Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
From:   Jens Axboe <axboe@kernel.dk>
To:     Jeff Moyer <jmoyer@redhat.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dan Carpenter <dan.carpenter@oracle.com>,
        kent.overstreet@gmail.com
References: <20181128183531.5139-1-willy@infradead.org>
 <x49va46e1p0.fsf@segfault.boston.devel.redhat.com>
 <x49pnuee1gm.fsf@segfault.boston.devel.redhat.com>
 <x49mupcm11r.fsf@segfault.boston.devel.redhat.com>
 <20181211175156.GF6830@bombadil.infradead.org>
 <x495zw0lz68.fsf@segfault.boston.devel.redhat.com>
 <0f77a532-0d88-78bc-b9cc-06bb203a0405@kernel.dk>
 <6b9a45c4-47a2-4c44-aa7e-6e5e90eff9df@kernel.dk>
Message-ID: <78dfbfa3-b2d1-4e9d-f59c-b263678a9866@kernel.dk>
Date:   Tue, 11 Dec 2018 11:36:41 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.2.1
MIME-Version: 1.0
In-Reply-To: <6b9a45c4-47a2-4c44-aa7e-6e5e90eff9df@kernel.dk>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 12/11/18 11:32 AM, Jens Axboe wrote:
> On 12/11/18 11:05 AM, Jens Axboe wrote:
>> On 12/11/18 11:02 AM, Jeff Moyer wrote:
>>> Matthew Wilcox <willy@infradead.org> writes:
>>>
>>>> On Tue, Dec 11, 2018 at 12:21:52PM -0500, Jeff Moyer wrote:
>>>>> I'm going to submit this version formally.  If you're interested in
>>>>> converting the ioctx_table to xarray, you can do that separately from a
>>>>> security fix.  I would include a performance analysis with that patch,
>>>>> though.  The idea of using a radix tree for the ioctx table was
>>>>> discarded due to performance reasons--see commit db446a08c23d5 ("aio:
>>>>> convert the ioctx list to table lookup v3").  I suspect using the xarray
>>>>> will perform similarly.
>>>>
>>>> There's a big difference between Octavian's patch and mine.  That patch
>>>> indexed into the radix tree by 'ctx_id' directly, which was pretty
>>>> much guaranteed to exhibit some close-to-worst-case behaviour from the
>>>> radix tree due to IDs being sparsely assigned.  My patch uses the ring
>>>> ID which _we_ assigned, and so is nicely behaved, being usually a very
>>>> small integer.
>>>
>>> OK, good to know.  I obviously didn't look too closely at the two.
>>>
>>>> What performance analysis would you find compelling?  Octavian's original
>>>> fio script:
>>>>
>>>>> rw=randrw; size=256k ;directory=/mnt/fio; ioengine=libaio; iodepth=1
>>>>> blocksize=1024; numjobs=512; thread; loops=100
>>>>>
>>>>> on an EXT2 filesystem mounted on top of a ramdisk
>>>>
>>>> or something else?
>>>
>>> I think the most common use case is a small number of ioctx-s, so I'd
>>> like to see that use case not regress (that should be easy, right?).
>>> Kent, what were the tests you were using when doing this work?  Jens,
>>> since you're doing performance work in this area now, are there any
>>> particular test cases you care about?
>>
>> I can give it a spin, ioctx lookup is in the fast path, and for "classic"
>> aio we do it twice for each IO...
> 
> Don't see any regressions. But if we're fiddling with it anyway, can't
> we do something smarter? Make the fast path just index a table, and put
> all the big hammers in setup/destroy. We're spending a non-substantial
> amount of time doing lookups, that's really no different before and
> after the patch.

Looks like it's the percpu ref get, in terms of "lookup" we already
look pretty good.

-- 
Jens Axboe