On Tue, May 22 2018 at 2:41am -0400, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Mon, May 21, 2018 at 07:38:55PM -0400, Kent Overstreet wrote: > > On Mon, May 21, 2018 at 02:24:32PM -0400, Mike Snitzer wrote: > > > Every single data structure change in this series should be reviewed for > > > unforeseen alignment consequences. Jens seemed to say that is > > > worthwhile. Not sure if he'll do it or we divide it up. If we divide > > > it up a temp topic branch should be published for others to inspect. > > > > > > Could be alignment hasn't been a historic concern for a bunch of the > > > data structures changed in this series.. if so then all we can do is fix > > > up any obvious potential for false sharing. > > > > Honestly, I almost never worry about alignment... the very few times I do care, > > I use __cacheline_aligned_in_smp. > > And that generally is the right stratgey. If Mike really doesn't want > a change we can just open code the kmalloc for the bio set there, the > important point is that we should not keep the old API around for no > good reason. Again, I never said I didn't want these changes. I just thought it worthwhile to mention the potential for alignment quirks arising. Reality is false sharing is quite uncommon. So my concern was/is pretty niche and unlikely to be applicable. That said, I did take the opportunity to look at all the DM structures that were modified. The bio_set and mempool_t structs are so large that they span multiple cachelines as is. So I focused on eliminating unnecessary spanning of cachelines (by non-bio_set and non-mempool_t members) and eliminated most holes in DM structs. This is the result: http://people.redhat.com/msnitzer/dm-4.18-struct-reorg/ Compare before.txt vs after_kent.txt vs after_mike.txt Nothing staggering or special. Just something I'll add once Kent's latest changes land. But, I also eliminated 2 4-byte holes in struct bio_set, Jens please feel free to pick this up (if you think it useful): From: Mike Snitzer <snitzer@xxxxxxxxxx> Subject: [PATCH] block: eliminate 2 4-byte holes in bio_set structure Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> --- include/linux/bio.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/bio.h b/include/linux/bio.h index 5e472fc..e6fc692 100644 --- a/include/linux/bio.h +++ b/include/linux/bio.h @@ -735,7 +735,6 @@ static inline void bio_inc_remaining(struct bio *bio) struct bio_set { struct kmem_cache *bio_slab; - unsigned int front_pad; mempool_t bio_pool; mempool_t bvec_pool; @@ -743,6 +742,7 @@ struct bio_set { mempool_t bio_integrity_pool; mempool_t bvec_integrity_pool; #endif + unsigned int front_pad; /* * Deadlock avoidance for stacking block drivers: see comments in -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
