Re: [PATCH] make crypto unplug fix V3

From: Chris Mason
Date: Fri Nov 13 2009 - 13:42:22 EST


On Fri, Nov 13, 2009 at 05:34:46PM +0000, Mel Gorman wrote:
> On Fri, Nov 13, 2009 at 07:58:12AM -0500, Chris Mason wrote:
> > This is still likely to set your dm data on fire. It is only meant for
> > testers that start with mkfs and don't have any valuable dm data.
> >
>
> The good news is that my room remains fire-free. Despite swap also
> running from dm-crypt, I had no corruption or instability issues.

Ok, definitely not so convincing I'd try and shove it into a late rc.

>
> Here is an updated set of results for fake-gitk running.
>
> X86
> 2.6.30-0000000-force-highorder Elapsed:12:08.908 Failures:0
> 2.6.31-0000000-force-highorder Elapsed:10:56.283 Failures:0
> 2.6.31-0000006-dm-crypt-unplug Elapsed:11:51.653 Failures:0
> 2.6.31-0000012-pgalloc-2.6.30 Elapsed:12:26.587 Failures:0
> 2.6.31-0000123-congestion-both Elapsed:10:55.298 Failures:0
> 2.6.31-0001234-kswapd-quick-recheck Elapsed:18:01.523 Failures:0
> 2.6.31-0123456-dm-crypt-unplug Elapsed:10:45.720 Failures:0
> 2.6.31-revert-8aa7e847 Elapsed:15:08.020 Failures:0
> 2.6.32-rc6-0000000-force-highorder Elapsed:16:20.765 Failures:4
> 2.6.32-rc6-0000006-dm-crypt-unplug Elapsed:13:42.920 Failures:0
> 2.6.32-rc6-0000012-pgalloc-2.6.30 Elapsed:16:13.380 Failures:1
> 2.6.32-rc6-0000123-congestion-both Elapsed:18:39.118 Failures:0
> 2.6.32-rc6-0001234-kswapd-quick-recheck Elapsed:15:04.398 Failures:0
> 2.6.32-rc6-0123456-dm-crypt-unplug Elapsed:12:50.438 Failures:0
> 2.6.32-rc6-revert-8aa7e847 Elapsed:20:50.888 Failures:0
>
> X86-64
> 2.6.30-0000000-force-highorder Elapsed:10:37.300 Failures:0
> 2.6.31-0000000-force-highorder Elapsed:08:49.338 Failures:0
> 2.6.31-0000006-dm-crypt-unplug Elapsed:09:37.840 Failures:0
> 2.6.31-0000012-pgalloc-2.6.30 Elapsed:15:49.690 Failures:0
> 2.6.31-0000123-congestion-both Elapsed:09:18.790 Failures:0
> 2.6.31-0001234-kswapd-quick-recheck Elapsed:08:39.268 Failures:0
> 2.6.31-0123456-dm-crypt-unplug Elapsed:08:20.965 Failures:0
> 2.6.31-revert-8aa7e847 Elapsed:08:07.457 Failures:0
> 2.6.32-rc6-0000000-force-highorder Elapsed:18:29.103 Failures:1
> 2.6.32-rc6-0000006-dm-crypt-unplug Elapsed:25:53.515 Failures:3
> 2.6.32-rc6-0000012-pgalloc-2.6.30 Elapsed:19:55.570 Failures:6
> 2.6.32-rc6-0000123-congestion-both Elapsed:17:29.255 Failures:2
> 2.6.32-rc6-0001234-kswapd-quick-recheck Elapsed:14:41.068 Failures:0
> 2.6.32-rc6-0123456-dm-crypt-unplug Elapsed:15:48.028 Failures:1
> 2.6.32-rc6-revert-8aa7e847 Elapsed:14:48.647 Failures:0
>
> The numbering in the kernel indicates what patches are applied. I tested
> the dm-crypt patch both in isolation and in combination with the patches
> in this series.
>
> Basically, the dm-crypt-unplug makes a small difference in performance
> overall, mostly slight gains and losses. There was one massive regression
> with the dm-crypt patch applied to 2.6.32-rc6 but at the moment, I don't
> know what that is.

How consistent are your numbers between runs? I was trying to match
this up with your last email and things were pretty different.

>
> In general, the patch reduces the amount of time direct reclaimers are
> spending on congestion_wait.
>
> > It includes my patch from last night, along with changes to force dm to
> > unplug when its IO queues empty.
> >
> > The problem goes like this:
> >
> > Process: submit read bio
> > dm: put bio onto work queue
> > process: unplug
> > dm: work queue finds bio, does a generic_make_request
> >
> > The end result is that we miss the unplug completely. dm-crypt needs to
> > unplug for sync bios. This patch also changes it to unplug whenever the
> > queue is empty, which is far from ideal but better than missing the
> > unplugs.
> >
> > This doesn't completely fix io stalls I'm seeing with dm-crypt, but its
> > my best guess. If it works, I'll break it up and submit for real to
> > the dm people.
> >
>
> Out of curiousity, how are you measuring IO stalls? In the tests I'm doing,
> the worker processes output their progress and it should be at a steady
> rate. I considered a stall to be an excessive delay between updates which
> is a pretty indirect measure.

I just setup a crypto disk and did dd if=/dev/zero of=/mnt/foo bs=1M

If you watch vmstat 1, there's supposed to be a constant steam of IO to
the disk. If a whole second goes by with zero IO, we're doing something
wrong, I get a number of multi-second stalls where we are just waiting
for IO to happen.

Most of the time I was able to catch a sysrq-w for it, someone was
waiting on a read to finish. It isn't completely clear to me if the
unplugging is working properly.

-chris
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