Re: [PATCH v3] mm: Add nodes= arg to memory.reclaim

[Muchun Song <muchun.song@linux.dev>]

> On Dec 3, 2022, at 06:35, Mina Almasry <almasrymina@google.com> wrote:
> 
> The nodes= arg instructs the kernel to only scan the given nodes for
> proactive reclaim. For example use cases, consider a 2 tier memory system:
> 
> nodes 0,1 -> top tier
> nodes 2,3 -> second tier
> 
> $ echo "1m nodes=0" > memory.reclaim
> 
> This instructs the kernel to attempt to reclaim 1m memory from node 0.
> Since node 0 is a top tier node, demotion will be attempted first. This
> is useful to direct proactive reclaim to specific nodes that are under
> pressure.
> 
> $ echo "1m nodes=2,3" > memory.reclaim
> 
> This instructs the kernel to attempt to reclaim 1m memory in the second tier,
> since this tier of memory has no demotion targets the memory will be
> reclaimed.
> 
> $ echo "1m nodes=0,1" > memory.reclaim
> 
> Instructs the kernel to reclaim memory from the top tier nodes, which can
> be desirable according to the userspace policy if there is pressure on
> the top tiers. Since these nodes have demotion targets, the kernel will
> attempt demotion first.
> 
> Since commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
> reclaim""), the proactive reclaim interface memory.reclaim does both
> reclaim and demotion. 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.
> 
> The 'nodes' arg is useful to allow the userspace to control demotion
> and reclaim independently according to its policy: if the memory.reclaim
> is called on a node with demotion targets, it will attempt demotion first;
> if it is called on a node without demotion targets, it will only attempt
> reclaim.
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Mina Almasry <almasrymina@google.com>

Acked-by: Muchun Song <songmuchun@bytedance.com>

Thanks.