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    SubjectRe: [PATCH 1/2] percpu-rwsem: use synchronize_sched_expedited


    On Thu, 29 Nov 2012, Andrew Morton wrote:

    > On Tue, 27 Nov 2012 22:59:52 -0500 (EST)
    > Mikulas Patocka <mpatocka@redhat.com> wrote:
    >
    > > percpu-rwsem: use synchronize_sched_expedited
    > >
    > > Use synchronize_sched_expedited() instead of synchronize_sched()
    > > to improve mount speed.
    > >
    > > This patch improves mount time from 0.500s to 0.013s.
    > >
    > > Note: if realtime people complain about the use
    > > synchronize_sched_expedited() and synchronize_rcu_expedited(), I suggest
    > > that they introduce an option CONFIG_REALTIME or
    > > /proc/sys/kernel/realtime and turn off these *_expedited functions if
    > > the option is enabled (i.e. turn synchronize_sched_expedited into
    > > synchronize_sched and synchronize_rcu_expedited into synchronize_rcu).
    > >
    > > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    >
    > So I read through this thread but I really didn't see a clear
    > description of why mount() got slower. The changelog for 4b05a1c74d1
    > is spectacularly awful :(
    >
    >
    > Apparently the slowdown occurred because a blockdev mount patch
    > 62ac665ff9fc07497ca524 ("blockdev: turn a rw semaphore into a percpu rw
    > semaphore") newly uses percpu rwsems, and percpu rwsems are slow on the
    > down_write() path.
    >
    > And using synchronize_sched_expedited() rather than synchronize_sched()
    > makes percpu_down_write() somewhat less slow. Correct?

    Yes.

    > Why is it OK to use synchronize_sched_expedited() here? If it's
    > faster, why can't we use synchronize_sched_expedited() everywhere and
    > zap synchronize_sched()?

    Because synchronize_sched_expedited sends interrupts to all processors and
    it is bad for realtime workloads.

    Peter Zijlstra once complained when I used synchronize_rcu_expedited in
    bdi_remove_from_list (but he left it there).

    I suggest that if it really hurts real time response for someone, let they
    introduce a switch to turn it into non-expedited call.

    Mikulas


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