Re: dm-crypt parallelization patches

From: Mikulas Patocka
Date: Tue Apr 09 2013 - 16:32:54 EST

On Tue, 9 Apr 2013, Tejun Heo wrote:

> On Tue, Apr 09, 2013 at 03:42:16PM -0400, Mikulas Patocka wrote:
> > If I drop ifdefs, it doesn't compile (because other cgroup stuff it
> > missing).
> >
> > So I enabled bio cgroups.
> >
> > bio_associate_current can't be used, because by the time we allocate the
> > outgoing write bio, we are no longer in the process that submitted the
> > original bio.
> Oh, I suppose it'd need some massaging to selectively turn off the
> cgroup part.
> > Anyway, I tried to reproduce in dm-crypt what bio_associate_current does -
> and we probably need to change that to bio_associate_task().

Generally, we shouldn't associate bios with "current" task in device
mapper targets. For example suppose that we have two stacked dm-crypt

In the "current" process pointer in lower dm-crypt target's request
function always points to the workqueue of the upper dm-crypt target that
submits the bios. So if we associate the bio with "current" in the lower
target, we are associating it with a preallocated workqueue and we already
lost the information who submitted it.

You should associate a bio with a task when you create the bio and "md"
and "dm" midlayers should just forward this association to lower layer

> > in the submitting process I record "ioc" and "css" fields in "dm_crypt_io"
> > structure and set these fields on all outgoing bios. It has no effect on
> > performance, it is as bad as if I hadn't done it.
> A good way to verify that the tagging is correct would be configuring
> io limits in block cgroup and see whether the limits are correctly
> applied when going through dm-crypt (please test with direct-io or
> reads, writeback is horribly broken, sorry).working correctly, maybe
> plugging is the overriding factor?
> Thanks.

It doesn't work because device mapper on the underlying layers ignores
bi_ioc and bi_css.

If I make device mapper forward bi_ioc and bi_css to outgoing bios, it
improves performance (from 2:30 to 1:30), but it is still far from



dm: forward cgroup context

This patch makes dm forward associated cgroup context to cloned bios.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

drivers/md/dm.c | 9 +++++++++
fs/bio.c | 2 ++
2 files changed, 11 insertions(+)

Index: linux-3.8.6-fast/drivers/md/dm.c
--- linux-3.8.6-fast.orig/drivers/md/dm.c 2013-04-09 22:00:36.000000000 +0200
+++ linux-3.8.6-fast/drivers/md/dm.c 2013-04-09 22:19:40.000000000 +0200
@@ -453,6 +453,10 @@ static void free_io(struct mapped_device

static void free_tio(struct mapped_device *md, struct dm_target_io *tio)
+ tio->clone.bi_ioc = NULL;
+ tio->clone.bi_css = NULL;

@@ -1124,6 +1128,11 @@ static struct dm_target_io *alloc_tio(st
clone = bio_alloc_bioset(GFP_NOIO, nr_iovecs, ci->md->bs);
tio = container_of(clone, struct dm_target_io, clone);

+ tio->clone.bi_ioc = ci->bio->bi_ioc;
+ tio->clone.bi_css = ci->bio->bi_css;
tio->io = ci->io;
tio->ti = ti;
memset(&tio->info, 0, sizeof(tio->info));
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