Re: [PATCH v2 00/28] The new cgroup slab memory controller

[Bharata B Rao]

On Tue, Sep 01, 2020 at 08:52:05AM -0400, Pavel Tatashin wrote:
> On Tue, Sep 1, 2020 at 1:28 AM Bharata B Rao <bharata@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Aug 28, 2020 at 12:47:03PM -0400, Pavel Tatashin wrote:
> > > There appears to be another problem that is related to the
> > > cgroup_mutex -> mem_hotplug_lock deadlock described above.
> > >
> > > In the original deadlock that I described, the workaround is to
> > > replace crash dump from piping to Linux traditional save to files
> > > method. However, after trying this workaround, I still observed
> > > hardware watchdog resets during machine  shutdown.
> > >
> > > The new problem occurs for the following reason: upon shutdown systemd
> > > calls a service that hot-removes memory, and if hot-removing fails for
> > > some reason systemd kills that service after timeout. However, systemd
> > > is never able to kill the service, and we get hardware reset caused by
> > > watchdog or a hang during shutdown:
> > >
> > > Thread #1: memory hot-remove systemd service
> > > Loops indefinitely, because if there is something still to be migrated
> > > this loop never terminates. However, this loop can be terminated via
> > > signal from systemd after timeout.
> > > __offline_pages()
> > >       do {
> > >           pfn = scan_movable_pages(pfn, end_pfn);
> > >                   # Returns 0, meaning there is nothing available to
> > >                   # migrate, no page is PageLRU(page)
> > >           ...
> > >           ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
> > >                                             NULL, check_pages_isolated_cb);
> > >                   # Returns -EBUSY, meaning there is at least one PFN that
> > >                   # still has to be migrated.
> > >       } while (ret);
> > >
> > > Thread #2: ccs killer kthread
> > >    css_killed_work_fn
> > >      cgroup_mutex  <- Grab this Mutex
> > >      mem_cgroup_css_offline
> > >        memcg_offline_kmem.part
> > >           memcg_deactivate_kmem_caches
> > >             get_online_mems
> > >               mem_hotplug_lock <- waits for Thread#1 to get read access
> > >
> > > Thread #3: systemd
> > > ksys_read
> > >  vfs_read
> > >    __vfs_read
> > >      seq_read
> > >        proc_single_show
> > >          proc_cgroup_show
> > >            mutex_lock -> wait for cgroup_mutex that is owned by Thread #2
> > >
> > > Thus, thread #3 systemd stuck, and unable to deliver timeout interrupt
> > > to thread #1.
> > >
> > > The proper fix for both of the problems is to avoid cgroup_mutex ->
> > > mem_hotplug_lock ordering that was recently fixed in the mainline but
> > > still present in all stable branches. Unfortunately, I do not see a
> > > simple fix in how to remove mem_hotplug_lock from
> > > memcg_deactivate_kmem_caches without using Roman's series that is too
> > > big for stable.
> >
> > We too are seeing this on Power systems when stress-testing memory
> > hotplug, but with the following call trace (from hung task timer)
> > instead of Thread #2 above:
> >
> > __switch_to
> > __schedule
> > schedule
> > percpu_rwsem_wait
> > __percpu_down_read
> > get_online_mems
> > memcg_create_kmem_cache
> > memcg_kmem_cache_create_func
> > process_one_work
> > worker_thread
> > kthread
> > ret_from_kernel_thread
> >
> > While I understand that Roman's new slab controller patchset will fix
> > this, I also wonder if infinitely looping in the memory unplug path
> > with mem_hotplug_lock held is the right thing to do? Earlier we had
> > a few other exit possibilities in this path (like max retries etc)
> > but those were removed by commits:
> >
> > 72b39cfc4d75: mm, memory_hotplug: do not fail offlining too early
> > ecde0f3e7f9e: mm, memory_hotplug: remove timeout from __offline_memory
> >
> > Or, is the user-space test is expected to induce a signal back-off when
> > unplug doesn't complete within a reasonable amount of time?
> 
> Hi Bharata,
> 
> Thank you for your input, it looks like you are experiencing the same
> problems that I observed.
> 
> What I found is that the reason why our machines did not complete
> hot-remove within the given time is because of this bug:
> https://lore.kernel.org/linux-mm/20200901124615.137200-1-pasha.tatashin@xxxxxxxxxx
> 
> Could you please try it and see if that helps for your case?

I am on an old codebase that already has the fix that you are proposing,
so I might be seeing someother issue which I will debug further.

So looks like the loop in __offline_pages() had a call to
drain_all_pages() before it was removed by

c52e75935f8d: mm: remove extra drain pages on pcp list

Regards,
Bharata.