From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: [PATCH 00/13] convert block layer to bioset_init()/mempool_init() Date: Mon, 21 May 2018 11:04:40 -0400 Message-ID: <20180521150439.GA19379@redhat.com> References: <20180520222558.7053-1-kent.overstreet@gmail.com> <20180521140348.GA19069@redhat.com> <686d7df6-c7d1-48a6-b7ff-48dc8aff6a62@kernel.dk> <20180521143132.GB19194@redhat.com> <2bbeeb1a-8b99-b06a-eb9b-eb8523c16460@kernel.dk> <20180521144703.GA19303@redhat.com> <4b343aef-e11c-73ba-1d88-7e73ca838cad@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <4b343aef-e11c-73ba-1d88-7e73ca838cad-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: drbd-dev-bounces-cunTk1MwBs8qoQakbn7OcQ@public.gmane.org Errors-To: drbd-dev-bounces-cunTk1MwBs8qoQakbn7OcQ@public.gmane.org To: Jens Axboe Cc: hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org, linux-raid-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, colyli-l3A5Bk7waGM@public.gmane.org, linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, clm-b10kYP2dOMg@public.gmane.org, neilb-IBi9RG/b67k@public.gmane.org, bacik-b10kYP2dOMg@public.gmane.org, Kent Overstreet , linux-btrfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, drbd-dev-cunTk1MwBs8qoQakbn7OcQ@public.gmane.org List-Id: linux-raid.ids On Mon, May 21 2018 at 10:52am -0400, Jens Axboe wrote: > On 5/21/18 8:47 AM, Mike Snitzer wrote: > > On Mon, May 21 2018 at 10:36am -0400, > > Jens Axboe wrote: > > > >> 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 :-) > > > > This isn't something that is easily caught upfront. Yes we can all be > > busy little beavers with pahole to audit alignment. But chances are > > most people won't do it. > > > > Reality is there is potential for a regression due to false sharing to > > creep in if a hot struct member suddenly starts straddling a cacheline. > > That type of NUMA performance killer is pretty insidious and somewhat > > tedious to hunt down even when looking for it with specialized tools: > > https://joemario.github.io/blog/2016/09/01/c2c-blog/ > > IMHO you're making a big deal out of something that should not be. I raised an issue that had seemingly not been considered at all. Not making a big deal. Raising it for others' benefit. > If the dm bits are that sensitive and cache line honed to perfection > already due to previous regressions in that area, then it might > not be a bad idea to have some compile checks for false cacheline > sharing between sensitive members, or spilling of a sub-struct > into multiple cachelines. > > It's not like this was pushed behind your back. It's posted for > review. It's quite possible the net change is a win for dm. Let's > focus on getting it reviewed, rather than pontificate on what > could potentially go all wrong with this. Why are you making this personal? Or purely about DM? I'm merely pointing out this change isn't something that can be given a quick blanket "looks good". Mike From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 21 May 2018 11:04:40 -0400 From: Mike Snitzer To: Jens Axboe 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 Subject: Re: [PATCH 00/13] convert block layer to bioset_init()/mempool_init() Message-ID: <20180521150439.GA19379@redhat.com> References: <20180520222558.7053-1-kent.overstreet@gmail.com> <20180521140348.GA19069@redhat.com> <686d7df6-c7d1-48a6-b7ff-48dc8aff6a62@kernel.dk> <20180521143132.GB19194@redhat.com> <2bbeeb1a-8b99-b06a-eb9b-eb8523c16460@kernel.dk> <20180521144703.GA19303@redhat.com> <4b343aef-e11c-73ba-1d88-7e73ca838cad@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <4b343aef-e11c-73ba-1d88-7e73ca838cad@kernel.dk> List-ID: On Mon, May 21 2018 at 10:52am -0400, Jens Axboe wrote: > On 5/21/18 8:47 AM, Mike Snitzer wrote: > > On Mon, May 21 2018 at 10:36am -0400, > > Jens Axboe wrote: > > > >> 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 :-) > > > > This isn't something that is easily caught upfront. Yes we can all be > > busy little beavers with pahole to audit alignment. But chances are > > most people won't do it. > > > > Reality is there is potential for a regression due to false sharing to > > creep in if a hot struct member suddenly starts straddling a cacheline. > > That type of NUMA performance killer is pretty insidious and somewhat > > tedious to hunt down even when looking for it with specialized tools: > > https://joemario.github.io/blog/2016/09/01/c2c-blog/ > > IMHO you're making a big deal out of something that should not be. I raised an issue that had seemingly not been considered at all. Not making a big deal. Raising it for others' benefit. > If the dm bits are that sensitive and cache line honed to perfection > already due to previous regressions in that area, then it might > not be a bad idea to have some compile checks for false cacheline > sharing between sensitive members, or spilling of a sub-struct > into multiple cachelines. > > It's not like this was pushed behind your back. It's posted for > review. It's quite possible the net change is a win for dm. Let's > focus on getting it reviewed, rather than pontificate on what > could potentially go all wrong with this. Why are you making this personal? Or purely about DM? I'm merely pointing out this change isn't something that can be given a quick blanket "looks good". Mike