On Thu, 2014-07-17 at 16:32 +0900, Yasuaki Ishimatsu wrote:
> llc_shared_map is not cleared even if CPU is offline or hot removed.
> So when hot-plugging CPU and assigning new CPU number to hot-added CPU,
> the mask has wrong value. The mask is used by CSF schduler to create
> sched_domain. 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 last level 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 last level cache of Socket#2 is shared with CPU#30-59
> and 90-104. So the mask has wrong value.
>
> At first, I cleared hot-removed CPU number's bit from llc_shared_map
> when hot removing CPU. But Borislav suggested that the problem will
> disappear if readded CPU is assigned same CPU number. And llc_shared_map
> must not be changed.
>
> So the patch assigns same CPU number to readded CPU by linking CPU
> number to APIC ID. And by the patch, the problem disappers.
>
> Signed-off-by: Yasuaki Ishimatsu <[email protected]>
> Suggested-by: Borislav Petkov <[email protected]>
> ---
> v2: change cpuid to cpunum
> v3: fix Borislav's email address of Suggested-by
> fix typo (ACPI ID to APIC ID)
> ---
> arch/x86/kernel/apic/apic.c | 33 ++++++++++++++++++++++++++++++++-
> 1 file changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index ad28db7..5dc3e50 100644
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -220,6 +220,23 @@ static void apic_pm_activate(void);
> static unsigned long apic_phys;
>
> /*
> + * Bind APIC ID to Logical CPU number
> + * Logical CPU number to APIC ID does not change by this array
> + * even if CPU is hotplugged. So don't clear the array even if
> + * CPU is hot-removed
> + */
> +static int apicid_to_cpunum[MAX_LOCAL_APIC] = {
> + [0 ... MAX_LOCAL_APIC-1] = -1,
> +};
> +
> +/*
> + * Represent Logical CPU number bound to APIC ID
> + * Don't clear a bit even if CPU is hot-removed
> + */
> +static DECLARE_BITMAP(cpu_used_bits, CONFIG_NR_CPUS);
> +static struct cpumask *const cpu_used_mask = to_cpumask(cpu_used_bits);
I think the name of cpu_used_mask is confusing since the term "used"
sounds some state of CPU, which is not the case here. How about
cpu_number_mask? This is more specific and avoids such confusion.
Otherwise, the change looks good.
Reviewed-by: Toshi Kani <[email protected]>
Thanks,
-Toshi
> +
> +/*
> * Get the LAPIC version
> */
^^
ps. I had a problem in applying this patch since there was an extra
blank character at the begging of unchanged lines. But this might be an
issue in my mailer.
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