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Subject: Re: [PATCH v9 5/5] lib/dlock-list: Scale dlock_lists_empty()
To:     Jan Kara <jack@suse.cz>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.com>,
        Jeff Layton <jlayton@poochiereds.net>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andi Kleen <andi@firstfloor.org>,
        Dave Chinner <dchinner@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Davidlohr Bueso <dbueso@suse.de>
References: <1536780532-4092-1-git-send-email-longman@redhat.com>
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 <20181004071600.GC29482@quack2.suse.cz>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <5bcdf2a2-6d03-df21-934d-6c989549253b@redhat.com>
Date:   Thu, 4 Oct 2018 09:41:56 -0400
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On 10/04/2018 03:16 AM, Jan Kara wrote:
> On Wed 12-09-18 15:28:52, Waiman Long wrote:
>> From: Davidlohr Bueso <dave@stgolabs.net>
>>
>> Instead of the current O(N) implementation, at the cost
>> of adding an atomic counter, we can convert the call to
>> an atomic_read(). The counter only serves for accounting
>> empty to non-empty transitions, and vice versa; therefore
>> only modified twice for each of the lists during the
>> lifetime of the dlock (while used).
>>
>> In addition, to be able to unaccount a list_del(), we
>> add a dlist pointer to each head, thus minimizing the
>> overall memory footprint.
>>
>> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
>> Acked-by: Waiman Long <longman@redhat.com>
> So I was wondering: Is this really worth it? AFAICS we have a single call
> site for dlock_lists_empty() and that happens during umount where we don't
> really care about this optimization. So it seems like unnecessary
> complication to me at this point? If someone comes up with a usecase that
> needs fast dlock_lists_empty(), then sure, we can do this...
>

Yes, that is true. We can skip this patch for the time being until a use
case comes up which requires dlock_lists_empty() to be used in the fast
path.

Cheers,
Longman