Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus

[Venki Pallipadi <venki@google.com>]

On Thu, Jan 19, 2012 at 12:43 PM, Srivatsa S. Bhat
<srivatsa.bhat@linux.vnet.ibm.com> wrote:
> Hi,
>
> On 01/20/2012 01:31 AM, Venkatesh Pallipadi wrote:
>
>> Does this look better? Will send separate patch to fix code
>> using num_possible_cpus() when they actually need nr_cpu_ids.
>>
>>
>>
>> Kernel's notion of possible cpus (from include/linux/cpumask.h)
>>  *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
>>
>>  *  The cpu_possible_mask is fixed at boot time, as the set of CPU id's
>>  *  that it is possible might ever be plugged in at anytime during the
>>  *  life of that system boot.
>>
>>  #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
>>
>> and on x86 cpumask_weight() calls hweight64 and hweight64 (on older kernels
>> and systems with !X86_FEATURE_POPCNT) or a popcnt based alternative.
>>
>> i.e, We needlessly go through this mask based calculation everytime
>> num_possible_cpus() is called.
>>
>> The problem is there with cpu_online_mask() as well, which is fixed value at
>> boot time in !CONFIG_HOTPLUG_CPU case and should not change that often even
>> in HOTPLUG case.
>>
>> Though most of the callers of these two routines are init time (with few
>> exceptions of runtime calls), it is cleaner to use variables
>> and not go through this repeated mask based calculation.
>>
>> Signed-off-by: Venkatesh Pallipadi <venki@google.com>
>> ---
>>  include/linux/cpumask.h |   10 ++++++++--
>>  kernel/cpu.c            |    5 +++++
>>  kernel/smp.c            |    4 ++++
>>  3 files changed, 17 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
>> index 4f7a632..ac3113b 100644
>> --- a/include/linux/cpumask.h
>> +++ b/include/linux/cpumask.h
>> @@ -23,10 +23,14 @@ typedef struct cpumask { DECLARE_BITMAP(bits, NR_CPUS); } cpumask_t;
>>
>>  #if NR_CPUS == 1
>>  #define nr_cpu_ids           1
>> +#define nr_possible_cpus     1
>>  #else
>>  extern int nr_cpu_ids;
>> +extern int nr_possible_cpus;
>>  #endif
>>
>> +extern int nr_online_cpus;
>> +
>>  #ifdef CONFIG_CPUMASK_OFFSTACK
>>  /* Assuming NR_CPUS is huge, a runtime limit is more efficient.  Also,
>>   * not all bits may be allocated. */
>> @@ -81,8 +85,10 @@ extern const struct cpumask *const cpu_present_mask;
>>  extern const struct cpumask *const cpu_active_mask;
>>
>>  #if NR_CPUS > 1
>> -#define num_online_cpus()    cpumask_weight(cpu_online_mask)
>> -#define num_possible_cpus()  cpumask_weight(cpu_possible_mask)
>> +
>> +#define num_online_cpus()    (nr_online_cpus)
>> +#define num_possible_cpus()  (nr_possible_cpus)
>> +
>>  #define num_present_cpus()   cpumask_weight(cpu_present_mask)
>>  #define num_active_cpus()    cpumask_weight(cpu_active_mask)
>>  #define cpu_online(cpu)              cpumask_test_cpu((cpu), cpu_online_mask)
>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>> index 2060c6e..f179baa 100644
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -622,6 +622,9 @@ static DECLARE_BITMAP(cpu_active_bits, CONFIG_NR_CPUS) __read_mostly;
>>  const struct cpumask *const cpu_active_mask = to_cpumask(cpu_active_bits);
>>  EXPORT_SYMBOL(cpu_active_mask);
>>
>> +int nr_online_cpus __read_mostly;
>> +EXPORT_SYMBOL(nr_online_cpus);
>> +
>>  void set_cpu_possible(unsigned int cpu, bool possible)
>>  {
>>       if (possible)
>
>
> Did you forget to add:
>
>        nr_possible_cpus = cpumask_weight(cpu_possible_mask);
>
> inside set_cpu_possible() ?

No. That was intentional as I have that coupled with nr_cpu_ids and
set once after all the bits are set in setup_nr_cpu_ids() instead of
doing for each bit set.

>
>
>> @@ -644,6 +647,8 @@ void set_cpu_online(unsigned int cpu, bool online)
>>               cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
>>       else
>>               cpumask_clear_cpu(cpu, to_cpumask(cpu_online_bits));
>> +
>> +     nr_online_cpus = cpumask_weight(cpu_online_mask);
>>  }
>>
>>  void set_cpu_active(unsigned int cpu, bool active)
>> diff --git a/kernel/smp.c b/kernel/smp.c
>> index db197d6..106e519 100644
>> --- a/kernel/smp.c
>> +++ b/kernel/smp.c
>> @@ -658,10 +658,14 @@ early_param("maxcpus", maxcpus);
>>  int nr_cpu_ids __read_mostly = NR_CPUS;
>>  EXPORT_SYMBOL(nr_cpu_ids);
>>
>> +int nr_possible_cpus __read_mostly = NR_CPUS;
>> +EXPORT_SYMBOL(nr_possible_cpus);
>> +
>>  /* An arch may set nr_cpu_ids earlier if needed, so this would be redundant */
>>  void __init setup_nr_cpu_ids(void)
>>  {
>>       nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
>> +     nr_possible_cpus = cpumask_weight(cpu_possible_mask);
>>  }
>>
>>  /* Called by boot processor to activate the rest. */
>
>
> Regards,
> Srivatsa S. Bhat
>