linux-kernel - Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace

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Date:   Fri, 30 Nov 2018 12:09:20 +1100
From:   NeilBrown <neilb@...e.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        paulmck@...ux.vnet.ibm.com, Florian Weimer <fweimer@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()

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@...ux.vnet.ibm.com> (from general rcu perspective)
>> Signed-off-by: NeilBrown <neilb@...e.com>
>> ---
>> 
>> 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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