linux-kernel - Re: [PATCH v2] cpu: Fix node state for whether it contains CPU

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Message-ID: <alpine.DEB.2.10.1608311534500.70776@chino.kir.corp.google.com>
Date:   Wed, 31 Aug 2016 15:38:09 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, Huang@...ux.intel.com,
        Ying <ying.huang@...el.com>, Andi Kleen <andi@...stfloor.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] cpu: Fix node state for whether it contains CPU

On Mon, 29 Aug 2016, 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.
> 
> 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;
> }
> 
> Fix this by checking each node for online CPU when we initialize
> vmstat that's responsible for maintaining node state.
> 

That would mean that when node_reclaim_mode is enabled that we weren't 
properly returning NODE_RECLAIM_NOSCAN if a remote node had its own cpus 
and PGDAT_RECLAIM_LOCKED wasn't already set, so this seems like it could 
result in a performance improvement.

> v2:
> 1. Fix the problem for all architectures in the generic path
> inside vmstat, not just for x86.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>

Acked-by: David Rientjes <rientjes@...gle.com>