Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable() helpers

From	"Wangnan (F)" <wangnan0@huawei.com>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable() helpers
Date	2015-10-13 06:00 +0200
Message-ID	<qiYUO-35B-3@gated-at.bofh.it> (permalink)
References	<qiHhh-2LN-29@gated-at.bofh.it> <qiHhh-2LN-27@gated-at.bofh.it> <qiQXg-8t1-13@gated-at.bofh.it> <qiYrL-2vS-7@gated-at.bofh.it> <qiYBr-2Ii-5@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 2015/10/13 11:39, Alexei Starovoitov wrote:
> On 10/12/15 8:27 PM, Wangnan (F) wrote:
>> Then how to avoid racing? For example, when one core disabling all 
>> events
>> in a map, another core is enabling all of them. This racing may causes
>> sereval
>> perf events in a map dump samples while other events not. To avoid such
>> racing
>> I think some locking must be introduced, then cost is even higher.
>>
>> The reason why we introduce an atomic pointer is because each operation
>> should
>> controls a set of events, not one event, due to the per-cpu manner of
>> perf events.
>
> why 'set disable' is needed ?
> the example given in cover letter shows the use case where you want
> to receive samples only within sys_write() syscall.
> The example makes sense, but sys_write() is running on this cpu, so just
> disabling it on the current one is enough.
>

Our real use case is control of the system-wide sampling. For example,
we need sampling all CPUs when smartphone start refershing its display.
We need all CPUs because in Android system there are plenty of threads
get involed into this behavior. We can't achieve this by controling
sampling on only one CPU. This is the reason we need 'set enable'
and 'set disable'.

Thank you.

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Thread

[RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable() helpers Kaixu Xia <xiakaixu@huawei.com> - 2015-10-12 11:10 +0200
  Re: [RFC PATCH 2/2] bpf: Implement  bpf_perf_event_sample_enable/disable() helpers Alexei Starovoitov <ast@plumgrid.com> - 2015-10-12 21:30 +0200
    Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable()  helpers "Wangnan (F)" <wangnan0@huawei.com> - 2015-10-13 05:30 +0200
      Re: [RFC PATCH 2/2] bpf: Implement  bpf_perf_event_sample_enable/disable() helpers Alexei Starovoitov <ast@plumgrid.com> - 2015-10-13 05:40 +0200
        Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable()  helpers "Wangnan (F)" <wangnan0@huawei.com> - 2015-10-13 06:00 +0200
          Re: [RFC PATCH 2/2] bpf: Implement  bpf_perf_event_sample_enable/disable() helpers Alexei Starovoitov <ast@plumgrid.com> - 2015-10-13 06:20 +0200
            Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable()  helpers "Wangnan (F)" <wangnan0@huawei.com> - 2015-10-13 06:40 +0200
              Re: [RFC PATCH 2/2] bpf: Implement  bpf_perf_event_sample_enable/disable() helpers Alexei Starovoitov <ast@plumgrid.com> - 2015-10-13 07:20 +0200
                Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable()  helpers "Wangnan (F)" <wangnan0@huawei.com> - 2015-10-13 09:10 +0200
                Re: [RFC PATCH 2/2] bpf: Implement  bpf_perf_event_sample_enable/disable() helpers He Kuang <hekuang@huawei.com> - 2015-10-13 13:00 +0200
                Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable()  helpers "Wangnan (F)" <wangnan0@huawei.com> - 2015-10-13 13:20 +0200
                Re: [RFC PATCH 2/2] bpf: Implement  bpf_perf_event_sample_enable/disable() helpers Alexei Starovoitov <ast@plumgrid.com> - 2015-10-14 07:20 +0200

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