On Mon, May 21 2018 at 11:09am -0400, Jens Axboe <axboe@xxxxxxxxx> wrote: > On 5/21/18 9:04 AM, Mike Snitzer wrote: > > On Mon, May 21 2018 at 10:52am -0400, > > Jens Axboe <axboe@xxxxxxxxx> wrote: > > > >> On 5/21/18 8:47 AM, Mike Snitzer wrote: > >>> On Mon, May 21 2018 at 10:36am -0400, > >>> Jens Axboe <axboe@xxxxxxxxx> wrote: > >>> > >>>> On 5/21/18 8:31 AM, Mike Snitzer wrote: > >>>>> On Mon, May 21 2018 at 10:19am -0400, > >>>>> Jens Axboe <axboe@xxxxxxxxx> wrote: > >>>>> > >>>>>> On 5/21/18 8:03 AM, Mike Snitzer wrote: > >>>>>>> On Sun, May 20 2018 at 6:25pm -0400, > >>>>>>> Kent Overstreet <kent.overstreet@xxxxxxxxx> 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". > > I'm not making this personal at all?! You raised a (valid) concern, > I'm merely stating why I don't think it's a high risk issue. I'm > assuming your worry is related to dm, as those are the reports > that would ultimately land on your desk. Then we'll just agree to disagree with what this implies: "It's not like this was pushed behind your back." Reality is I'm fine with the change. Just think there is follow-on work (now or later) that is needed. Enough said.
