linux-kernel - Re: [Patch V3 3/9] sgi-xp: Replace cpu_to_node() with cpu_to

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Date:	Wed, 19 Aug 2015 06:52:59 -0500
From:	Robin Holt <robinmholt@...il.com>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Tang Chen <tangchen@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
	Cliff Whickman <cpw@....com>, Tony Luck <tony.luck@...el.com>,
	linux-mm@...ck.org, linux-hotplug@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: [Patch V3 3/9] sgi-xp: Replace cpu_to_node() with cpu_to_mem() to
 support memoryless node

On Sun, Aug 16, 2015 at 10:19 PM, Jiang Liu <jiang.liu@...ux.intel.com> wrote:
> Function xpc_create_gru_mq_uv() allocates memory with __GFP_THISNODE
> flag set, which may cause permanent memory allocation failure on
> memoryless node. So replace cpu_to_node() with cpu_to_mem() to better
> support memoryless node. For node with memory, cpu_to_mem() is the same
> as cpu_to_node().
>
> Signed-off-by: Jiang Liu <jiang.liu@...ux.intel.com>
> ---
>  drivers/misc/sgi-xp/xpc_uv.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/misc/sgi-xp/xpc_uv.c b/drivers/misc/sgi-xp/xpc_uv.c
> index 95c894482fdd..9210981c0d5b 100644
> --- a/drivers/misc/sgi-xp/xpc_uv.c
> +++ b/drivers/misc/sgi-xp/xpc_uv.c
> @@ -238,7 +238,7 @@ xpc_create_gru_mq_uv(unsigned int mq_size, int cpu, char *irq_name,
>
>         mq->mmr_blade = uv_cpu_to_blade_id(cpu);
>
> -       nid = cpu_to_node(cpu);
> +       nid = cpu_to_mem(cpu);

I would recommend rejecting this.  First, SGI's UV system does not and
can not support memory-less nodes.  Additionally the hardware _REALLY_
wants the memory to be local to the CPU.  We will register this memory
region with the node firmware.  That will set the hardware up to watch
this memory block and raise an IRQ targeting the registered CPU when
anything is written into the memory block.  This is all part of how
cross-partition communications expects to work.

Additionally, the interrupt handler will read the memory region, so
having node-local memory is extremely helpful.

Thanks,
Robin
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