[PATCH] ppc64 boot: Wait for boot cpu to show up if nr_cpus limit is about to hit.

Denis Kirjanov kda at linux-powerpc.org
Wed Feb 3 00:58:33 AEDT 2016


On 2/1/16, Mahesh J Salgaonkar <mahesh at linux.vnet.ibm.com> wrote:
> From: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
>
> The kernel boot parameter 'nr_cpus=' allows one to specify number of
> possible cpus in the system. In the normal scenario the first cpu (cpu0)
> that shows up is the boot cpu and hence it gets covered under nr_cpus
> limit.
>
> But this assumption will be broken in kdump scenario where kdump kenrel
> after a crash can boot up on an non-zero boot cpu. The paca structure
> allocation depends on value of nr_cpus and is indexed using logical cpu
> ids. This definetly will be an issue if boot cpu id > nr_cpus
And what happend in this case? Have you tried it out?
>
> This patch modifies allocate_pacas() and smp_setup_cpu_maps() to
> accommodate boot cpu for the case where boot_cpuid > nr_cpu_ids.
>
> This change would help to reduce the memory reservation requirement for
> kdump on ppc64.
>
> Signed-off-by: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/paca.h    |    3 +++
>  arch/powerpc/include/asm/smp.h     |    1 +
>  arch/powerpc/kernel/paca.c         |   23 ++++++++++++++++++----
>  arch/powerpc/kernel/prom.c         |   37
> +++++++++++++++++++++++++++++++++++-
>  arch/powerpc/kernel/setup-common.c |   25 ++++++++++++++++++++++++
>  5 files changed, 83 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/paca.h
> b/arch/powerpc/include/asm/paca.h
> index 70bd438..9be48b4 100644
> --- a/arch/powerpc/include/asm/paca.h
> +++ b/arch/powerpc/include/asm/paca.h
> @@ -41,6 +41,9 @@ extern unsigned int debug_smp_processor_id(void); /* from
> linux/smp.h */
>  #define get_lppaca()	(get_paca()->lppaca_ptr)
>  #define get_slb_shadow()	(get_paca()->slb_shadow_ptr)
>
> +/* Maximum number of threads per core. */
> +#define	MAX_SMT		8
> +
>  struct task_struct;
>
>  /*
> diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
> index 825663c..0a5b99f 100644
> --- a/arch/powerpc/include/asm/smp.h
> +++ b/arch/powerpc/include/asm/smp.h
> @@ -30,6 +30,7 @@
>  #include <asm/percpu.h>
>
>  extern int boot_cpuid;
> +extern int boot_hw_cpuid;
>  extern int spinning_secondaries;
>
>  extern void cpu_die(void);
> diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
> index 01ea0ed..96e5715 100644
> --- a/arch/powerpc/kernel/paca.c
> +++ b/arch/powerpc/kernel/paca.c
> @@ -206,6 +206,7 @@ void __init allocate_pacas(void)
>  {
>  	u64 limit;
>  	int cpu;
> +	int nr_cpus;
>
>  	limit = ppc64_rma_size;
>
> @@ -218,20 +219,32 @@ void __init allocate_pacas(void)
>  	limit = min(0x10000000ULL, limit);
>  #endif
>
> -	paca_size = PAGE_ALIGN(sizeof(struct paca_struct) * nr_cpu_ids);
> +	/*
> +	 * Always align up the nr_cpu_ids to SMT threads and allocate
> +	 * the paca. This will help us to prepare for a situation where
> +	 * boot cpu id > nr_cpus_id. We will use the last nthreads
> +	 * slots (nthreads == threads per core) to accommodate a core
> +	 * that contains boot cpu thread.
> +	 *
> +	 * Do not change nr_cpu_ids value here. Let us do that in
> +	 * early_init_dt_scan_cpus() where we know exact value
> +	 * of threads per core.
> +	 */
> +	nr_cpus = _ALIGN_UP(nr_cpu_ids, MAX_SMT);
> +	paca_size = PAGE_ALIGN(sizeof(struct paca_struct) * nr_cpus);
>
>  	paca = __va(memblock_alloc_base(paca_size, PAGE_SIZE, limit));
>  	memset(paca, 0, paca_size);
>
>  	printk(KERN_DEBUG "Allocated %u bytes for %d pacas at %p\n",
> -		paca_size, nr_cpu_ids, paca);
> +		paca_size, nr_cpus, paca);
>
> -	allocate_lppacas(nr_cpu_ids, limit);
> +	allocate_lppacas(nr_cpus, limit);
>
> -	allocate_slb_shadows(nr_cpu_ids, limit);
> +	allocate_slb_shadows(nr_cpus, limit);
>
>  	/* Can't use for_each_*_cpu, as they aren't functional yet */
> -	for (cpu = 0; cpu < nr_cpu_ids; cpu++)
> +	for (cpu = 0; cpu < nr_cpus; cpu++)
>  		initialise_paca(&paca[cpu], cpu);
>  }
>
> diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
> index 7030b03..9d1568f 100644
> --- a/arch/powerpc/kernel/prom.c
> +++ b/arch/powerpc/kernel/prom.c
> @@ -291,6 +291,29 @@ static void __init
> check_cpu_feature_properties(unsigned long node)
>  	}
>  }
>
> +/*
> + * Adjust the logical id of a boot cpu to fall under nr_cpu_ids. Map it to
> + * last core slot in the allocated paca array.
> + *
> + * e.g. on SMT=8 system, kernel booted with nr_cpus=1 and boot cpu = 33,
> + * align nr_cpu_ids to MAX_SMT value 8. Allocate paca array to hold up-to
> + * MAX_SMT=8 cpus. Since boot cpu 33 is greater than nr_cpus (8), adjust
> + * its logical id so that new id becomes less than nr_cpu_ids. Make sure
> + * that boot cpu's new logical id is aligned to its thread id and falls
> + * under last nthreads slots available in paca array. In this case the
> + * boot cpu 33 is adjusted to new boot cpu id 1.
> + *
> + */
> +static inline void adjust_boot_cpuid(int nthreads, int phys_id)
> +{
> +	boot_hw_cpuid = phys_id;
> +	if (boot_cpuid >= nr_cpu_ids) {
> +		boot_cpuid = (boot_cpuid % nthreads) + (nr_cpu_ids - nthreads);
> +		pr_info("Adjusted logical boot cpu id: logical %d physical %d\n",
> +			boot_cpuid, phys_id);
> +	}
> +}
> +
>  static int __init early_init_dt_scan_cpus(unsigned long node,
>  					  const char *uname, int depth,
>  					  void *data)
> @@ -315,6 +338,16 @@ static int __init early_init_dt_scan_cpus(unsigned long
> node,
>  	nthreads = len / sizeof(int);
>
>  	/*
> +	 * Now that we know threads per core lets align nr_cpu_ids to
> +	 * correct SMT value.
> +	 */
> +	if (nr_cpu_ids % nthreads) {
> +		nr_cpu_ids = _ALIGN_UP(nr_cpu_ids, nthreads);
> +		pr_info("Aligned nr_cpus to SMT=%d, nr_cpu_ids = %d\n",
> +				 nthreads, nr_cpu_ids);
> +	}
> +
> +	/*
>  	 * Now see if any of these threads match our boot cpu.
>  	 * NOTE: This must match the parsing done in smp_setup_cpu_maps.
>  	 */
> @@ -352,7 +385,9 @@ static int __init early_init_dt_scan_cpus(unsigned long
> node,
>  	DBG("boot cpu: logical %d physical %d\n", found,
>  	    be32_to_cpu(intserv[found_thread]));
>  	boot_cpuid = found;
> -	set_hard_smp_processor_id(found, be32_to_cpu(intserv[found_thread]));
> +	adjust_boot_cpuid(nthreads, be32_to_cpu(intserv[found_thread]));
> +	set_hard_smp_processor_id(boot_cpuid,
> +					be32_to_cpu(intserv[found_thread]));
>
>  	/*
>  	 * PAPR defines "logical" PVR values for cpus that
> diff --git a/arch/powerpc/kernel/setup-common.c
> b/arch/powerpc/kernel/setup-common.c
> index 44c8d03..93c7e5f 100644
> --- a/arch/powerpc/kernel/setup-common.c
> +++ b/arch/powerpc/kernel/setup-common.c
> @@ -77,6 +77,7 @@ struct machdep_calls *machine_id;
>  EXPORT_SYMBOL(machine_id);
>
>  int boot_cpuid = -1;
> +int boot_hw_cpuid = -1;
>  EXPORT_SYMBOL_GPL(boot_cpuid);
>
>  unsigned long klimit = (unsigned long) _end;
> @@ -444,6 +445,7 @@ void __init smp_setup_cpu_maps(void)
>  	struct device_node *dn = NULL;
>  	int cpu = 0;
>  	int nthreads = 1;
> +	bool boot_cpu_added = false;
>
>  	DBG("smp_setup_cpu_maps()\n");
>
> @@ -470,6 +472,24 @@ void __init smp_setup_cpu_maps(void)
>  		}
>
>  		nthreads = len / sizeof(int);
> +		/*
> +		 * If boot cpu hasn't been added to paca and there are only
> +		 * last nthreads slots available in paca array then wait
> +		 * for boot cpu to show up.
> +		 */
> +		if (!boot_cpu_added && (cpu + nthreads) >= nr_cpu_ids) {
> +			int found = 0;
> +
> +			DBG("Holding last nthreads paca slots for boot cpu\n");
> +			for (j = 0; j < nthreads && cpu < nr_cpu_ids; j++) {
> +				if (boot_hw_cpuid == be32_to_cpu(intserv[j])) {
> +					found = 1;
> +					break;
> +				}
> +			}
> +			if (!found)
> +				continue;
> +		}
>
>  		for (j = 0; j < nthreads && cpu < nr_cpu_ids; j++) {
>  			bool avail;
> @@ -485,6 +505,11 @@ void __init smp_setup_cpu_maps(void)
>  			set_cpu_present(cpu, avail);
>  			set_hard_smp_processor_id(cpu, be32_to_cpu(intserv[j]));
>  			set_cpu_possible(cpu, true);
> +			if (boot_hw_cpuid == be32_to_cpu(intserv[j])) {
> +				DBG("Boot cpu %d (hard id %d) added to paca\n",
> +				    cpu, be32_to_cpu(intserv[j]));
> +				boot_cpu_added = true;
> +			}
>  			cpu++;
>  		}
>  	}
>
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