On Thu, Nov 29 2018, Al Viro wrote:
> On Fri, Nov 30, 2018 at 10:33:18AM +1100, NeilBrown wrote:
>>
>> The synchronize_rcu() in namespace_unlock() is called every time
>> a filesystem is unmounted. If a great many filesystems are mounted,
>> this can cause a noticable slow-down in, for example, system shutdown.
>>
>> The sequence:
>> mkdir -p /tmp/Mtest/{0..5000}
>> time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
>> time umount /tmp/Mtest/*
>>
>> on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
>> 100 seconds to unmount them.
>>
>> Boot the same VM with 1 CPU and it takes 18 seconds to mount the
>> tmpfs filesystems, but only 36 to unmount.
>>
>> If we change the synchronize_rcu() to synchronize_rcu_expedited()
>> the umount time on a 4-cpu VM drop to 0.6 seconds
>>
>> I think this 200-fold speed up is worth the slightly high system
>> impact of using synchronize_rcu_expedited().
>>
>> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> (from general rcu perspective)
>> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
>> ---
>>
>> Al Viro says "I can live with this one" but this still hasn't landed.
>> Maybe someone else could apply it?
>
> Applied (in work.misc, once I push it out)
Excellent - thanks a lot :-)
NeilBrown
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