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    SubjectRe: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)

    * Benjamin <> wrote:

    > LWP is highly limited in its ability's to support more than
    > one "LWP-Instance" being active for a thread, IOW it is not
    > possible.

    That's OK, we can deal with various PMU constraints just fine.

    > You can't activate LWP from a threads context and
    > simultaneously activate lwp-system-wide-profiling in the way
    > you suggested it, Ingo. Either do the first xor do the last,

    We have other PMU resources that are exclusive in that sense.

    > because you only have one xsave-area/msr/lwpcb that is read by
    > the hardware and only one LWP-Buffer that is written by the
    > hw.

    That's similar to PEBS (which we already support), there's only
    one Debug Store per CPU, obviously.

    > So, if one thread is running LWP, because he wants to
    > (selfmonitoring and stuff [like for what lwp was designed])
    > and a su or u would activate this system-wide-monitoring, both
    > would frequently interfere with the each other. I don't think
    > you want this to be possible at all.

    THe LWPCB is designed to allow multiple events, and the LWP
    ring-buffer is shared between these events.

    If the kernel properly manages the lwpcb then no such
    'interference' happens during normal use - both outside and
    self-installed events can be activated at once, up to the event
    limit - similar to how we handle regular PMU events.

    [ This is why the threshold IRQ support i requested is key: it
    is needed for the flow of events and for the kernel
    event-demultiplexer to work transparently. ]

    > Frankly, it was already a pain to get LWP running from
    > in-kernel, like it is done now. I would expect a much higher
    > pain, if you would want to do this with a transparent buffer,
    > that gets passed around each scheduling (and this would
    > permanently eliminate the "lightweight" in "LWP").

    There's no heavyweight 'passing around' of a buffer needed at
    context switch time. The buffer context has to be flipped - part
    of the job of context switching.

    So no, i don't think any of your objections have any merit.



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