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
