Messages in this thread | | | Date | Thu, 29 Nov 2012 22:00:53 -0500 (EST) | From | Mikulas Patocka <> | Subject | Re: [PATCH 1/2] percpu-rwsem: use synchronize_sched_expedited |
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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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