From: Tejun Heo <tj@kernel.org>
To: Kent Overstreet <koverstreet@google.com>
Cc: linux-kernel@vger.kernel.org, linux-aio@kvack.org,
linux-fsdevel@vger.kernel.org, zab@redhat.com, bcrl@kvack.org,
jmoyer@redhat.com, axboe@kernel.dk, viro@zeniv.linux.org.uk,
tytso@mit.edu, Oleg Nesterov <oleg@redhat.com>,
srivatsa.bhat@linux.vnet.ibm.com,
Christoph Lameter <cl@linux.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH 23/32] Generic dynamic per cpu refcounting
Date: Mon, 28 Jan 2013 10:18:39 -0800 [thread overview]
Message-ID: <20130128181839.GB22465@mtj.dyndns.org> (raw)
In-Reply-To: <20130128174858.GW26407@google.com>
Hello, Kent.
On Mon, Jan 28, 2013 at 09:48:58AM -0800, Kent Overstreet wrote:
> > Ooh, I forgot one thing. We might not gain much by replacing file
> > refcnt w/ this. You can't really get cheaper than fget_light().
>
> I've seen fget() show up when profiling the aio code - it's not high
> enough to be a big concern when not doing stupid stuff, but high enough
> that making it percpu would be worth it if it was easy. Which it's not,
> for plenty of reasons.
Yeah, aio wouldn't be able to use fget_light().
> > So, I'm now back to "do we need dynamic allocation". What else do we
> > have to convert?
>
> I dunno. There's a lot of random refcounts scattered around, though.
>
> The way I see it, if it's always percpu when joe random dev needs a
> refcount, he's going to weigh whether the overhead of a percpu refcount
> is worth it.
>
> With dynamic allocation, it's 16 bytes if you don't need it to be
> percpu, vs. 4 for an atomic_t - so you never need to think about it, you
> can just always use this for your refcounts and never have to think
> about if it's going to be a fast path thing or not.
Yes, that is an appealing thought and it would actually improve
something unlike kref silliness. One concern is that when not
converted to percpu, there will be overhead for extra bookkeeping. It
probably can be offset by flipping to percpu mode earlier. Anyways,
it would be great if both the percpu and non-percpu fast paths light.
Thanks.
--
tejun
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next prev parent reply other threads:[~2013-01-28 18:18 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-27 1:59 [PATCH 00/32] AIO performance improvements/cleanups, v3 Kent Overstreet
2012-12-27 1:59 ` [PATCH 01/32] mm: remove old aio use_mm() comment Kent Overstreet
2012-12-27 1:59 ` [PATCH 02/32] aio: remove dead code from aio.h Kent Overstreet
2012-12-27 1:59 ` [PATCH 03/32] gadget: remove only user of aio retry Kent Overstreet
2012-12-27 1:59 ` [PATCH 04/32] aio: remove retry-based AIO Kent Overstreet
2012-12-29 7:36 ` Hillf Danton
2013-01-07 22:12 ` Kent Overstreet
2012-12-29 7:47 ` Hillf Danton
2013-01-07 22:15 ` Kent Overstreet
2012-12-27 1:59 ` [PATCH 05/32] char: add aio_{read,write} to /dev/{null,zero} Kent Overstreet
2012-12-27 1:59 ` [PATCH 06/32] aio: Kill return value of aio_complete() Kent Overstreet
2012-12-27 1:59 ` [PATCH 07/32] aio: kiocb_cancel() Kent Overstreet
2012-12-27 1:59 ` [PATCH 08/32] aio: Move private stuff out of aio.h Kent Overstreet
2012-12-27 1:59 ` [PATCH 09/32] aio: dprintk() -> pr_debug() Kent Overstreet
2012-12-27 1:59 ` [PATCH 10/32] aio: do fget() after aio_get_req() Kent Overstreet
2012-12-27 1:59 ` [PATCH 11/32] aio: Make aio_put_req() lockless Kent Overstreet
2012-12-27 1:59 ` [PATCH 12/32] aio: Refcounting cleanup Kent Overstreet
2012-12-27 1:59 ` [PATCH 13/32] wait: Add wait_event_hrtimeout() Kent Overstreet
2012-12-27 10:37 ` Fubo Chen
2013-01-03 23:08 ` Andrew Morton
2013-01-08 0:09 ` Kent Overstreet
2012-12-27 1:59 ` [PATCH 14/32] aio: Make aio_read_evt() more efficient, convert to hrtimers Kent Overstreet
2013-01-03 23:19 ` Andrew Morton
2013-01-08 0:28 ` Kent Overstreet
2013-01-08 1:00 ` Andrew Morton
2013-01-08 1:28 ` Kent Overstreet
2012-12-27 1:59 ` [PATCH 15/32] aio: Use flush_dcache_page() Kent Overstreet
2012-12-27 1:59 ` [PATCH 16/32] aio: Use cancellation list lazily Kent Overstreet
2012-12-27 1:59 ` [PATCH 17/32] aio: Change reqs_active to include unreaped completions Kent Overstreet
2012-12-27 1:59 ` [PATCH 18/32] aio: Kill batch allocation Kent Overstreet
2012-12-27 1:59 ` [PATCH 19/32] aio: Kill struct aio_ring_info Kent Overstreet
2012-12-27 1:59 ` [PATCH 20/32] aio: Give shared kioctx fields their own cachelines Kent Overstreet
2013-01-03 23:25 ` Andrew Morton
2013-01-07 23:48 ` Kent Overstreet
2012-12-27 2:00 ` [PATCH 21/32] aio: reqs_active -> reqs_available Kent Overstreet
2012-12-27 2:00 ` [PATCH 22/32] aio: percpu reqs_available Kent Overstreet
2012-12-27 2:00 ` [PATCH 23/32] Generic dynamic per cpu refcounting Kent Overstreet
2013-01-03 22:48 ` Andrew Morton
2013-01-07 23:47 ` Kent Overstreet
2013-01-08 1:03 ` [PATCH] percpu-refcount: Sparse fixes Kent Overstreet
2013-01-25 0:51 ` [PATCH 23/32] Generic dynamic per cpu refcounting Tejun Heo
2013-01-25 1:13 ` Kent Overstreet
2013-01-25 2:03 ` Tejun Heo
2013-01-25 2:09 ` Tejun Heo
2013-01-28 17:48 ` Kent Overstreet
2013-01-28 18:18 ` Tejun Heo [this message]
2013-01-25 6:15 ` Rusty Russell
2013-01-28 17:53 ` Kent Overstreet
2013-01-28 17:59 ` Tejun Heo
2013-01-28 18:32 ` Kent Overstreet
2013-01-28 18:57 ` Christoph Lameter
2013-02-08 14:44 ` Tejun Heo
2013-02-08 14:49 ` Jens Axboe
2013-02-08 17:50 ` Andrew Morton
2013-02-08 21:27 ` Kent Overstreet
2013-02-11 14:21 ` Jeff Moyer
2013-02-08 21:17 ` Kent Overstreet
2012-12-27 2:00 ` [PATCH 24/32] aio: Percpu ioctx refcount Kent Overstreet
2012-12-27 2:00 ` [PATCH 25/32] aio: use xchg() instead of completion_lock Kent Overstreet
2013-01-03 23:34 ` Andrew Morton
2013-01-07 23:21 ` Kent Overstreet
2013-01-07 23:35 ` Andrew Morton
2013-01-08 0:01 ` Kent Overstreet
2012-12-27 2:00 ` [PATCH 26/32] aio: Don't include aio.h in sched.h Kent Overstreet
2012-12-27 2:00 ` [PATCH 27/32] aio: Kill ki_key Kent Overstreet
2012-12-27 2:00 ` [PATCH 28/32] aio: Kill ki_retry Kent Overstreet
2012-12-27 2:00 ` [PATCH 29/32] block, aio: Batch completion for bios/kiocbs Kent Overstreet
2013-01-04 9:22 ` Jens Axboe
2013-01-07 23:34 ` Kent Overstreet
2013-01-08 15:33 ` Jeff Moyer
2013-01-08 16:06 ` Kent Overstreet
2013-01-08 16:15 ` Jeff Moyer
2013-01-08 16:48 ` Kent Overstreet
2012-12-27 2:00 ` [PATCH 30/32] virtio-blk: Convert to batch completion Kent Overstreet
2012-12-27 2:00 ` [PATCH 31/32] mtip32xx: " Kent Overstreet
2012-12-27 2:00 ` [PATCH 32/32] aio: Smoosh struct kiocb Kent Overstreet
2013-01-04 9:22 ` [PATCH 00/32] AIO performance improvements/cleanups, v3 Jens Axboe
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