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

[Jon Doron <arilou@gmail.com>]

On 02/02/2023, Andrii Nakryiko wrote:
>On Wed, Feb 1, 2023 at 10:26 PM Jon Doron <arilou@gmail.com> wrote:
>>
>> From: Jon Doron <jond@wiz.io>
>>
>> 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@wiz.io>
>> ---
>>  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
>>