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    SubjectRe: [PATCH 3/4] trace: add ability to collect call chain of non-current task.
    On 09/28/2011 12:55 AM, Frederic Weisbecker wrote:
    > On Tue, Sep 27, 2011 at 04:05:51PM +0200, Peter Zijlstra wrote:
    >> On Mon, 2011-09-26 at 19:55 +0400, Andrew Vagin wrote:
    >>> Know issues:
    >>> * Now call chains for non-current tasks are collected on x86 only,
    >>> but it may be done for other architectures simply.
    >>> * It collects only kernel call chains, because we can't get direct
    >>> access to memory of other processes and this operation should be
    >>> fast enough.
    >> Also, it changes the semantics of tracepoint, normally you return the
    >> callchain leading to the tracepoint, this changes that.
    >> I'm not entirely against it, as I can see the use, but I would like to
    >> solicit other opinions.
    > There is also a problem in perf tools because we are dealing with a
    > callchain that doesn't belong to the current thread memory mapping and symbol
    > space. I believe that's a problem once we deal with the userspace part of
    > the callchain.
    > That and the fact there are other ways to get that callchain like taking
    > the one of the last sched_switch event from the waked thread.
    > Perhaps we should use the sched_switch event and a post_sched_switch
    > event, compute the time difference as a weight and use the callchain of any of
    > those. Perhaps as a plugin in perf report?
    > In any case that rather sounds like something that should be done in post-processing
    > in userspace, in perf tools.
    > We should probably avoid the remote callchains, sounds like asking for complications
    > everywhere.
    Thank you for comments and suggestions. I take timeout to think over them.

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