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SubjectRe: [patch] mm, oom: make a last minute check to prevent unnecessary memcg oom kills
On Tue, 10 Mar 2020, Michal Hocko wrote:

> On Tue 10-03-20 14:55:50, David Rientjes wrote:
> > Killing a user process as a result of hitting memcg limits is a serious
> > decision that is unfortunately needed only when no forward progress in
> > reclaiming memory can be made.
> >
> > Deciding the appropriate oom victim can take a sufficient amount of time
> > that allows another process that is exiting to actually uncharge to the
> > same memcg hierarchy and prevent unnecessarily killing user processes.
> >
> > An example is to prevent *multiple* unnecessary oom kills on a system
> > with two cores where the oom kill occurs when there is an abundance of
> > free memory available:
> >
> > Memory cgroup out of memory: Killed process 628 (repro) total-vm:41944kB, anon-rss:40888kB, file-rss:496kB, shmem-rss:0kB, UID:0 pgtables:116kB oom_score_adj:0
> > <immediately after>
> > repro invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=0, oom_score_adj=0
> > CPU: 1 PID: 629 Comm: repro Not tainted 5.6.0-rc5+ #130
> > Call Trace:
> > dump_stack+0x78/0xb6
> > dump_header+0x55/0x240
> > oom_kill_process+0xc5/0x170
> > out_of_memory+0x305/0x4a0
> > try_charge+0x77b/0xac0
> > mem_cgroup_try_charge+0x10a/0x220
> > mem_cgroup_try_charge_delay+0x1e/0x40
> > handle_mm_fault+0xdf2/0x15f0
> > do_user_addr_fault+0x21f/0x420
> > async_page_fault+0x2f/0x40
> > memory: usage 61336kB, limit 102400kB, failcnt 74
> >
> > Notice the second memcg oom kill shows usage is >40MB below its limit of
> > 100MB but a process is still unnecessarily killed because the decision has
> > already been made to oom kill by calling out_of_memory() before the
> > initial victim had a chance to uncharge its memory.
>
> Could you be more specific about the specific workload please?
>

Robert, could you elaborate on the user-visible effects of this issue that
caused it to initially get reported?

> > Make a last minute check to determine if an oom kill is really needed to
> > prevent unnecessary oom killing.
>
> I really see no reason why the memcg oom should behave differently from
> the global case. In both cases there will be a point of no return.
> Where-ever it is done it will be racy and the oom victim selection will
> play the race window role. There is simply no way around that without
> making the whole thing completely synchronous. This all looks like a
> micro optimization and I would really like to see a relevant real world
> usecase presented before new special casing is added.
>

The patch certainly prevents unnecessary oom kills when there is a pending
victim that uncharges its memory between invoking the oom killer and
finding MMF_OOM_SKIP in the list of eligible tasks and its much more
common on systems with limited cpu cores.

Adding support for the global case is more difficult because we rely on
multiple heuristics, not only watermarks, to determine if we can allocate.
It would likely require using a lot of the logic from the page allocator
(alloc flags, watermark check, mempolicy awareness, cpusets) to make it
work reliably and not miss a corner-case where we actually don't end up on
oom killing anything at all.

Memcg charging, on the other hand, is much simpler as exhibited by this
patch since it's only about the number of pages to charge and avoiding
unnecessarily oom killing a user process is of paramount importance to
that user.

Basically: it becomes very difficult for a cloud provider to say "look,
your process was oom killed and it shows usage is 60MB in a memcg limited
to 100MB" :)

> >
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: David Rientjes <rientjes@google.com>
> > ---
> > include/linux/memcontrol.h | 7 +++++++
> > mm/memcontrol.c | 2 +-
> > mm/oom_kill.c | 16 +++++++++++++---
> > 3 files changed, 21 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -445,6 +445,8 @@ void mem_cgroup_iter_break(struct mem_cgroup *, struct mem_cgroup *);
> > int mem_cgroup_scan_tasks(struct mem_cgroup *,
> > int (*)(struct task_struct *, void *), void *);
> >
> > +unsigned long mem_cgroup_margin(struct mem_cgroup *memcg);
> > +
> > static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
> > {
> > if (mem_cgroup_disabled())
> > @@ -945,6 +947,11 @@ static inline int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
> > return 0;
> > }
> >
> > +static inline unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
> > +{
> > + return 0;
> > +}
> > +
> > static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
> > {
> > return 0;
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1286,7 +1286,7 @@ void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru,
> > * Returns the maximum amount of memory @mem can be charged with, in
> > * pages.
> > */
> > -static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
> > +unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
> > {
> > unsigned long margin = 0;
> > unsigned long count;
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -972,9 +972,6 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> > }
> > task_unlock(victim);
> >
> > - if (__ratelimit(&oom_rs))
> > - dump_header(oc, victim);
> > -
> > /*
> > * Do we need to kill the entire memory cgroup?
> > * Or even one of the ancestor memory cgroups?
> > @@ -982,6 +979,19 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> > */
> > oom_group = mem_cgroup_get_oom_group(victim, oc->memcg);
> >
> > + if (is_memcg_oom(oc)) {
> > + cond_resched();
> > +
> > + /* One last check: do we *really* need to kill? */
> > + if (mem_cgroup_margin(oc->memcg) >= (1 << oc->order)) {
> > + put_task_struct(victim);
> > + return;
> > + }
> > + }
> > +
> > + if (__ratelimit(&oom_rs))
> > + dump_header(oc, victim);
> > +
> > __oom_kill_process(victim, message);
> >
> > /*
>
> --
> Michal Hocko
> SUSE Labs
>

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