Re: ZFS deadlocks triggered by HDD timeouts

On Wed, Dec 1, 2021 at 3:48 PM Warner Losh <imp@bsdimp.com> wrote:
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> On Wed, Dec 1, 2021, 3:36 PM Alan Somers <asomers@freebsd.org> wrote:
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>> On Wed, Dec 1, 2021 at 2:46 PM Warner Losh <imp@bsdimp.com> wrote:
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>> >
>> >
>> >
>> > On Wed, Dec 1, 2021, 2:36 PM Alan Somers <asomers@freebsd.org> wrote:
>> >>
>> >> On Wed, Dec 1, 2021 at 1:56 PM Warner Losh <imp@bsdimp.com> wrote:
>> >> >
>> >> >
>> >> >
>> >> > On Wed, Dec 1, 2021 at 1:47 PM Alan Somers <asomers@freebsd.org> wrote:
>> >> >>
>> >> >> On Wed, Dec 1, 2021 at 1:37 PM Warner Losh <imp@bsdimp.com> wrote:
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> > On Wed, Dec 1, 2021 at 1:28 PM Alan Somers <asomers@freebsd.org> wrote:
>> >> >> >>
>> >> >> >> On Wed, Dec 1, 2021 at 11:25 AM Warner Losh <imp@bsdimp.com> wrote:
>> >> >> >> >
>> >> >> >> >
>> >> >> >> >
>> >> >> >> > On Wed, Dec 1, 2021, 11:16 AM Alan Somers <asomers@freebsd.org> wrote:
>> >> >> >> >>
>> >> >> >> >> On a stable/13 build from 16-Sep-2021 I see frequent ZFS deadlocks
>> >> >> >> >> triggered by HDD timeouts.  The timeouts are probably caused by
>> >> >> >> >> genuine hardware faults, but they didn't lead to deadlocks in
>> >> >> >> >> 12.2-RELEASE or 13.0-RELEASE.  Unfortunately I don't have much
>> >> >> >> >> additional information.  ZFS's stack traces aren't very informative,
>> >> >> >> >> and dmesg doesn't show anything besides the usual information about
>> >> >> >> >> the disk timeout.  I don't see anything obviously related in the
>> >> >> >> >> commit history for that time range, either.
>> >> >> >> >>
>> >> >> >> >> Has anybody else observed this phenomenon?  Or does anybody have a
>> >> >> >> >> good way to deliberately inject timeouts?  CAM makes it easy enough to
>> >> >> >> >> inject an error, but not a timeout.  If it did, then I could bisect
>> >> >> >> >> the problem.  As it is I can only reproduce it on production servers.
>> >> >> >> >
>> >> >> >> >
>> >> >> >> > What SIM? Timeouts are tricky because they have many sources, some of which are nonlocal...
>> >> >> >> >
>> >> >> >> > Warner
>> >> >> >>
>> >> >> >> mpr(4)
>> >> >> >
>> >> >> >
>> >> >> > Is this just a single drive that's acting up, or is the controller initialized as part of the error recovery?
>> >> >>
>> >> >> I'm not doing anything fancy with mprutil or sas3flash, if that's what
>> >> >> you're asking.
>> >> >
>> >> >
>> >> > No. I'm asking if you've enabled debugging on the recovery messages and see that we enter any kind of
>> >> > controller reset when the timeouts occur.
>> >>
>> >> No.  My CAM setup is the default except that I enabled CAM_IO_STATS
>> >> and changed the following two sysctls:
>> >> kern.cam.da.retry_count=2
>> >> kern.cam.da.default_timeout=10
>> >>
>> >>
>> >> >
>> >> >>
>> >> >> > If a single drive,
>> >> >> > are there multiple timeouts that happen at the same time such that we timeout a request while we're waiting for
>> >> >> > the abort command we send to the firmware to be acknowledged?
>> >> >>
>> >> >> I don't know.
>> >> >
>> >> >
>> >> > OK.
>> >> >
>> >> >>
>> >> >> > Would you be able to run a kgdb script to see
>> >> >> > if you're hitting a situation that I fixed in mpr that would cause I/O to never complete in this rather odd circumstance?
>> >> >> > If you can, and if it is, then there's a change I can MFC :).
>> >> >>
>> >> >> Possibly.  When would I run this kgdb script?  Before ZFS locks up,
>> >> >> after, or while the problematic timeout happens?
>> >> >
>> >> >
>> >> > After the timeouts. I've been doing 'kgdb' followed by 'source mpr-hang.gdb' to run this.
>> >> >
>> >> > What you are looking for is anything with a qfrozen_cnt > 0.. The script is imperfect and racy
>> >> > with normal operations (but not in a bad way), so you may need to run it a couple of times
>> >> > to get consistent data. On my systems, there'd be one or two devices with a frozen count > 1
>> >> > and no I/O happened on those drives and processes hung. That might not be any different than
>> >> > a deadlock :)
>> >> >
>> >> > Warner
>> >> >
>> >> > P.S. here's the mpr-hang.gdb script. Not sure if I can make an attachment survive the mailing lists :)
>> >>
>> >> Thanks, I'll try that.  If this is the problem, do you have any idea
>> >> why it wouldn't happen on 12.2-RELEASE (I haven't seen it on
>> >> 13.0-RELEASE, but maybe I just don't have enough runtime on that
>> >> version).
>> >
>> >
>> > 9781c28c6d63 was merged to stable/13 as a996b55ab34c on Sept 2nd. I fixed a bug
>> > with that version in current as a8837c77efd0, but haven't merged it. I kinda expect that
>> > this might be the cause of the problem. But in Netflix's fleet we've seen this maybe a
>> > couple of times a week over many thousands of machines, so I've been a little cautious
>> > in merging it to make sure that it's really fixed. So far, the jury is out.
>> >
>> > Warner
>>
>> Well, I'm experiencing this error much more frequently than you then.
>> I've seen it on about 10% of similarly-configured servers and they've
>> only been running that release for 1 week.
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> You can run my script soon then to see if it's the same thing.
>
> Warner
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>> -Alan

That confirms it.  I hit the deadlock again, and qfrozen_cnt was
between 1 and 3 for four devices: two da devices (we use multipath)
and their accompanying pass devices.  So I should try merging
a8837c77efd0 next?