Re: [PATCH] x86,cpu-hotplug: clear llc_shared_mask at CPU hotplug

[Mike Galbraith <umgwanakikbuti@gmail.com>]

(CC Peter)

On Wed, 2014-07-02 at 15:41 +0900, Yasuaki Ishimatsu wrote: 
> llc_shared_mask is not cleared even if cpu is offline or hot removed.
> So when hot-plugging CPU, the mask has wrong value. The mask is used
> by CSF schduler. So it breaks CFS scheduler.
> 
> Here is a example on my system.
> My system has 4 sockets and each socket has 15 cores and HT is enabled.
> In this case, each core of sockes is numbered as follows:
> 
>          | CPU#
> Socket#0 | 0-14 , 60-74
> Socket#1 | 15-29, 75-89
> Socket#2 | 30-44, 90-104
> Socket#3 | 45-59, 105-119
> 
> Then llc_shared_mask of CPU#30 has 0x3fff80000001fffc0000000.
> It means that cache of Socket#2 is shared with CPU#30-44 and 90-104.
> 
> When hot-removing socket#2 and #3, each core of sockets is numbered
> as follows:
> 
>          | CPU#
> Socket#0 | 0-14 , 60-74
> Socket#1 | 15-29, 75-89
> 
> But llc_shared_mask is not cleared. So llc_shared_mask of CPU#30 remains
> having 0x3fff80000001fffc0000000.
> 
> After that, when hot-adding socket#2 and #3, each core of sockets is
> numbered as follows:
> 
>          | CPU#
> Socket#0 | 0-14 , 60-74
> Socket#1 | 15-29, 75-89
> Socket#2 | 30-59
> Socket#3 | 90-119
> 
> Then llc_shared_mask of CPU#30 becomes 0x3fff8000fffffffc0000000.
> It means that cache of Socket#2 is shared with CPU#30-59 and 90-104.
> So the mask has wrong value.
> 
> This patch fixes above problem by clearing llc_shared_mask bit of
> offlined cpu.
> 
> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
> ---
>  arch/x86/kernel/smpboot.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 5492798..893cd2b 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1279,6 +1279,7 @@ __init void prefill_possible_map(void)
>  static void remove_siblinginfo(int cpu)
>  {
>  	int sibling;
> +	int llc_shared;
>  	struct cpuinfo_x86 *c = &cpu_data(cpu);
> 
>  	for_each_cpu(sibling, cpu_core_mask(cpu)) {
> @@ -1290,9 +1291,12 @@ static void remove_siblinginfo(int cpu)
>  			cpu_data(sibling).booted_cores--;
>  	}
> 
> +	for_each_cpu(llc_shared, cpu_llc_shared_mask(cpu))
> +		cpumask_clear_cpu(cpu, cpu_llc_shared_mask(llc_shared));
>  	for_each_cpu(sibling, cpu_sibling_mask(cpu))
>  		cpumask_clear_cpu(cpu, cpu_sibling_mask(sibling));
>  	cpumask_clear(cpu_sibling_mask(cpu));
> +	cpumask_clear(cpu_llc_shared_mask(cpu));
>  	cpumask_clear(cpu_core_mask(cpu));
>  	c->phys_proc_id = 0;
>  	c->cpu_core_id = 0;
> 
> --
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