On Wed 20-03-24 18:55:05, Pavel Tikhomirov wrote: > > > On 20/03/2024 18:28, Michal Hocko wrote: > > On Wed 20-03-24 18:03:30, Pavel Tikhomirov wrote: > > > In memory_max_write() we first set memcg->memory.max and only then > > > try to enforce it in loop. What if while we are in loop someone else > > > have changed memcg->memory.max but we are still trying to enforce > > > the old value? I believe this can lead to nasty consequence like getting > > > an oom on perfectly fine cgroup within it's limits or excess reclaim. > > > > I would argue that uncoordinated hard limit configuration can cause > > problems on their own. > > Sorry, didn't know that. Well, just consider potential over-reclaim as a result of several competing actors to set the same limit. Or completely indeterministic final output of the limit setting depending on timing. This simply cannot work reliably. > > Beside how is this any different from changing > > the high limit while we are inside the reclaim loop? > > I believe reclaim loop rereads limits on each iteration, e.g. in > reclaim_high(), so it should always be enforcing the right limit. Reclaim loop might happen to take quite some time... > > > We also have exactly the same thing in memory_high_write(). > > > > > > So let's stop enforcing old limits if we already have a new ones. > > > > I do see any reasons why this would be harmful I just do not see why > > this is a real thing or why the new behavior is any better for racing > > updaters as those are not deterministic anyway. If you have any actual > > usecase then more details would really help to justify this change. > > > > The existing behavior makes some sense as it enforces the given limit > > deterministically. > > I don't have any actual problem, usecase or reproduce at hand, I only see a > potential problem: > > Let's imagine that: > > a) We set cgroup max limit to some small value, memory_max_write updates > memcg->memory.max and starts spinning in loop as it wants to reclaim some > memory which does not fit in new limit. > > b) We don't need small limit anymore and we raise the limit to a big value, > but memory_max_write() from (a) is still spinning. And if we are lucky > enough and processes of cgroup are constantly consuming memory, to > compensate effect from memory_max_write() from (a), so that it will continue > spinning there forever. This is a killable operation, so if you decide to change mind about limit setting and the current update is still in progress then just terminate it rather then override by a different process. > Yes it is not that bad, because memory_max/high_write() also constantly > checks for pending signals in loop so they won't actually get irreversibly > stuck. But I just thought it was worth fixing. If we want to fix this parallel limits setting then we should also think about a reasonable and predictable behavior and that would likely require some sort of locking IMO. -- Michal Hocko SUSE Labs
