Re: [PATCH v2] x86,cpu-hotplug: assign same CPU number to readded CPU

[Yasuaki Ishimatsu]

(2014/07/16 20:11), Borislav Petkov wrote:
> On Wed, Jul 16, 2014 at 04:33:03PM +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 <isimatu.yasuaki@xxxxxxxxxxxxxx>
> > Suggested-by: Borislav Petkov <bp@xxxxxxxxxx>
>
> You can remove the "1" above as the domain is .de :-)

Sorry for my mistake. I'll update soon.

> Ok, so after looking at the dmesg you sent me, it looks like it works as
> expected.
>
> Apparently, the cores get enumerated in the order they're listed in SRAT
> - this also explains the mapping between APIC ID and core number in
> Linux.
>
> Your patch looks simple enough as you're basically making that mapping
> explicit with apicid_to_cpunum and make sure it remains stable when the
> cores reappear.
>
> And since APIC ID doesn't change across physical hotplug (does it?), it
> all works as expected.
>
> But, since this is BIOS and BIOS does crazy hacks to accomodate b0rked
> OSes, I'd like it if Tony looked at this too, whether it makes sense and
> whether that solution is fine.
>
> @Tony: the text should explain it all, leaving in the rest.

Thank you for reviewing my patch and confirming dmesg.
I wait Tony's comment.

Thanks,
Yasuaki Ishimatsu

> Thanks.
>
> > ---
> > v2: change cpuid to cpunum
> > ---
> >   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 ACPI 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);
> > +
> > +/*
> >    * Get the LAPIC version
> >    */
> >   static inline int lapic_get_version(void)
> > @@ -2122,6 +2139,17 @@ void disconnect_bsp_APIC(int virt_wire_setup)
> >   	apic_write(APIC_LVT1, value);
> >   }
> >
> > +static int get_cpunum(int apicid)
> > +{
> > +	int cpu;
> > +
> > +	cpu = apicid_to_cpunum[apicid];
> > +	if (cpu < 0)
> > +		cpu = cpumask_next_zero(-1, cpu_used_mask);
> > +
> > +	return cpu;
> > +}
> > +
> >   int generic_processor_info(int apicid, int version)
> >   {
> >   	int cpu, max = nr_cpu_ids;
> > @@ -2199,7 +2227,9 @@ int generic_processor_info(int apicid, int version)
> >   		 */
> >   		cpu = 0;
> >   	} else
> > -		cpu = cpumask_next_zero(-1, cpu_present_mask);
> > +		cpu = get_cpunum(apicid);
> > +
> > +	apicid_to_cpunum[apicid] = cpu;
> >
> >   	/*
> >   	 * Validate version
> > @@ -2228,6 +2258,7 @@ int generic_processor_info(int apicid, int version)
> >   	early_per_cpu(x86_cpu_to_logical_apicid, cpu) =
> >   		apic->x86_32_early_logical_apicid(cpu);
> >   #endif
> > +	cpumask_set_cpu(cpu, cpu_used_mask);
> >   	set_cpu_possible(cpu, true);
> >   	set_cpu_present(cpu, true);


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