Messages in this thread | | | Date | Fri, 30 Nov 2012 19:57:09 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/2] percpu-rwsem: use synchronize_sched_expedited |
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On 11/29, 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? > > 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()?
synchronize_sched_expedited() is much faster but it is heavyweight, try_stop_cpus() affects the whole system.
We briefly discussed this possibility with Paul, personally I think it would be better to add percpu_rw_semaphore->use_expedited or add the percpu_down_write_expedited().
But this change was already applied. I hope we can do this later.
> Oleg, this has implications for your > percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.patch. > I've changed that to use synchronize_sched_expedited() everywhere - > please have a think and a retest.
Thanks a lot Andrew. The fix looks obviously fine, but I'll try to test anyway.
> Note that elsewhere in this thread, > percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.patch > was found to slow mount down again somewhat, because it adds an > additional synchronize_sched[_expedited]() to the percpu_down_write() > path.
Cough... this patch _removes_ one synchronize_sched[_expedited]() and removes msleep ;) Although the main point was not-block-the-readers.
And I think we can remove another one, but this is not trivial and percpu_free_rwsem() should be might_sleep().
Oleg.
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