Date:   Wed, 9 Feb 2022 12:10:07 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Tony Lu <tonylu@...ux.alibaba.com>
Cc:     Stefan Raspl <raspl@...ux.ibm.com>, kgraul@...ux.ibm.com,
        kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org,
        RDMA mailing list <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH net-next] net/smc: Allocate pages of SMC-R on ibdev NUMA
 node

On Wed, Feb 09, 2022 at 04:00:34PM +0800, Tony Lu wrote:
> On Tue, Feb 08, 2022 at 11:32:23AM +0200, Leon Romanovsky wrote:
> > On Tue, Feb 08, 2022 at 10:10:55AM +0100, Stefan Raspl wrote:
> > > On 2/7/22 14:49, Leon Romanovsky wrote:
> > > > On Mon, Feb 07, 2022 at 05:59:58PM +0800, Tony Lu wrote:
> > > > > On Mon, Jan 31, 2022 at 09:20:52AM +0200, Leon Romanovsky wrote:
> > > > > > On Mon, Jan 31, 2022 at 03:03:00AM +0800, Tony Lu wrote:
> > > > > > > Currently, pages are allocated in the process context, for its NUMA node
> > > > > > > isn't equal to ibdev's, which is not the best policy for performance.
> > > > > > > 
> > > > > > > Applications will generally perform best when the processes are
> > > > > > > accessing memory on the same NUMA node. When numa_balancing enabled
> > > > > > > (which is enabled by most of OS distributions), it moves tasks closer to
> > > > > > > the memory of sndbuf or rmb and ibdev, meanwhile, the IRQs of ibdev bind
> > > > > > > to the same node usually. This reduces the latency when accessing remote
> > > > > > > memory.
> > > > > > 
> > > > > > It is very subjective per-specific test. I would expect that
> > > > > > application will control NUMA memory policies (set_mempolicy(), ...)
> > > > > > by itself without kernel setting NUMA node.
> > > > > > 
> > > > > > Various *_alloc_node() APIs are applicable for in-kernel allocations
> > > > > > where user can't control memory policy.
> > > > > > 
> > > > > > I don't know SMC-R enough, but if I judge from your description, this
> > > > > > allocation is controlled by the application.
> > > > > 
> > > > > The original design of SMC doesn't handle the memory allocation of
> > > > > different NUMA node, and the application can't control the NUMA policy
> > > > > in SMC.
> > > > > 
> > > > > It allocates memory according to the NUMA node based on the process
> > > > > context, which is determined by the scheduler. If application process
> > > > > runs on NUMA node 0, SMC allocates on node 0 and so on, it all depends
> > > > > on the scheduler. If RDMA device is attached to node 1, the process runs
> > > > > on node 0, it allocates memory on node 0.
> > > > > 
> > > > > This patch tries to allocate memory on the same NUMA node of RDMA
> > > > > device. Applications can't know the current node of RDMA device. The
> > > > > scheduler knows the node of memory, and can let applications run on the
> > > > > same node of memory and RDMA device.
> > > > 
> > > > I don't know, everything explained above is controlled through memory
> > > > policy, where application needs to run on same node as ibdev.
> > > 
> > > The purpose of SMC-R is to provide a drop-in replacement for existing TCP/IP
> > > applications. The idea is to avoid almost any modification to the
> > > application, just switch the address family. So while what you say makes a
> > > lot of sense for applications that intend to use RDMA, in the case of SMC-R
> > > we can safely assume that most if not all applications running it assume
> > > they get connectivity through a non-RDMA NIC. Hence we cannot expect the
> > > applications to think about aspects such as NUMA, and we should do the right
> > > thing within SMC-R.
> > 
> > And here comes the problem, you are doing the right thing for very
> > specific and narrow use case, where application and ibdev run on
> > same node. It is not true for multi-core systems as application will
> > be scheduled on less load node (in very simplistic form).
> > 
> > In general case, the application will get CPU and memory based on scheduler
> > heuristic as you don't use memory policy to restrict it. The assumption
> > that allocations need to be close to ibdev and not to applications can
> > lead to worse performance.
> > 
> 
> Yes, the applications cannot run faster if they always access remote
> memory. There are something complex in SMC, so choose to bind to the
> RDMA device.
> 
> As Stefan mentioned, SMC is to provide a drop-in replacement for TCP.

If I'm looking on the right piece of code (net/core/skbuff.c:build_skb),
even SKB is not allocated close to ehternet device. I'm not convinced that
SMC should be different here.

Thanks

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