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 10:28:46AM +0200, Juergen Gross wrote:
> On 01/04/16 09:43, Peter Zijlstra wrote:
> > 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.
> 
> Main reason for disabling preemption was to avoid any suspend/resume
> cycles while vcpu pinning is active.
> 
> With the switch to workqueues this might not be necessary, if I've read
> try_to_freeze_tasks() correctly. Can you confirm, please?

This is not something we should worry about; the caller should ensure
the CPU stays valid; typically I would expect a caller to do
get_online_cpus() before 'computing' what CPU to send the function to.

> > 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.
> 
> Okay.
> 
> > And then build a phys variant on top.
> 
> Hmm, I'm not sure I understand what you are suggesting here.
> 
> Should this phys variant make use of smp_call_on_cpu() via an
> intermediate function called on the dedicated cpu which is doing the
> pinning and calling the user function then?
> 
> Or do you want the phys variant to either use smp_call_on_cpu() or to
> do the pinning and call the user function by itself depending on the
> environment (pinning supported)?

Yeah, uhmm.. not sure on the details; my brain is having a hard time
engaging this morning.

Maybe just make the vpin thing an option like:

        smp_call_on_cpu(int (*func)(void *), int phys_cpu);

Also; is something like the vpin thing possible on KVM? because if we're
going to expose it to generic code like this we had maybe look at wider
support.

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