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>

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 3ede0981f4acfb1b92804a4110054128e0ca2a31..718fa939b1a7d6accf50dc11d98163e7100f1a6a 100644 (file)
--- 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);