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

From: "Wangnan (F)" <[email-masked]>
Date: Tue, 13 Oct 2015 11:27:27 +0800

On 2015/10/13 3:29, Alexei Starovoitov wrote:
> On 10/12/15 2:02 AM, Kaixu Xia wrote:
>> +extern const struct bpf_func_proto bpf_perf_event_sample_enable_proto;
>> +extern const struct bpf_func_proto bpf_perf_event_sample_disable_proto;
>
> externs are unnecessary. Just make them static.
> Also I prefer single helper that takes a flag, so we can extend it
> instead of adding func_id for every little operation.
>
> To avoid conflicts if you touch kernel/bpf/* or bpf.h please always
> base your patches of net-next.
>
> > +    atomic_set(&map->perf_sample_disable, 0);
>
> global flag per map is no go.
> events are independent and should be treated as such.
>

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.

Thank you.

> Please squash these two patches, since they're part of one logical
> feature. Splitting them like this only makes review harder.
>
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