Re: [Xen-devel] [PATCH v2] xen: remove on-stack cpumask from stop_machine_run()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.05.19 at 13:53, <jgross@xxxxxxxx> wrote:
> The "allbutself" cpumask in stop_machine_run() is not needed. Instead
> of allocating it on the stack it can easily be avoided.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one further remark:

> --- a/xen/common/stop_machine.c
> +++ b/xen/common/stop_machine.c
> @@ -69,8 +69,8 @@ static void stopmachine_wait_state(void)
>  
>  int stop_machine_run(int (*fn)(void *), void *data, unsigned int cpu)
>  {
> -    cpumask_t allbutself;
>      unsigned int i, nr_cpus;
> +    unsigned int this = smp_processor_id();
>      int ret;
>  
>      BUG_ON(!local_irq_is_enabled());
> @@ -79,9 +79,9 @@ int stop_machine_run(int (*fn)(void *), void *data, 
> unsigned int cpu)
>      if ( !get_cpu_maps() )
>          return -EBUSY;
>  
> -    cpumask_andnot(&allbutself, &cpu_online_map,
> -                   cpumask_of(smp_processor_id()));
> -    nr_cpus = cpumask_weight(&allbutself);
> +    nr_cpus = num_online_cpus();
> +    if ( cpu_online(this) )
> +        nr_cpus--;
>  
>      /* Must not spin here as the holder will expect us to be descheduled. */
>      if ( !spin_trylock(&stopmachine_lock) )
> @@ -100,8 +100,9 @@ int stop_machine_run(int (*fn)(void *), void *data, 
> unsigned int cpu)
>  
>      smp_wmb();
>  
> -    for_each_cpu ( i, &allbutself )
> -        tasklet_schedule_on_cpu(&per_cpu(stopmachine_tasklet, i), i);
> +    for_each_online_cpu ( i )
> +        if ( i != this )
> +            tasklet_schedule_on_cpu(&per_cpu(stopmachine_tasklet, i), i);
>  
>      stopmachine_set_state(STOPMACHINE_PREPARE);
>      stopmachine_wait_state();

A few lines down from here there's another use of smp_processor_id().
If I end up committing this I may take the liberty and also change that
to "this", unless you object.

Jan



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