Re: [PATCH bpf-next v2] libbpf: Add wakeup_events to creation options

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On 02/02/2023, Andrii Nakryiko wrote:
On Wed, Feb 1, 2023 at 10:26 PM Jon Doron <arilou@xxxxxxxxx> wrote:

From: Jon Doron <jond@xxxxxx>

Add option to set when the perf buffer should wake up, by default the
perf buffer becomes signaled for every event that is being pushed to it.

In case of a high throughput of events it will be more efficient to wake
up only once you have X events ready to be read.

So your application can wakeup once and drain the entire perf buffer.

Signed-off-by: Jon Doron <jond@xxxxxx>
---
 tools/lib/bpf/libbpf.c | 4 ++--
 tools/lib/bpf/libbpf.h | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index eed5cec6f510..6b30ff13922b 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11719,8 +11719,8 @@ struct perf_buffer *perf_buffer__new(int map_fd, size_t page_cnt,
        attr.config = PERF_COUNT_SW_BPF_OUTPUT;
        attr.type = PERF_TYPE_SOFTWARE;
        attr.sample_type = PERF_SAMPLE_RAW;
-       attr.sample_period = 1;
-       attr.wakeup_events = 1;
+       attr.sample_period = OPTS_GET(opts, wakeup_events, 1);
+       attr.wakeup_events = OPTS_GET(opts, wakeup_events, 1);

I suspect the case of

LIBBPF_OPTS(perf_buffer_opts, opts);

perf_buffer__new(...., &opts);

is not handled correctly and you end up with sample_period == wakeup_events == 0

Can you please add BPF selftests that's setting wakeup_events to zero
and separately to >1?


Hi Andrii,

I'm not sure what we are testing, when you have sample_period == wakeup_events == 0, it basically means to never wakeup, so let's say you would wait on the poll_fd infinitely it will never wake you up.

When you have let's say wakeup_event != 0, you will wakeup after the ring buffer in the perf buffer has more events than wakeup_events.

I do see your point that if someone is using the macro to build the opts they will end with something unexpected, would you like me to treat 0 as 1 in that case?

-- Jon.


        p.attr = &attr;
        p.sample_cb = sample_cb;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 8777ff21ea1d..e83c0a915dc7 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -1246,8 +1246,9 @@ typedef void (*perf_buffer_lost_fn)(void *ctx, int cpu, __u64 cnt);
 /* common use perf buffer options */
 struct perf_buffer_opts {
        size_t sz;
+       __u32 wakeup_events;
 };
-#define perf_buffer_opts__last_field sz
+#define perf_buffer_opts__last_field wakeup_events

 /**
  * @brief **perf_buffer__new()** creates BPF perfbuf manager for a specified
--
2.39.1




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