Re: [patch] mm, oom: prevent soft lockup on memcg oom for UP systems

[David Rientjes]

On Thu, 12 Mar 2020, Michal Hocko wrote:

> > I think the changelog clearly states that we need to guarantee that a 
> > reclaimer will yield the processor back to allow a victim to exit.  This 
> > is where we make the guarantee.  If it helps for the specific reason it 
> > triggered in my testing, we could add:
> > 
> > "For example, mem_cgroup_protected() can prohibit reclaim and thus any 
> > yielding in page reclaim would not address the issue."
> 
> I would suggest something like the following:
> "
> The reclaim path (including the OOM) relies on explicit scheduling
> points to hand over execution to tasks which could help with the reclaim
> process.

Are there other examples where yielding in the reclaim path would "help 
with the reclaim process" other than oom victims?  This sentence seems 
vague.

> Currently it is mostly shrink_page_list which yields CPU for
> each reclaimed page. This might be insuficient though in some
> configurations. E.g. when a memcg OOM path is triggered in a hierarchy
> which doesn't have any reclaimable memory because of memory reclaim
> protection (MEMCG_PROT_MIN) then there is possible to trigger a soft
> lockup during an out of memory situation on non preemptible kernels
> <PUT YOUR SOFT LOCKUP SPLAT HERE>
> 
> Fix this by adding a cond_resched up in the reclaim path and make sure
> there is a yield point regardless of reclaimability of the target
> hierarchy.
> "
>