Re: [PATCH] VFS: use synchronize_rcu_expedited() in namespace_unlock()

[NeilBrown <neilb@suse.com>]

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On Thu, Oct 26 2017, Paul E. McKenney wrote:

> On Thu, Oct 26, 2017 at 01:26:37PM +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 is 8 seconds to mount and 0.6 to
>> unmount.
>> 
>> I think this 200-fold speed up is worth the slightly higher system
>> impact of use synchronize_rcu_expedited().
>> 
>> Signed-off-by: NeilBrown <neilb@suse.com>
>> ---
>> 
>> Cc: to Paul and Josh in case they'll correct me if using _expedited()
>> is really bad here.
>
> I suspect that filesystem unmount is pretty rare in production real-time
> workloads, which are the ones that might care.  So I would guess that
> this is OK.
>
> If the real-time guys ever do want to do filesystem unmounts while their
> real-time applications are running, they might modify this so that it can
> use synchronize_rcu() instead for real-time builds of the kernel.

Thanks for the confirmation Paul.

>
> But just for completeness, one way to make this work across the board
> might be to instead use call_rcu(), with the callback function kicking
> off a workqueue handler to do the rest of the unmount.  Of course,
> in saying that, I am ignoring any mutexes that you might be holding
> across this whole thing, and also ignoring any problems that might arise
> when returning to userspace with some portion of the unmount operation
> still pending.  (For example, someone unmounting a filesystem and then
> immediately remounting that same filesystem.)

I had briefly considered that option, but it doesn't work.
The purpose of this synchronize_rcu() is to wait for any filename lookup
which might be locklessly touching the mountpoint to complete.
It is only after that that the real meat of unmount happen - the
filesystem is told that the last reference is gone, and it gets to
flush any saved changes out to disk etc.
That stuff really has to happen before the umount syscall returns.

Thanks,
NeilBrown

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