On Thu, Apr 30, 2020 at 10:57 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > On Wed 29-04-20 12:56:27, Johannes Weiner wrote: > [...] > > I think to address this, we need a more comprehensive solution and > > introduce some form of serialization. I'm not sure yet how that would > > look like yet. > > Yeah, that is what I've tried to express earlier and that is why I would > rather go with an uglier workaround for now and think about a more > robust effective values calculation on top. > Agreed. If there's a more robust effective values calculation on top, then we don't need to hack it here and there. > > I'm still not sure it's worth having a somewhat ugly workaround in > > mem_cgroup_protection() to protect against half of the bug. If you > > think so, the full problem should at least be documented and marked > > XXX or something. > > Yes, this makes sense to me. What about the following? Many thanks for the explaination on this workaround. With this explanation, I think the others will have a clear idea why we must add this ugly workaround here. > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index 1b4150ff64be..50ffbc17cdd8 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -350,6 +350,42 @@ static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg, > if (mem_cgroup_disabled()) > return 0; > > + /* > + * There is no reclaim protection applied to a targeted reclaim. > + * We are special casing this specific case here because > + * mem_cgroup_protected calculation is not robust enough to keep > + * the protection invariant for calculated effective values for > + * parallel reclaimers with different reclaim target. This is > + * especially a problem for tail memcgs (as they have pages on LRU) > + * which would want to have effective values 0 for targeted reclaim > + * but a different value for external reclaim. > + * > + * Example > + * Let's have global and A's reclaim in parallel: > + * | > + * A (low=2G, usage = 3G, max = 3G, children_low_usage = 1.5G) > + * |\ > + * | C (low = 1G, usage = 2.5G) > + * B (low = 1G, usage = 0.5G) > + * > + * For the global reclaim > + * A.elow = A.low > + * B.elow = min(B.usage, B.low) because children_low_usage <= A.elow > + * C.elow = min(C.usage, C.low) > + * > + * With the effective values resetting we have A reclaim > + * A.elow = 0 > + * B.elow = B.low > + * C.elow = C.low > + * > + * If the global reclaim races with A's reclaim then > + * B.elow = C.elow = 0 because children_low_usage > A.elow) > + * is possible and reclaiming B would be violating the protection. > + * > + */ > + if (memcg == root) > + return 0; > + > if (in_low_reclaim) > return READ_ONCE(memcg->memory.emin); > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 05b4ec2c6499..df88a22f09bc 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -6385,6 +6385,14 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, > > if (!root) > root = root_mem_cgroup; > + > + /* > + * Effective values of the reclaim targets are ignored so they > + * can be stale. Have a look at mem_cgroup_protection for more > + * details. > + * TODO: calculation should be more robust so that we do not need > + * that special casing. > + */ > if (memcg == root) > return MEMCG_PROT_NONE; > > > > In practice, I doubt this matters all that much because limit reclaim > > and global reclaim tend to occur in complementary > > containerization/isolation strategies, not heavily simultaneously. > > I would expect that as well but this is always hard to tell. > > -- > Michal Hocko > SUSE Labs -- Thanks Yafang
