Re: [patch] mm, oom: make a last minute check to prevent unnecessary memcg oom kills

[Michal Hocko <mhocko@kernel.org>]

On Tue 10-03-20 15:54:44, David Rientjes wrote:
> 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?

Yes please, real life usecases are important when adding hacks like this
one and we should have a clear data to support the check actually helps
(in how many instances etc...)
-- 
Michal Hocko
SUSE Labs