linux-kernel - Re: [PATCH] Allocate idle task for a CPU always on its local node

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Date:	Tue, 17 May 2016 14:13:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] Allocate idle task for a CPU always on its local node

On Tue, 17 May 2016 06:44:54 -0700 Andi Kleen <andi@...stfloor.org> wrote:

> From: Andi Kleen <ak@...ux.intel.com>
> 
> Linux pre-allocates the task structs of the idle tasks for all possible CPUs.
> This currently means they all end up on node 0. This also implies
> that the cache line of MWAIT, which is around the flags field in the task
> struct, are all located in node 0.
> 
> We see a noticeable performance improvement on Knights Landing CPUs when
> the cache lines used for MWAIT are located in the local nodes of the CPUs
> using them. I would expect this to give a (likely slight) improvement
> on other systems too.
> 
> The patch implements placing the idle task in the node of
> its CPUs, by passing the right target node to copy_process()
> 

Looks nice.

This is nicer ;)


From: Andrew Morton <akpm@...ux-foundation.org>
Subject: allocate-idle-task-for-a-cpu-always-on-its-local-node-fix

use NUMA_NO_NODE, not a bare -1

Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 kernel/fork.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff -puN kernel/fork.c~allocate-idle-task-for-a-cpu-always-on-its-local-node-fix kernel/fork.c
--- a/kernel/fork.c~allocate-idle-task-for-a-cpu-always-on-its-local-node-fix
+++ a/kernel/fork.c
@@ -346,7 +346,7 @@ static struct task_struct *dup_task_stru
 	struct thread_info *ti;
 	int err;
 
-	if (node < 0)
+	if (node == NUMA_NO_NODE)
 		node = tsk_fork_get_node(orig);
 	tsk = alloc_task_struct_node(node);
 	if (!tsk)
@@ -1754,7 +1754,7 @@ long _do_fork(unsigned long clone_flags,
 	}
 
 	p = copy_process(clone_flags, stack_start, stack_size,
-			 child_tidptr, NULL, trace, tls, -1);
+			 child_tidptr, NULL, trace, tls, NUMA_NO_NODE);
 	/*
 	 * Do this prior waking up the new thread - the thread pointer
 	 * might get invalid after that point, if the thread exits quickly.
_