Re: [RFC PATCH V1] mm: Disable demotion from proactive reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

Hello Mina,

On Tue, Nov 22, 2022 at 12:38:45PM -0800, Mina Almasry wrote:
> Since commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
> reclaim""), the proactive reclaim interface memory.reclaim does both
> reclaim and demotion. This is likely fine for us for latency critical
> jobs where we would want to disable proactive reclaim entirely, and is
> also fine for latency tolerant jobs where we would like to both
> proactively reclaim and demote.
> 
> However, for some latency tiers in the middle we would like to demote but
> not reclaim. This is because reclaim and demotion incur different latency
> costs to the jobs in the cgroup. Demoted memory would still be addressable
> by the userspace at a higher latency, but reclaimed memory would need to
> incur a pagefault.
> 
> To address this, I propose having reclaim-only and demotion-only
> mechanisms in the kernel. There are a couple possible
> interfaces to carry this out I considered:
> 
> 1. Disable demotion in the memory.reclaim interface and add a new
>    demotion interface (memory.demote).
> 2. Extend memory.reclaim with a "demote=<int>" flag to configure the demotion
>    behavior in the kernel like so:
>    	- demote=0 would disable demotion from this call.
> 	- demote=1 would allow the kernel to demote if it desires.
> 	- demote=2 would only demote if possible but not attempt any
> 	  other form of reclaim.

Unfortunately, our proactive reclaim stack currently relies on
memory.reclaim doing both. It may not stay like that, but I'm a bit
wary of changing user-visible semantics post-facto.

In patch 2, you're adding a node interface to memory.demote. Can you
add this to memory.reclaim instead? This would allow you to control
demotion and reclaim independently as you please: if you call it on a
node with demotion targets, it will demote; if you call it on a node
without one, it'll reclaim. And current users will remain unaffected.