Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Dec 13 2007, Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
> > Jens Axboe wrote:
> > >On Thu, Dec 13 2007, Mark Lord wrote:
> > >>Mark Lord wrote:
> > >>>Jens Axboe wrote:
> > >>>>On Thu, Dec 13 2007, Mark Lord wrote:
> > >>>>>Matthew Wilcox wrote:
> > >>>>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> > >>>>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> > >>>>>>>64KB for libata,
> > >>>>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> > >>>>>>Just a suspicion ... could this be slab vs slub?  ie check your 
> > >>>>>>configs
> > >>>>>>are the same / similar between the two kernels.
> > >>>>>..
> > >>>>>
> > >>>>>Mmmm.. a good thought, that one.
> > >>>>>But I just rechecked, and both have CONFIG_SLAB=y
> > >>>>>
> > >>>>>My guess is that something got changed around when Jens
> > >>>>>reworked the block layer for 2.6.24.
> > >>>>>I'm going to dig around in there now.
> > >>>>I didn't rework the block layer for 2.6.24 :-). The core block layer
> > >>>>changes since 2.6.23 are:
> > >>>>
> > >>>>- Support for empty barriers. Not a likely candidate.
> > >>>>- Shared tag queue fixes. Totally unlikely.
> > >>>>- sg chaining support. Not likely.
> > >>>>- The bio changes from Neil. Of the bunch, the most likely suspects in
> > >>>> this area, since it changes some of the code involved with merges and
> > >>>> blk_rq_map_sg().
> > >>>>- Lots of simple stuff, again very unlikely.
> > >>>>
> > >>>>Anyway, it sounds odd for this to be a block layer problem if you do see
> > >>>>occasional segments being merged. So it sounds more like the input data
> > >>>>having changed.
> > >>>>
> > >>>>Why not just bisect it?
> > >>>..
> > >>>
> > >>>Because the early 2.6.24 series failed to boot on this machine
> > >>>due to bugs in the block layer -- so the code that caused this regression
> > >>>is probably in the stuff from before the kernels became usable here.
> > >>..
> > >>
> > >>That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
> > >>the first couple of -rc* ones failed here because of incompatibilities
> > >>between the block/bio changes and libata.
> > >>
> > >>That's better, I think! 
> > >
> > >No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> > >
> > >If I were you, I'd just start from the first -rc that booted for you. If
> > >THAT has the bug, then we'll think of something else. If you don't get
> > >anywhere, I can run some tests tomorrow and see if I can reproduce it
> > >here.
> > ..
> > 
> > I believe that *anyone* can reproduce it, since it's broken long before
> > the requests ever get to SCSI or libata.  Which also means that *anyone*
> > who wants to can bisect it, as well.
> > 
> > I don't do "bisects".
> 
> It was just a suggestion on how to narrow it down, do as you see fit.
> 
> > But I will dig a bit more and see if I can find the culprit.
> 
> Sure, I'll dig around as well.

Just tried something simple. I only see one 12kb segment so far, so not
a lot by any stretch. I also DONT see any missed merges signs, so it
would appear that the pages in the request are simply not contigious
physically.

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e30b1a4..1e34b6f 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1330,6 +1330,8 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 				goto new_segment;
 
 			sg->length += nbytes;
+			if (sg->length > 8192)
+				printk("sg_len=%d\n", sg->length);
 		} else {
 new_segment:
 			if (!sg)
@@ -1349,6 +1351,8 @@ new_segment:
 				sg = sg_next(sg);
 			}
 
+			if (bvprv && (page_address(bvprv->bv_page) + bvprv->bv_len == page_address(bvec->bv_page)))
+				printk("missed merge\n");
 			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
 			nsegs++;
 		}

-- 
Jens Axboe

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