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    SubjectRe: [PATCH 3/3] tracing/function-return-tracer: add the overrun field

    * Frédéric Weisbecker <fweisbec@gmail.com> wrote:

    > 2008/11/21 Ingo Molnar <mingo@elte.hu>:
    > >
    > > * Frédéric Weisbecker <fweisbec@gmail.com> wrote:
    > >
    > >> When the tracer will be launched, I will hold the tasklist_lock to
    > >> allocate/insert the dynamic arrays. So in this atomic context, I
    > >> will not be able to call kmalloc with GFP_KERNEL. And I fear that
    > >> using GFP_ATOMIC for possible hundreds of tasks would be clearly
    > >> unacceptable.
    > >>
    > >> What do you think of this way:
    > >>
    > >> _tracer activates
    > >> _a function enters the tracer entry-hooker. If the array is allocated
    > >> for the current task, that's well. If not I launch a kernel thread
    > >> that will later allocate an array for the current task (I will pass
    > >> the pid as a parameter). So the current task will be soon be traced.
    > >> _ when a process forks, I can allocate a dynamic array for the new
    > >> task without problem (I hope).
    > >>
    > >> So some tasks will not be traced at the early beggining of tracing
    > >> but they will soon all be traced.... There is perhaps a problem with
    > >> tasks that are sleeping for long times... There will be some losses
    > >> once they will be awaken...
    > >
    > > i'd suggest a different approach that is simpler:
    > >
    > > - step0: set flag that "all newly created tasks need the array
    > > allocated from now on".
    > >
    > > - step1: allocate N arrays outside tasklist_lock
    > >
    > > - step2: take tasklist_lock, loop over all tasks that exist and pass
    > > in the N arrays to all tasks that still need it.
    > >
    > > If tasks were 'refilled', drop tasklist_lock and go back to step 1.
    > >
    > > - step3: free N (superfluously allocated) arrays
    > >
    > > Make N something like 32 to not get into a bad quadratic nr_tasks
    > > double loop in practice. (Possibly allocate arrays[32] dynamically as
    > > well at step0 and not have it on the kernel stack - so 32 can be
    > > changed to 128 or so.)
    > >
    > > Ingo
    > >
    >
    > Ok. I thought about this method but wondered about the fact that
    > kmalloc can schedule and then I could run in an infinite loop (or a
    > too long one).

    the retry loop should solve that aspect - and the chunking solves the
    "dont run too long with a lock held" problem.

    > I will try this. Thanks.

    looks good, applied :)

    Ingo
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