linux-kernel - Re: [PATCH 2/2] virtio-scsi: reset virtqueue affinity when doing cpu hotplug

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Message-ID: <50F624BA.20208@cn.fujitsu.com>
Date:	Wed, 16 Jan 2013 11:55:38 +0800
From:	Wanlong Gao <gaowanlong@...fujitsu.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	virtualization@...ts.linux-foundation.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	hutao@...fujitsu.com, linux-scsi@...r.kernel.org, mst@...hat.com,
	asias@...hat.com, stefanha@...hat.com, nab@...ux-iscsi.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>
Subject: Re: [PATCH 2/2] virtio-scsi: reset virtqueue affinity when doing
 cpu hotplug

On 01/16/2013 11:31 AM, Rusty Russell wrote:
> Wanlong Gao <gaowanlong@...fujitsu.com> writes:
>> Add hot cpu notifier to reset the request virtqueue affinity
>> when doing cpu hotplug.
> 
> You need to be careful to get_online_cpus() and put_online_cpus() here,
> so CPUs can't go up and down in the middle of operations.
> 
> In particular, get_online_cpus()/put_online_cpus() around calls to
> virtscsi_set_affinity() (except within notifiers).

Yes, I'll take care of this, thank you.

> 
>> +static int virtscsi_cpu_callback(struct notifier_block *nfb,
>> +				 unsigned long action, void *hcpu)
>> +{
>> +	struct virtio_scsi *vscsi = container_of(nfb, struct virtio_scsi, nb);
>> +	switch(action) {
>> +	case CPU_ONLINE:
>> +	case CPU_ONLINE_FROZEN:
>> +	case CPU_DEAD:
>> +	case CPU_DEAD_FROZEN:
>> +		virtscsi_set_affinity(vscsi, true);
>> +		break;
>> +	default:
>> +		break;
>> +	}
>> +	return NOTIFY_OK;
>> +}
> 
> You probably want to remove affinities *before* the CPU goes down, and
> restore it after the CPU comes up.
> 
> So, how about:
> 
>         switch (action & ~CPU_TASKS_FROZEN) {
>         case CPU_ONLINE:
>         case CPU_DOWN_FAILED:
>                 virtscsi_set_affinity(vscsi, true, -1);
>                 break;
>         case CPU_DOWN_PREPARE:
>                 virtscsi_set_affinity(vscsi, true, (unsigned long)hcpu);
>                 break;
>         }
> 
> The extra argument to virtscsi_set_affinity() is to tell it to ignore a
> cpu which seems online (because it's going down).

OK, thank you very much for teaching this.

> 
>>  static void virtscsi_init_vq(struct virtio_scsi_vq *virtscsi_vq,
>>  			     struct virtqueue *vq)
>>  {
>> @@ -888,6 +909,13 @@ static int __devinit virtscsi_probe(struct virtio_device *vdev)
>>  	if (err)
>>  		goto virtscsi_init_failed;
>>  
>> +	vscsi->nb.notifier_call = &virtscsi_cpu_callback;
>> +	err = register_hotcpu_notifier(&vscsi->nb);
>> +	if (err) {
>> +		pr_err("virtio_scsi: registering cpu notifier failed\n");
>> +		goto scsi_add_host_failed;
>> +	}
>> +
> 
> You have to clean this up if scsi_add_host() fails, I think.

Yeah, will do, thank you very much. 


Regards,
Wanlong Gao

> 
> Cheers,
> Rusty.
> --
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