Re: Re: [PATCH RESEND net-next 1/2] net-memcg: Scopify the indicators of sockmem pressure

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Jul 26, 2023 at 04:44:24PM +0800, Abel Wu wrote:
> On 7/26/23 10:56 AM, Roman Gushchin wrote:
> > On Mon, Jul 24, 2023 at 11:47:02AM +0800, Abel Wu wrote:
> > > Hi Roman, thanks for taking time to have a look!
> > > > 
> > > > > When in legacy mode aka. cgroupv1, the socket memory is charged
> > > > > into a separate counter memcg->tcpmem rather than ->memory, so
> > > > > the reclaim pressure of the memcg has nothing to do with socket's
> > > > > pressure at all.
> > > > 
> > > > But we still might set memcg->socket_pressure and propagate the pressure,
> > > > right?
> > > 
> > > Yes, but the pressure comes from memcg->socket_pressure does not mean
> > > pressure in socket memory in cgroupv1, which might lead to premature
> > > reclamation or throttling on socket memory allocation. As the following
> > > example shows:
> > > 
> > > 			->memory	->tcpmem
> > > 	limit		10G		10G
> > > 	usage		9G		4G
> > > 	pressure	true		false
> > 
> > Yes, now it makes sense to me. Thank you for the explanation.
> 
> Cheers!
> 
> > 
> > Then I'd organize the patchset in the following way:
> > 1) cgroup v1-only fix to not throttle tcpmem based on the vmpressure
> > 2) a formal code refactoring
> 
> OK, I will take a try to re-organize in next version.

Thank you!
> 
> > > > 
> > > > Overall I think it's a good idea to clean these things up and thank you
> > > > for working on this. But I wonder if we can make the next step and leave only
> > > > one mechanism for both cgroup v1 and v2 instead of having this weird setup
> > > > where memcg->socket_pressure is set differently from different paths on cgroup
> > > > v1 and v2.
> > > 
> > > There is some difficulty in unifying the mechanism for both cgroup
> > > designs. Throttling socket memory allocation when memcg is under
> > > pressure only makes sense when socket memory and other usages are
> > > sharing the same limit, which is not true for cgroupv1. Thoughts?
> > 
> > I see... Generally speaking cgroup v1 is considered frozen, so we can leave it
> > as it is, except when it creates an unnecessary complexity in the code.
> 
> Are you suggesting that the 2nd patch can be ignored and keep
> ->tcpmem_pressure as it is? Or keep the 2nd patch and add some
> explanation around as you suggested in last reply?

I suggest to split a code refactoring (which is not expected to bring any
functional changes) and an actual change of the behavior on cgroup v1.
Re the refactoring: I see a lot of value in adding comments and make the
code more readable, I don't see that much value in merging two variables.
But if it comes organically with the code simplification - nice.

> 
> > 
> > I'm curious, was your work driven by some real-world problem or a desire to clean
> > up the code? Both are valid reasons of course.
> 
> We (a cloud service provider) are migrating users to cgroupv2,
> but encountered some problems among which the socket memory
> really puts us in a difficult situation. There is no specific
> threshold for socket memory in cgroupv2 and relies largely on
> workloads doing traffic control themselves.
> 
> Say one workload behaves fine in cgroupv1 with 10G of ->memory
> and 1G of ->tcpmem, but will suck (or even be OOMed) in cgroupv2
> with 11G of ->memory due to burst memory usage on socket.
> 
> It's rational for the workloads to build some traffic control
> to better utilize the resources they bought, but from kernel's
> point of view it's also reasonable to suppress the allocation
> of socket memory once there is a shortage of free memory, given
> that performance degradation is better than failure.

Yeah, I can see it. But Idk if it's too workload-specific to have
a single-policy-fits-all-cases approach.
E.g. some workloads might prefer to have a portion of pagecache
being reclaimed.
What do you think?

> 
> Currently the mechanism of net-memcg's pressure doesn't work as
> we expected, please check the discussion in [1]. Besides this,
> we are also working on mitigating the priority inversion issue
> introduced by the net protocols' global shared thresholds [2],
> which has something to do with the net-memcg's pressure. This
> patchset and maybe some other are byproducts of the above work.
> 
> [1] https://lore.kernel.org/netdev/20230602081135.75424-1-wuyun.abel@bytedance.com/
> [2] https://lore.kernel.org/netdev/20230609082712.34889-1-wuyun.abel@bytedance.com/

Thanks for the clarification!