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    SubjectRe: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

    * Rik van Riel <riel@redhat.com> wrote:

    > On 05/07/2015 08:29 AM, Ingo Molnar wrote:
    > >
    > > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
    > >
    > >>>> We cannot take the lock_trace(task) from irq context, and we
    > >>>> probably do not need to anyway, since we do not care about a
    > >>>> precise stack trace for the task.
    > >>>
    > >>> So one worry with this and similar approaches of statistically
    > >>> detecting user mode would be the fact that on the way out to
    > >>> user-space we don't really destroy the previous call trace - we
    > >>> just pop off the stack (non-destructively), restore RIPs and are
    > >>> gone.
    > >>>
    > >>> We'll need that percpu flag I suspect.
    > >>
    > >> Note we have the context tracking state which tells where the
    > >> current task is: user/system/guest.
    > >
    > > Yes, but that overhead is what I'm suggesting we get rid of, I thought
    > > Rik was trying to find a mechanism that would be independent of that?
    >
    > One thing at a time :)
    >
    > I am working on the timer sampling stuff, which should be easy to
    > adapt to a different user/system/guest/irq/softirq/... tracking
    > thing, if somebody else comes up with a more efficient way to do
    > that.

    So if you make the timer sampling use a percpu variable, and set that
    variable from the existing callbacks, then we could do this gradually:
    first the timer sampling uses the flag, then RCU could use it, and
    finally we could push it out to minimal assembly code.

    But it's important to start out with a percpu flag to track this all.

    Thanks,

    Ingo


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