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    SubjectRe: [RFC] perf_events: support for uncore a.k.a. nest units
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    On Wed, Mar 31, 2010 at 4:01 PM, Peter Zijlstra <peterz@infradead.org> wrote:
    > On Tue, 2010-03-30 at 15:12 -0700, Corey Ashford wrote:
    >>
    >> > Initially I'd not allow per-pmu-per-task contexts
    >> > because then things like perf_event_task_sched_out() would get rather
    >> > complex.
    >>
    >> Definitely.  I don't think it makes sense to have per-task context on
    >> nest/uncore PMUs.  At least we haven't found any justification for it.
    >
    > For uncore no, but there is also the hw-breakpoint stuff that is being
    > presented as a pmu, for those it would make sense to have a separate
    > per-task context.
    >
    > But doing multiple per-task contexts is something for a next step
    > indeed.
    >
    >> > For RR we can move away from perf_event_task_tick and let the pmu
    >> > install a (hr)timer for this on their own.
    >>
    >> This is necessary I think, because of the access time for some of the PMU's.  I
    >> wonder though if it should, perhaps optionally, be off-loaded to a high-priority
    >> task to do the switching so that access latency to the PMU can be controlled.
    >>
    >> As I mentioned when we met, some of the Wire-Speed processor nest PMU control
    >> registers are accessed via SCOM, which is an internal, 200 MHz serial bus.  We
    >> are being quoted ~525 SCOM bus ticks to do a PMU control register access, which
    >> comes out to about 2.5 microseconds.  If you figure 5 accesses to rotate the
    >> events on a PMU, that's a minimum of 12.5 microseconds.
    >
    > Yeah, you mentioned that.. for those things we need some changes anyway,
    > since currently we install per-cpu counters using IPIs and expect the
    > pmu::enable() method to be synchronous (it has a return value). It would
    > be totally unacceptable to do 2.5ms pokes with IRQs disabled.
    >
    > The RR thing would be the easiest to solve, just let the timer wake up a
    > thread instead of doing the work itself, that's fully isolated to how
    > the pmu chooses to implement that. The above mentioned issue however
    > would be much more challenging to fix nicely.
    >
    >
    Also some of perf_enable()/perf_disable() would have to be per PMU and
    not global like they are today.
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