Re: [RFC v1] mm: add page preemption

[Michal Hocko <mhocko@kernel.org>]

On Tue 22-10-19 22:28:02, Hillf Danton wrote:
> 
> On Tue, 22 Oct 2019 14:42:41 +0200 Michal Hocko wrote:
> > 
> > On Tue 22-10-19 20:14:39, Hillf Danton wrote:
> > > 
> > > On Mon, 21 Oct 2019 14:27:28 +0200 Michal Hocko wrote:
> > [...]
> > > > Why do we care and which workloads would benefit and how much.
> > > 
> > > Page preemption, disabled by default, should be turned on by those
> > > who wish that the performance of their workloads can survive memory
> > > pressure to certain extent.
> > 
> > I am sorry but this doesn't say anything to me. How come not all
> > workloads would fit that description?
> 
> That means pp plays a role when kswapd becomes active, and it may
> prevent too much jitters in active lru pages.

This is still too vague to be useful in any way.

> > > The number of pp users is supposed near the people who change the
> > > nice value of their apps either to -1 or higher at least once a week,
> > > less than vi users among UK's undergraduates.
> > > 
> > > > And last but not least why the existing infrastructure doesn't help
> > > > (e.g. if you have clearly defined workloads with different
> > > > memory consumption requirements then why don't you use memory cgroups to
> > > > reflect the priority).
> > > 
> > > Good question:)
> > > 
> > > Though pp is implemented by preventing any task from reclaiming as many
> > > pages as possible from other tasks that are higher on priority, it is
> > > trying to introduce prio into page reclaiming, to add a feature.
> > > 
> > > Page and memcg are different objects after all; pp is being added at
> > > the page granularity. It should be an option available in environments
> > > without memcg enabled.
> > 
> > So do you actually want to establish LRUs per priority?
> 
> No, no change other than the prio for every lru page was added. LRU per prio
> is too much to implement.

Well, considering that per page priority is a no go as already pointed
out by Willy then you do not have other choice right?

> > Why using memcgs is not an option?
> 
> I have plan to add prio in memcg. As you see, I sent a rfc before v0 with
> nice added in memcg, and realised a couple days ago that its dependence on
> soft limit reclaim is not acceptable.
> 
> But we can't do that without determining how to define memcg's prio.
> What is in mind now is the highest (or lowest) prio of tasks in a memcg
> with a knob offered to userspace.
> 
> If you like, I want to have a talk about it sometime later.

This doesn't really answer my question. Why cannot you use memcgs as
they are now. Why exactly do you need a fixed priority?

> > This is the main facility to partition reclaimable
> > memory in the first place. You should really focus on explaining on why
> > a much more fine grained control is needed much more thoroughly.
> > 
> > > What is way different from the protections offered by memory cgroup
> > > is that pages protected by memcg:min/low can't be reclaimed regardless
> > > of memory pressure. Such guarantee is not available under pp as it only
> > > suggests an extra factor to consider on deactivating lru pages.
> > 
> > Well, low limit can be breached if there is no eliglible memcg to be
> > reclaimed. That means that you can shape some sort of priority by
> > setting the low limit already.
> > 
> > [...]
> > 
> > > What was added on the reclaimer side is
> > > 
> > > 1, kswapd sets pgdat->kswapd_prio, the switch between page reclaimer
> > >    and allocator in terms of prio, to the lowest value before taking
> > >    a nap.
> > > 
> > > 2, any allocator is able to wake up the reclaimer because of the
> > >    lowest prio, and it starts reclaiming pages using the waker's prio.
> > > 
> > > 3, allocator comes while kswapd is active, its prio is checked and
> > >    no-op if kswapd is higher on prio; otherwise switch is updated
> > >    with the higher prio.
> > > 
> > > 4, every time kswapd raises sc.priority that starts with DEF_PRIORITY,
> > >    it is checked if there is pending update of switch; and kswapd's
> > >    prio steps up if there is a pending one, thus its prio never steps
> > >    down. Nor prio inversion. 
> > > 
> > > 5, goto 1 when kswapd finishes its work.
> > 
> > What about the direct reclaim?
> 
> Their prio will not change before reclaiming finishes, so leave it be.

This doesn't answer my question.

> > What if pages of a lower priority are
> > hard to reclaim? Do you want a process of a higher priority stall more
> > just because it has to wait for those lower priority pages?
> 
> The problems above are not introduced by pp, let Mr. Kswapd take care of
> them.

No, this is not an answer.

-- 
Michal Hocko
SUSE Labs