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    SubjectRe: [RFC/PATCH 17/38] perf tools: Maintain map groups list in a leader thread
    On Fri, Oct 09, 2015 at 03:58:49PM +0900, Namhyung Kim wrote:

    SNIP

    >
    >
    > >
    > > some comment on how we treat map groups in general (for fork/clone/exit)
    > > would be awesome ;-)
    >
    > I admit that this code is subtle and confusing.. How about this?
    >
    >
    > Managing map groups is subtle in that we basically want to share a map
    > groups between threads in a process. When a new process is created
    > (forked), the child clones (current) map groups from the parent. But
    > if a new thread is called it only gets a reference of the leader's mg.
    >
    > Complication comes from the exec as we also want to keep the history
    > of a thread's execution, so the map groups are now managed by mg_list.
    > This mg_list is maintained by leader threads only, and non-leader
    > threads have a reference a mg at the time in the mg_list. It uses a
    > timestamp at the event to find out the correct mg in the mg_list.
    >
    > One corner case is when exec is called from a non-leader thread. We
    > want to add a new mg to the mg_list in the thread. But it doesn't
    > have a mg_list since it was not a leader. So it sets up a mg_list and
    > insert a cloned mg from the old leader. Now it can handle exec as
    > usual - create a new mg and insert it to the mg_list.

    seems ok, thanks

    jirka


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