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

[Pavel Tatashin <pasha.tatashin@soleen.com>]

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.

Thanks,
Pasha

On Wed, Aug 12, 2020 at 8:31 PM Pavel Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> On Wed, Aug 12, 2020 at 8:04 PM Roman Gushchin <guro@fb.com> wrote:
> >
> > On Wed, Aug 12, 2020 at 07:16:08PM -0400, Pavel Tatashin wrote:
> > > Guys,
> > >
> > > There is a convoluted deadlock that I just root caused, and that is
> > > fixed by this work (at least based on my code inspection it appears to
> > > be fixed); but the deadlock exists in older and stable kernels, and I
> > > am not sure whether to create a separate patch for it, or backport
> > > this whole thing.
> >
>
> Hi Roman,
>
> > Hi Pavel,
> >
> > wow, it's a quite complicated deadlock. Thank you for providing
> > a perfect analysis!
>
> Thank you, it indeed took me a while to fully grasp the deadlock.
>
> >
> > Unfortunately, backporting the whole new slab controller isn't an option:
> > it's way too big and invasive.
>
> This is what I thought as well, this is why I want to figure out what
> is the best way forward.
>
> > Do you already have a standalone fix?
>
> Not yet, I do not have a standalone fix. I suspect the best fix would
> be to address fix css_killed_work_fn() stack so we never have:
> cgroup_mutex -> mem_hotplug_lock. Either decoupling them or reverse
> the order would work. If you have suggestions since you worked on this
> code recently, please let me know.
>
> Thank you,
> Pasha
>
> >
> > Thanks!
> >
> >
> > >
> > > Thread #1: Hot-removes memory
> > > device_offline
> > >   memory_subsys_offline
> > >     offline_pages
> > >       __offline_pages
> > >         mem_hotplug_lock <- write access
> > >       waits for Thread #3 refcnt for pfn 9e5113 to get to 1 so it can
> > > migrate it.
> > >
> > > 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: crashing userland program
> > > do_coredump
> > >   elf_core_dump
> > >       get_dump_page() -> get page with pfn#9e5113, and increment refcnt
> > >       dump_emit
> > >         __kernel_write
> > >           __vfs_write
> > >             new_sync_write
> > >               pipe_write
> > >                 pipe_wait   -> waits for Thread #4 systemd-coredump to
> > > read the pipe
> > >
> > > Thread #4: systemd-coredump
> > > ksys_read
> > >   vfs_read
> > >     __vfs_read
> > >       seq_read
> > >         proc_single_show
> > >           proc_cgroup_show
> > >             cgroup_mutex -> waits from Thread #2 for this lock.
> >
> > >
> > > In Summary:
> > > Thread#1 waits for Thread#3 for refcnt, Thread#3 waits for Thread#4 to
> > > read pipe. Thread#4 waits for Thread#2 for cgroup_mutex lock; Thread#2
> > > waits for Thread#1 for mem_hotplug_lock rwlock.
> > >
> > > This work appears to fix this deadlock because cgroup_mutex is not
> > > called anymore before mem_hotplug_lock (unless I am missing it), as it
> > > removes memcg_deactivate_kmem_caches.
> > >
> > > Thank you,
> > > Pasha
> > >
> > > On Wed, Jan 29, 2020 at 9:42 PM Roman Gushchin <guro@fb.com> wrote:
> > > >
> > > > On Thu, Jan 30, 2020 at 07:36:26AM +0530, Bharata B Rao wrote:
> > > > > On Mon, Jan 27, 2020 at 09:34:25AM -0800, Roman Gushchin wrote:
> > > > > > The existing cgroup slab memory controller is based on the idea of
> > > > > > replicating slab allocator internals for each memory cgroup.
> > > > > > This approach promises a low memory overhead (one pointer per page),
> > > > > > and isn't adding too much code on hot allocation and release paths.
> > > > > > But is has a very serious flaw: it leads to a low slab utilization.
> > > > > >
> > > > > > Using a drgn* script I've got an estimation of slab utilization on
> > > > > > a number of machines running different production workloads. In most
> > > > > > cases it was between 45% and 65%, and the best number I've seen was
> > > > > > around 85%. Turning kmem accounting off brings it to high 90s. Also
> > > > > > it brings back 30-50% of slab memory. It means that the real price
> > > > > > of the existing slab memory controller is way bigger than a pointer
> > > > > > per page.
> > > > > >
> > > > > > The real reason why the existing design leads to a low slab utilization
> > > > > > is simple: slab pages are used exclusively by one memory cgroup.
> > > > > > If there are only few allocations of certain size made by a cgroup,
> > > > > > or if some active objects (e.g. dentries) are left after the cgroup is
> > > > > > deleted, or the cgroup contains a single-threaded application which is
> > > > > > barely allocating any kernel objects, but does it every time on a new CPU:
> > > > > > in all these cases the resulting slab utilization is very low.
> > > > > > If kmem accounting is off, the kernel is able to use free space
> > > > > > on slab pages for other allocations.
> > > > > >
> > > > > > Arguably it wasn't an issue back to days when the kmem controller was
> > > > > > introduced and was an opt-in feature, which had to be turned on
> > > > > > individually for each memory cgroup. But now it's turned on by default
> > > > > > on both cgroup v1 and v2. And modern systemd-based systems tend to
> > > > > > create a large number of cgroups.
> > > > > >
> > > > > > This patchset provides a new implementation of the slab memory controller,
> > > > > > which aims to reach a much better slab utilization by sharing slab pages
> > > > > > between multiple memory cgroups. Below is the short description of the new
> > > > > > design (more details in commit messages).
> > > > > >
> > > > > > Accounting is performed per-object instead of per-page. Slab-related
> > > > > > vmstat counters are converted to bytes. Charging is performed on page-basis,
> > > > > > with rounding up and remembering leftovers.
> > > > > >
> > > > > > Memcg ownership data is stored in a per-slab-page vector: for each slab page
> > > > > > a vector of corresponding size is allocated. To keep slab memory reparenting
> > > > > > working, instead of saving a pointer to the memory cgroup directly an
> > > > > > intermediate object is used. It's simply a pointer to a memcg (which can be
> > > > > > easily changed to the parent) with a built-in reference counter. This scheme
> > > > > > allows to reparent all allocated objects without walking them over and
> > > > > > changing memcg pointer to the parent.
> > > > > >
> > > > > > Instead of creating an individual set of kmem_caches for each memory cgroup,
> > > > > > two global sets are used: the root set for non-accounted and root-cgroup
> > > > > > allocations and the second set for all other allocations. This allows to
> > > > > > simplify the lifetime management of individual kmem_caches: they are
> > > > > > destroyed with root counterparts. It allows to remove a good amount of code
> > > > > > and make things generally simpler.
> > > > > >
> > > > > > The patchset* has been tested on a number of different workloads in our
> > > > > > production. In all cases it saved significant amount of memory, measured
> > > > > > from high hundreds of MBs to single GBs per host. On average, the size
> > > > > > of slab memory has been reduced by 35-45%.
> > > > >
> > > > > Here are some numbers from multiple runs of sysbench and kernel compilation
> > > > > with this patchset on a 10 core POWER8 host:
> > > > >
> > > > > ==========================================================================
> > > > > Peak usage of memory.kmem.usage_in_bytes, memory.usage_in_bytes and
> > > > > meminfo:Slab for Sysbench oltp_read_write with mysqld running as part
> > > > > of a mem cgroup (Sampling every 5s)
> > > > > --------------------------------------------------------------------------
> > > > >                               5.5.0-rc7-mm1   +slab patch     %reduction
> > > > > --------------------------------------------------------------------------
> > > > > memory.kmem.usage_in_bytes    15859712        4456448         72
> > > > > memory.usage_in_bytes         337510400       335806464       .5
> > > > > Slab: (kB)                    814336          607296          25
> > > > >
> > > > > memory.kmem.usage_in_bytes    16187392        4653056         71
> > > > > memory.usage_in_bytes         318832640       300154880       5
> > > > > Slab: (kB)                    789888          559744          29
> > > > > --------------------------------------------------------------------------
> > > > >
> > > > >
> > > > > Peak usage of memory.kmem.usage_in_bytes, memory.usage_in_bytes and
> > > > > meminfo:Slab for kernel compilation (make -s -j64) Compilation was
> > > > > done from bash that is in a memory cgroup. (Sampling every 5s)
> > > > > --------------------------------------------------------------------------
> > > > >                               5.5.0-rc7-mm1   +slab patch     %reduction
> > > > > --------------------------------------------------------------------------
> > > > > memory.kmem.usage_in_bytes    338493440       231931904       31
> > > > > memory.usage_in_bytes         7368015872      6275923968      15
> > > > > Slab: (kB)                    1139072         785408          31
> > > > >
> > > > > memory.kmem.usage_in_bytes    341835776       236453888       30
> > > > > memory.usage_in_bytes         6540427264      6072893440      7
> > > > > Slab: (kB)                    1074304         761280          29
> > > > >
> > > > > memory.kmem.usage_in_bytes    340525056       233570304       31
> > > > > memory.usage_in_bytes         6406209536      6177357824      3
> > > > > Slab: (kB)                    1244288         739712          40
> > > > > --------------------------------------------------------------------------
> > > > >
> > > > > Slab consumption right after boot
> > > > > --------------------------------------------------------------------------
> > > > >                               5.5.0-rc7-mm1   +slab patch     %reduction
> > > > > --------------------------------------------------------------------------
> > > > > Slab: (kB)                    821888          583424          29
> > > > > ==========================================================================
> > > > >
> > > > > Summary:
> > > > >
> > > > > With sysbench and kernel compilation,  memory.kmem.usage_in_bytes shows
> > > > > around 70% and 30% reduction consistently.
> > > > >
> > > > > Didn't see consistent reduction of memory.usage_in_bytes with sysbench and
> > > > > kernel compilation.
> > > > >
> > > > > Slab usage (from /proc/meminfo) shows consistent 30% reduction and the
> > > > > same is seen right after boot too.
> > > >
> > > > That's just perfect!
> > > >
> > > > memory.usage_in_bytes was most likely the same because the freed space
> > > > was taken by pagecache.
> > > >
> > > > Thank you very much for testing!
> > > >
> > > > Roman