Re: [Xen-devel] [PATCH] xen/sched: avoid cpumasks on stack in sched/core.c

2020-01-23 Thread Jürgen Groß 

On 24.01.20 01:01, Dario Faggioli wrote:

On Thu, 2020-01-23 at 10:03 +0100, Juergen Gross wrote:

There are still several instances of cpumask_t on the stack in
scheduling code. Avoid them as far as possible.

Signed-off-by: Juergen Gross 


Reviewed-by: Dario Faggioli 

Just curious...


--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -2586,11 +2582,11 @@ static void schedule(void)
  
  if ( gran > 1 )

  {
-cpumask_t mask;
+cpumask_t *mask = cpumask_scratch_cpu(cpu);
  
  prev->rendezvous_in_cnt = gran;

-cpumask_andnot(, sr->cpus, cpumask_of(cpu));
-cpumask_raise_softirq(, SCHED_SLAVE_SOFTIRQ);
+cpumask_andnot(mask, sr->cpus, cpumask_of(cpu));
+cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ);


... why are we keeping the temporary variable (mask) ?


per_cpu accesses are more expensive than those to local variables.
mask is used twice.


Juergen

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Re: [Xen-devel] [PATCH] xen/sched: avoid cpumasks on stack in sched/core.c

2020-01-23 Thread Dario Faggioli 
On Thu, 2020-01-23 at 10:03 +0100, Juergen Gross wrote:
> There are still several instances of cpumask_t on the stack in
> scheduling code. Avoid them as far as possible.
> 
> Signed-off-by: Juergen Gross 
>
Reviewed-by: Dario Faggioli 

Just curious...

> --- a/xen/common/sched/core.c
> +++ b/xen/common/sched/core.c
> @@ -2586,11 +2582,11 @@ static void schedule(void)
>  
>  if ( gran > 1 )
>  {
> -cpumask_t mask;
> +cpumask_t *mask = cpumask_scratch_cpu(cpu);
>  
>  prev->rendezvous_in_cnt = gran;
> -cpumask_andnot(, sr->cpus, cpumask_of(cpu));
> -cpumask_raise_softirq(, SCHED_SLAVE_SOFTIRQ);
> +cpumask_andnot(mask, sr->cpus, cpumask_of(cpu));
> +cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ);
>
... why are we keeping the temporary variable (mask) ?

Thanks and Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
---
<> (Raistlin Majere)



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