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Date: Mon, 29 Aug 2016 15:36:12 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
        "H. Peter Anvin" <hpa@zytor.com>, "Huang, Ying" <ying.huang@intel.com>,
        Andi Kleen <andi@firstfloor.org>, Dave Hansen <dave.hansen@intel.com>,
        Dan Williams <dan.j.williams.intel.com@linux.intel.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86, cpu: Fix node state for whether it contains CPU
Message-ID: <20160829133612.GQ10153@twins.programming.kicks-ass.net>
References: <20160824232647.GA21759@linux.intel.com>
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On Wed, Aug 24, 2016 at 04:26:49PM -0700, Tim Chen wrote:
> In current kernel code, we only call node_set_state(cpu_to_node(cpu),
> N_CPU) when a cpu is hot plugged.  But we do not set the node state for
> N_CPU when the cpus are brought online during boot.
> 
> So this could lead to failure when we check to see
> if a node contains cpu with node_state(node_id, N_CPU).
> 
> One use case is in the node_reclaime function:
> 
>         /*
>          * Only run node reclaim on the local node or on nodes that do
>          * not
>          * have associated processors. This will favor the local
>          * processor
>          * over remote processors and spread off node memory allocations
>          * as wide as possible.
>          */
>         if (node_state(pgdat->node_id, N_CPU) && pgdat->node_id !=
> 		numa_node_id())
>                 return NODE_RECLAIM_NOSCAN;
> 
> I instrumented the kernel to call this function after boot and it
> always returns 0 on a x86 desktop machine until I apply
> the attached patch.
> 
> static int num_cpu_node(void)
> {
>        int i, nr_cpu_nodes = 0;
> 
>        for_each_node(i) {
>                if (node_state(i, N_CPU))
>                        ++ nr_cpu_nodes;
>        }
> 
>        return nr_cpu_nodes;
> }
> 
> I have not tested other architectues but they are likely
> to have similar issue.
> 
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  arch/x86/kernel/smpboot.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index d8f7d01..04c0574 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -259,6 +259,7 @@ static void notrace start_secondary(void *unused)
>  	lock_vector_lock();
>  	setup_vector_irq(smp_processor_id());
>  	set_cpu_online(smp_processor_id(), true);
> +	node_set_state(cpu_to_node(smp_processor_id()), N_CPU);
>  	unlock_vector_lock();
>  	cpu_set_state_online(smp_processor_id());
>  	x86_platform.nmi_init();

Would it not be easier to register the vmstat_notifier earlier, before
SMP bringup? Because with this change, we need to go fix all
architectures.