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

Jeff Chua
2012-11-28T14:19:08Z

On Wed, Nov 28, 2012 at 11:59 AM, Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
> On Tue, 27 Nov 2012, Jeff Chua wrote:
>
>> On Tue, Nov 27, 2012 at 3:38 PM, Jens Axboe <axboe@kernel.dk> wrote:
>> > On 2012-11-27 06:57, Jeff Chua wrote:
>> >> On Sun, Nov 25, 2012 at 7:23 AM, Jeff Chua <jeff.chua.linux@gmail.com> wrote:
>> >>> On Sun, Nov 25, 2012 at 5:09 AM, Mikulas Patocka <mpatocka@redhat.com> wrote:
>> >>>> So it's better to slow down mount.
>> >>>
>> >>> I am quite proud of the linux boot time pitting against other OS. Even
>> >>> with 10 partitions. Linux can boot up in just a few seconds, but now
>> >>> you're saying that we need to do this semaphore check at boot up. By
>> >>> doing so, it's inducing additional 4 seconds during boot up.
>> >>
>> >> By the way, I'm using a pretty fast SSD (Samsung PM830) and fast CPU
>> >> (2.8GHz). I wonder if those on slower hard disk or slower CPU, what
>> >> kind of degradation would this cause or just the same?
>> >
>> > It'd likely be the same slow down time wise, but as a percentage it
>> > would appear smaller on a slower disk.
>> >
>> > Could you please test Mikulas' suggestion of changing
>> > synchronize_sched() in include/linux/percpu-rwsem.h to
>> > synchronize_sched_expedited()?
>>
>> Tested. It seems as fast as before, but may be a "tick" slower. Just
>> perception. I was getting pretty much 0.012s with everything reverted.
>> With synchronize_sched_expedited(), it seems to be 0.012s ~ 0.013s.
>> So, it's good.
>>
>>
>> > linux-next also has a re-write of the per-cpu rw sems, out of Andrews
>> > tree. It would be a good data point it you could test that, too.
>>
>> Tested. It's slower. 0.350s. But still faster than 0.500s without the patch.
>>
>> # time mount /dev/sda1 /mnt; sync; sync; umount /mnt
>>
>>
>> So, here's the comparison ...
>>
>> 0.500s 3.7.0-rc7
>> 0.168s 3.7.0-rc2
>> 0.012s 3.6.0
>> 0.013s 3.7.0-rc7 + synchronize_sched_expedited()
>> 0.350s 3.7.0-rc7 + Oleg's patch.
>>
>>
>> Thanks,
>> Jeff.
>
> OK, I'm seinding two patches to reduce mount times. If it is possible to
> put them to 3.7.0, put them there.
>
> Mikulas
>
> ---
>
> 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>
>
> ---
> include/linux/percpu-rwsem.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> Index: linux-3.7-rc7/include/linux/percpu-rwsem.h
> ===================================================================
> --- linux-3.7-rc7.orig/include/linux/percpu-rwsem.h 2012-11-28 02:41:03.000000000 +0100
> +++ linux-3.7-rc7/include/linux/percpu-rwsem.h 2012-11-28 02:41:15.000000000 +0100
> @@ -13,7 +13,7 @@ struct percpu_rw_semaphore {
> };
>
> #define light_mb() barrier()
> -#define heavy_mb() synchronize_sched()
> +#define heavy_mb() synchronize_sched_expedited()
>
> static inline void percpu_down_read(struct percpu_rw_semaphore *p)
> {
> @@ -51,7 +51,7 @@ static inline void percpu_down_write(str
> {
> mutex_lock(&p->mtx);
> p->locked = true;
> - synchronize_sched(); /* make sure that all readers exit the rcu_read_lock_sched region */
> + synchronize_sched_expedited(); /* make sure that all readers exit the rcu_read_lock_sched region */
> while (__percpu_count(p->counters))
> msleep(1);
> heavy_mb(); /* C, between read of p->counter and write to data, paired with B */

Mikulas,

Tested this one and this is good! Back to 3.6.0 behavior.

As for the 2nd patch (block_dev.c), it didn't really make any
difference for ext2/3/4, but for reiserfs, it does. So, won't just the
patch about(synchronize_sched_expedited) be good enough?

Thanks,
Jeff