Re: [Xen-devel] [PATCH v3 5/6] virt, sched: add cpu pinning to smp_call_sync_on_phys_cpu()

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, Apr 01, 2016 at 09:14:33AM +0200, Juergen Gross wrote:
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -14,6 +14,7 @@
>  #include <linux/smp.h>
>  #include <linux/cpu.h>
>  #include <linux/sched.h>
> +#include <linux/hypervisor.h>
>  
>  #include "smpboot.h"
>  
> @@ -758,9 +759,14 @@ struct smp_sync_call_struct {
>  static void smp_call_sync_callback(struct work_struct *work)
>  {
>       struct smp_sync_call_struct *sscs;
> +     unsigned int cpu;
>  
>       sscs = container_of(work, struct smp_sync_call_struct, work);
> +     cpu = get_cpu();
> +     hypervisor_pin_vcpu(cpu);
>       sscs->ret = sscs->func(sscs->data);
> +     hypervisor_pin_vcpu(-1);
> +     put_cpu();
>  
>       complete(&sscs->done);
>  }

So I don't really like this; it adds the requirement that the function
cannot schedule, which greatly limits the utility of the construct. At
this point you might as well use the regular IPI stuff.

You can easily avoid this constraint by using:

        hypervisor_pin_vcpu(smp_processor_id());

Also, for the vpinning stuff, the UP version below is sufficient, even
on SMP systems (with the current !preempt constraint). Which seems to
suggest we're not having the right interface for this.

So I would propose you add:

        smp_call_on_cpu()

As per patch 2. No promises about physical or anything. This means it
can be used freely by anyone that wants to run a function on another
cpu -- a much more useful thing.

And then build a phys variant on top.


> diff --git a/kernel/up.c b/kernel/up.c
> index afd395c..725ec44 100644
> --- a/kernel/up.c
> +++ b/kernel/up.c
> @@ -6,6 +6,7 @@
>  #include <linux/kernel.h>
>  #include <linux/export.h>
>  #include <linux/smp.h>
> +#include <linux/hypervisor.h>
>  
>  int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
>                               int wait)
> @@ -85,9 +86,17 @@ EXPORT_SYMBOL(on_each_cpu_cond);
>  
>  int smp_call_sync_on_phys_cpu(unsigned int cpu, int (*func)(void *), void 
> *par)
>  {
> +     int ret;
> +
>       if (cpu != 0)
>               return -EINVAL;
>  
> -     return func(par);
> +     preempt_disable();
> +     hypervisor_pin_vcpu(0);
> +     ret = func(par);
> +     hypervisor_pin_vcpu(-1);
> +     preempt_enable();
> +
> +     return ret;
>  }
>  EXPORT_SYMBOL_GPL(smp_call_sync_on_phys_cpu);
> -- 
> 2.6.2
> 

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