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From: Davidlohr Bueso <dave@stgolabs.net>
To: Waiman Long <longman@redhat.com>
Cc: Jan Kara <jack@suse.cz>, 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 <dbueso@suse.de>
Subject: Re: [PATCH v9 5/5] lib/dlock-list: Scale dlock_lists_empty()
Date: Tue, 16 Oct 2018 19:51:47 -0700	[thread overview]
Message-ID: <20181017025147.wfk7cktcn3emlb6b@linux-r8p5> (raw)
In-Reply-To: <5bcdf2a2-6d03-df21-934d-6c989549253b@redhat.com>

On Thu, 04 Oct 2018, Waiman Long wrote:

>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.

So fyi I ended up porting the epoll ready-list to this api, where
dlock_lists_empty() performance _does_ matter. However, the list
iteration is common enough operation to put perform the benefits of
the percpu add/delete operations. For example, when sending ready
events to userspace (ep_send_events_proc()), each item must drop the
iter lock, and also do a delete operation -- similarly for checking
for ready events (ep_read_events_proc). This ends hurting more than
benefiting workloads.

Anyway, so yeah I have no need for this patch, and the added complexity +
atomics is unjustified.

Thanks,
Davidlohr

  reply	other threads:[~2018-10-17 10:45 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-12 19:28 [PATCH v9 0/5] vfs: Use dlock list for SB's s_inodes list Waiman Long
2018-09-12 19:28 ` [PATCH v9 1/5] lib/dlock-list: Distributed and lock-protected lists Waiman Long
2018-09-12 19:28 ` [PATCH v9 2/5] vfs: Remove unnecessary list_for_each_entry_safe() variants Waiman Long
2018-09-12 19:28 ` [PATCH v9 3/5] vfs: Use dlock list for superblock's inode list Waiman Long
2018-09-17 14:15   ` Davidlohr Bueso
2018-09-17 14:46     ` Waiman Long
2018-09-12 19:28 ` [PATCH v9 4/5] lib/dlock-list: Make sibling CPUs share the same linked list Waiman Long
2018-09-12 19:28 ` [PATCH v9 5/5] lib/dlock-list: Scale dlock_lists_empty() Waiman Long
2018-10-04  7:16   ` Jan Kara
2018-10-04 13:41     ` Waiman Long
2018-10-17  2:51       ` Davidlohr Bueso [this message]
2018-09-17 15:18 ` [PATCH v9 6/6] prefetch: Remove spin_lock_prefetch() Waiman Long

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