From 8904b18046c2f050107f6449e887e7c1142b9ab9 Mon Sep 17 00:00:00 2001 From: Stephane Eranian Date: Fri, 20 Nov 2009 22:19:57 +0100 Subject: [PATCH] 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 Signed-off-by: Peter Zijlstra Cc: Paul Mackerras LKML-Reference: <20091120212509.344964101@chello.nl> Signed-off-by: Ingo Molnar LKML-Reference: <1256302576-6169-1-git-send-email-eranian@gmail.com> --- kernel/perf_event.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/perf_event.c b/kernel/perf_event.c index 3ede0981f4ac..718fa939b1a7 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);