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

[Roman Gushchin]

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 Pavel,

wow, it's a quite complicated deadlock. Thank you for providing
a perfect analysis!

Unfortunately, backporting the whole new slab controller isn't an option:
it's way too big and invasive.
Do you already have a standalone fix?

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@xxxxxx> 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