Re: [RFC v3] memcg: add memcg lru

[Michal Hocko]

On Mon 18-11-19 20:50:14, Hillf Danton wrote:
> 
> On Mon, 18 Nov 2019 11:29:50 +0100 Michal Hocko wrote:
> > 
> > On Sun 17-11-19 19:35:26, Hillf Danton wrote:
> > > 
> > > Currently soft limit reclaim (slr) is frozen, see
> > > Documentation/admin-guide/cgroup-v2.rst for reasons.
> > > 
> > > This work adds memcg hook into kswapd's logic to bypass slr, paving
> > > a brick for its cleanup later.
> > > 
> > > After b23afb93d317 ("memcg: punt high overage reclaim to
> > > return-to-userland path"), high limit breachers (hlb) are reclaimed
> > > one after another spiraling up through the memcg hierarchy before
> > > returning to userspace.
> > > 
> > > The current memcg high work helps to add the lru because we get to
> > > collect hlb at zero price and in particular without adding changes
> > > to the high work's behavior.
> > > 
> > > Then a fifo list, which is essencially a simple copy of the page lru,
> > > is needed to facilitate queuing up hlb and ripping pages off them in
> > > round robin once kswapd starts doing its job.
> > > 
> > > Finally new hook is added with slr's two problems addressed i.e.
> > > hierarchy-unaware reclaim and overreclaim.
> > > 
> > > Thanks to Rong Chen for testing.
> > 
> Hey Michal
> 
> Thanks for your comments, this time and previous.
> 
> > You have ignored the previous review feedback again [1]. I have nacked
> > the patch on grounds that it is completely missing any real use case
> > scenario or any numbers suggesting there is an actual improvement.
> > 
> You are right though around half.
> 
> After another peep at your comment on v2, I think you didn't approve
> the change added in high work to defer reclaim until kswapd becomes
> active with good reasoning. That defer is cut in v3.

OK, that part was obviously broken in the previous version. But please
read the whole feedback I (and Johannes) have provided.
Besides that I would consider it polite to summarize the previous
version which received to NAKs from maintainers and explain why you
believe the code has addressed that problem.

> The added lru will take the place of the current slr, so slr's use
> cases apply to it with no exception, yes? Please feel free let us
> know what use cases else you may have interests in.

Let me ask differently. There must be a reason you have spent time on
developing this feature. There must be a usecase you are targetting.
Can you describe it so that we can evaluate pros and cons?
-- 
Michal Hocko
SUSE Labs