Comment 54 for bug 1328727

On Tue 02-04-19 16:25:00, Andrew Morton wrote:
>
> I cc'ed a bunch of people from bugzilla.
>
> Folks, please please please remember to reply via emailed
> reply-to-all. Don't use the bugzilla interface!
>
> On Mon, 16 Jun 2014 18:29:26 +0200 "Rafael J. Wysocki"
> <email address hidden> wrote:
>
> > On 6/13/2014 6:55 AM, Johannes Weiner wrote:
> > > On Fri, Jun 13, 2014 at 01:50:47AM +0200, Rafael J. Wysocki wrote:
> > >> On 6/13/2014 12:02 AM, Johannes Weiner wrote:
> > >>> On Tue, May 06, 2014 at 01:45:01AM +0200, Rafael J. Wysocki wrote:
> > >>>> On 5/6/2014 1:33 AM, Johannes Weiner wrote:
> > >>>>> Hi Oliver,
> > >>>>>
> > >>>>> On Mon, May 05, 2014 at 11:00:13PM +0200, Oliver Winker wrote:
> > >>>>>> Hello,
> > >>>>>>
> > >>>>>> 1) Attached a full function-trace log + other SysRq outputs, see [1]
> > >>>>>> attached.
> > >>>>>>
> > >>>>>> I saw bdi_...() calls in the s2disk paths, but didn't check in
> detail
> > >>>>>> Probably more efficient when one of you guys looks directly.
> > >>>>> Thanks, this looks interesting. balance_dirty_pages() wakes up the
> > >>>>> bdi_wq workqueue as it should:
> > >>>>>
> > >>>>> [ 249.148009] s2disk-3327 2.... 48550413us :
> global_dirty_limits <-balance_dirty_pages_ratelimited
> > >>>>> [ 249.148009] s2disk-3327 2.... 48550414us :
> global_dirtyable_memory <-global_dirty_limits
> > >>>>> [ 249.148009] s2disk-3327 2.... 48550414us :
> writeback_in_progress <-balance_dirty_pages_ratelimited
> > >>>>> [ 249.148009] s2disk-3327 2.... 48550414us :
> bdi_start_background_writeback <-balance_dirty_pages_ratelimited
> > >>>>> [ 249.148009] s2disk-3327 2.... 48550414us :
> mod_delayed_work_on <-balance_dirty_pages_ratelimited
> > >>>>> but the worker wakeup doesn't actually do anything:
> > >>>>> [ 249.148009] kworker/-3466 2d... 48550431us : finish_task_switch
> <-__schedule
> > >>>>> [ 249.148009] kworker/-3466 2.... 48550431us : _raw_spin_lock_irq
> <-worker_thread
> > >>>>> [ 249.148009] kworker/-3466 2d... 48550431us :
> need_to_create_worker <-worker_thread
> > >>>>> [ 249.148009] kworker/-3466 2d... 48550432us : worker_enter_idle
> <-worker_thread
> > >>>>> [ 249.148009] kworker/-3466 2d... 48550432us : too_many_workers
> <-worker_enter_idle
> > >>>>> [ 249.148009] kworker/-3466 2.... 48550432us : schedule
> <-worker_thread
> > >>>>> [ 249.148009] kworker/-3466 2.... 48550432us : __schedule
> <-worker_thread
> > >>>>>
> > >>>>> My suspicion is that this fails because the bdi_wq is frozen at this
> > >>>>> point and so the flush work never runs until resume, whereas before
> my
> > >>>>> patch the effective dirty limit was high enough so that image could
> be
> > >>>>> written in one go without being throttled; followed by an fsync()
> that
> > >>>>> then writes the pages in the context of the unfrozen s2disk.
> > >>>>>
> > >>>>> Does this make sense? Rafael? Tejun?
> > >>>> Well, it does seem to make sense to me.
> > >>> From what I see, this is a deadlock in the userspace suspend model and
> > >>> just happened to work by chance in the past.
> > >> Well, it had been working for quite a while, so it was a rather large
> > >> opportunity
> > >> window it seems. :-)
> > > No doubt about that, and I feel bad that it broke. But it's still a
> > > deadlock that can't reasonably be accommodated from dirty throttling.
> > >
> > > It can't just put the flushers to sleep and then issue a large amount
> > > of buffered IO, hoping it doesn't hit the dirty limits. Don't shoot
> > > the messenger, this bug needs to be addressed, not get papered over.
> > >
> > >>> Can we patch suspend-utils as follows?
> > >> Perhaps we can. Let's ask the new maintainer.
> > >>
> > >> Rodolfo, do you think you can apply the patch below to suspend-utils?
> > >>
> > >>> Alternatively, suspend-utils
> > >>> could clear the dirty limits before it starts writing and restore them
> > >>> post-resume.
> > >> That (and the patch too) doesn't seem to address the problem with
> existing
> > >> suspend-utils
> > >> binaries, however.
> > > It's userspace that freezes the system before issuing buffered IO, so
> > > my conclusion was that the bug is in there. This is arguable. I also
> > > wouldn't be opposed to a patch that sets the dirty limits to infinity
> > > from the ioctl that freezes the system or creates the image.
> >
> > OK, that sounds like a workable plan.
> >
> > How do I set those limits to infinity?
>
> Five years have passed and people are still hitting this.
>
> Killian described the workaround in comment 14 at
> https://bugzilla.kernel.org/show_bug.cgi?id=75101.
>
> People can use this workaround manually by hand or in scripts. But we
> really should find a proper solution. Maybe special-case the freezing
> of the flusher threads until all the writeout has completed. Or
> something else.

I've refreshed my memory wrt this bug and I believe the bug is really on
the side of suspend-utils (uswsusp or however it is called). They are low
level system tools, they ask the kernel to freeze all processes
(SNAPSHOT_FREEZE ioctl), and then they rely on buffered writeback (which is
relatively heavyweight infrastructure) to work. That is wrong in my
opinion.

I can see Johanness was suggesting in comment 11 to use O_SYNC in
suspend-utils which worked but was too slow. Indeed O_SYNC is rather big
hammer but using O_DIRECT should be what they need and get better
performance - no additional buffering in the kernel, no dirty throttling,
etc. They only need their buffer & device offsets sector aligned - they
seem to be even page aligned in suspend-utils so they should be fine. And
if the performance still sucks (currently they appear to do mostly random
4k writes so it probably would for rotating disks), they could use AIO DIO
to get multiple pages in flight (as many as they dare to allocate buffers)
and then the IO scheduler will reorder things as good as it can and they
should get reasonable performance.

Is there someone who works on suspend-utils these days? Because the repo
I've found on kernel.org seems to be long dead (last commit in 2012).

        Honza