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

[Mikulas Patocka]

On Thu, 29 Nov 2012, Andrew Morton wrote:

> On Tue, 27 Nov 2012 22:59:52 -0500 (EST)
> Mikulas Patocka <mpatocka@xxxxxxxxxx> 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@xxxxxxxxxx>
> 
> 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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