Ingo Molnar <mingo@elte.hu> LKML-Reference: <1256302576-6169-1-git-send-email-eranian@gmail.com>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/perf_event.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 3ede0981f4a..718fa939b1a 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -2340,7 +2340,7 @@ perf_mmap_data_init(struct perf_event *event, struct perf_mmap_data *data)
}
if (!data->watermark)
- data->watermark = max_t(long, PAGE_SIZE, max_size / 2);
+ data->watermark = max_size / 2;
rcu_assign_pointer(event->data, data);