From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: [PATCH 00/13] convert block layer to bioset_init()/mempool_init() Date: Mon, 21 May 2018 08:36:46 -0600 Message-ID: <2bbeeb1a-8b99-b06a-eb9b-eb8523c16460@kernel.dk> References: <20180520222558.7053-1-kent.overstreet@gmail.com> <20180521140348.GA19069@redhat.com> <686d7df6-c7d1-48a6-b7ff-48dc8aff6a62@kernel.dk> <20180521143132.GB19194@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180521143132.GB19194@redhat.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Mike Snitzer Cc: Kent Overstreet , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, hch@infradead.org, colyli@suse.de, darrick.wong@oracle.com, clm@fb.com, bacik@fb.com, linux-xfs@vger.kernel.org, drbd-dev@lists.linbit.com, linux-btrfs@vger.kernel.org, linux-raid@vger.kernel.org, neilb@suse.com List-Id: linux-raid.ids On 5/21/18 8:31 AM, Mike Snitzer wrote: > On Mon, May 21 2018 at 10:19am -0400, > Jens Axboe wrote: > >> On 5/21/18 8:03 AM, Mike Snitzer wrote: >>> On Sun, May 20 2018 at 6:25pm -0400, >>> Kent Overstreet wrote: >>> >>>> Jens - this series does the rest of the conversions that Christoph wanted, and >>>> drops bioset_create(). >>>> >>>> Only lightly tested, but the changes are pretty mechanical. Based on your >>>> for-next tree. >>> >>> By switching 'mempool_t *' to 'mempool_t' and 'bio_set *' to 'bio_set' >>> you've altered the alignment of members in data structures. So I'll >>> need to audit all the data structures you've modified in DM. >>> >>> Could we get the backstory on _why_ you're making this change? >>> Would go a long way to helping me appreciate why this is a good use of >>> anyone's time. >> >> Yeah, it's in the first series, it gets rid of a pointer indirection. > > "Allows mempools to be embedded in other structs, getting rid of a > pointer indirection from allocation fastpaths." > > So this is about using contiguous memory or avoiding partial allocation > failure? Or both? > > Or more to it? Just trying to fully appreciate the theory behind the > perceived associated benefit. It's about avoiding a pointer indirection. Instead of having to follow a pointer to get to that struct, it's simple offset math off your main structure. > I do think the increased risk of these embedded bio_set and mempool_t > themselves crossing cachelines, or struct members that follow them doing > so, really detracts from these types of changes. Definitely something to look out for, though most of them should be per-dev structures and not in-flight structures. That makes it a bit less sensitive. But can't hurt to audit the layouts and adjust if necessary. This is why it's posted for review :-) -- Jens Axboe