Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern

[Uladzislau Rezki <urezki@gmail.com>]

> 
> Yes, I mean __GFP_MEMALLOC. Sorry, the patch was just to show the idea and
> marked as RFC.
> 
> Good point on the atomic aspect of this path, you are right we cannot sleep.
> I believe the GFP_NOWAIT I mentioned in my last reply will take care of that?
> 
I think there should be GFP_ATOMIC used, because it has more chance to
return memory then GFP_NOWAIT. I see that Michal has same view on it.

> > As for removing __GFP_NOWARN. Actually it is expectable that an
> > allocation can fail, if so we follow last emergency case. You
> > can see the trace but what would you do with that information?
> 
> Yes, the benefit of the trace/warning is that the user can switch to a
> non-headless API and avoid the synchronize_rcu(), that would help them get
> faster kfree_rcu() performance instead of having silent slowdowns.
> 
Agree. What about just adding WARN_ON_ONCE()? I am just thinking if it
could be harmful or not.

>
> It also tells us whether the headless API is worth it in the long run, I
> think it is worth it because we will likely never hit the synchronize_rcu()
> failsafe. But if we hit it a lot, at least it wont happen silently.
> 
Agree.

> Paul was concerned about following scenario with hitting synchronize_rcu():
> 1. Consider a system under memory pressure.
> 2. Consider some other subsystem X depending on another system Y which uses
>    kfree_rcu(). If Y doesn't complete the operation in time, X accumulates
>    more memory.
> 3. Since kfree_rcu() on Y hits synchronize_rcu() a lot, it slows it down.
>    This causes X to further allocate memory, further causing a chain
>    reaction.
> Paul, please correct me if I'm wrong.
> 
I see your point and agree that in theory it can happen. So, we should
make it more tight when it comes to rcu_head attachment logic.

--
Vlad Rezki