linux-kernel - Re: [PATCH v3 5/6] virt, sched: add cpu pinning to smp_call_sync_on_phys

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Date:	Fri, 1 Apr 2016 10:28:46 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
	david.vrabel@...rix.com, mingo@...hat.com,
	Douglas_Warzecha@...l.com, pali.rohar@...il.com, jdelvare@...e.com,
	linux@...ck-us.net, tglx@...utronix.de, hpa@...or.com,
	jeremy@...p.org, chrisw@...s-sol.org, akataria@...are.com,
	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	x86@...nel.org
Subject: Re: [PATCH v3 5/6] virt, sched: add cpu pinning to
 smp_call_sync_on_phys_cpu()

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?

> 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.

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)?


Juergen