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SubjectRe: [PATCH RFC] sched: add notifier for process migration
On 10/14/2009 06:26 PM, Peter Zijlstra wrote:
>> We already have one event notifier there - look at the
>> perf_swcounter_event() callback. Why add a second one for essentially
>> the same thing?
>> We should only put a single callback there - a tracepoint defined via
>> TRACE_EVENT() - and any secondary users can register a callback to the
>> tracepoint itself.
>> There's many similar places in the kernel - with notifier chains and
>> also with a need to get tracepoints there. The fastest (and most
>> consistent) solution is to add just a single event callback facility.
> But that would basically mandate tracepoints to be always enabled, do we
> want to go there?
> I don't think the overhead of tracepoints is understood well enough,
> Jason you poked at that, do you have anything solid on that?
> Also, I can imagine the embedded people to not want that.
> I really like perf and tracepoints to not become co-dependent until
> tracepoint become mandatory for all configurations.

It would be cleanest to have both pvclock and tracepoints select
migration notifiers, defaulting to off. Similarly both perf and kvm
should use preemption notifiers (they do the same thing - switch
per-task values into and out of cpu registers).

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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