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

[Juergen Gross <jgross@xxxxxxxx>]

On 01/04/16 10:44, Peter Zijlstra wrote:
> 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.

Okay.

> 
>>> 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);

Okay.

> 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.

It is necessary for dom0 under Xen. I don't think there is a need to do
this on KVM as a guest has no direct access to e.g. BIOS functions of
the real hardware and the host system needs no vcpu pinning. I'm not
sure about VMWare.

Juergen


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