Linux Virtualization - Nov 2017 - [PATCH-tip v2 2/2] x86/xen: Deprecate xen_nopvspin

On 11/01/2017 04:58 PM, Waiman Long wrote:
> +/* TODO: To be removed in a future kernel version */
>  static __init int xen_parse_nopvspin(char *arg)
>  {
> -	xen_pvspin = false;
> +	pr_warn("xen_nopvspin is deprecated, replace it with
\"pvlock_type=queued\"!\n");
> +	if (!pv_spinlock_type)
> +		pv_spinlock_type = locktype_queued;
Since we currently end up using unfair locks and because you are
deprecating xen_nopvspin I wonder whether it would be better to set this
to locktype_unfair so that current behavior doesn't change. (Sorry, I
haven't responded to your earlier message before you posted this). Juergen?

I am also not sure I agree with making pv_spinlock an enum *and* a
bitmask at the same time. I understand that it makes checks easier but I
think not assuming a value or a pattern would be better, especially
since none of the uses is on a critical path.

(For example, !pv_spinlock_type is the same as locktype_auto, which is
defined but never used)


-boris
>  	return 0;
>  }
>  early_param("xen_nopvspin", xen_parse_nopvspin);
> -

On 11/01/2017 06:01 PM, Boris Ostrovsky wrote:
> On 11/01/2017 04:58 PM, Waiman Long wrote:
>> +/* TODO: To be removed in a future kernel version */
>>  static __init int xen_parse_nopvspin(char *arg)
>>  {
>> -	xen_pvspin = false;
>> +	pr_warn("xen_nopvspin is deprecated, replace it with
\"pvlock_type=queued\"!\n");
>> +	if (!pv_spinlock_type)
>> +		pv_spinlock_type = locktype_queued;
> Since we currently end up using unfair locks and because you are
> deprecating xen_nopvspin I wonder whether it would be better to set this
> to locktype_unfair so that current behavior doesn't change. (Sorry, I
> haven't responded to your earlier message before you posted this).
Juergen?
I think the latest patch from Juergen in tip is to use native qspinlock
when xen_nopvspin is specified. Right? That is why I made the current
choice. I can certainly change to unfair if it is what you guys want.
> I am also not sure I agree with making pv_spinlock an enum *and* a
> bitmask at the same time. I understand that it makes checks easier but I
> think not assuming a value or a pattern would be better, especially
> since none of the uses is on a critical path.
>
> (For example, !pv_spinlock_type is the same as locktype_auto, which is
> defined but never used)
OK, I will take out the enum and make explicit use of locktype_auto.

Cheers,
Longman

On 02/11/17 14:25, Waiman Long wrote:
> On 11/01/2017 06:01 PM, Boris Ostrovsky wrote:
>> On 11/01/2017 04:58 PM, Waiman Long wrote:
>>> +/* TODO: To be removed in a future kernel version */
>>>  static __init int xen_parse_nopvspin(char *arg)
>>>  {
>>> -	xen_pvspin = false;
>>> +	pr_warn("xen_nopvspin is deprecated, replace it with
\"pvlock_type=queued\"!\n");
>>> +	if (!pv_spinlock_type)
>>> +		pv_spinlock_type = locktype_queued;
>> Since we currently end up using unfair locks and because you are
>> deprecating xen_nopvspin I wonder whether it would be better to set
this
>> to locktype_unfair so that current behavior doesn't change. (Sorry,
I
>> haven't responded to your earlier message before you posted this).
Juergen?
> 
> I think the latest patch from Juergen in tip is to use native qspinlock
> when xen_nopvspin is specified. Right? That is why I made the current
> choice. I can certainly change to unfair if it is what you guys want.
No, when we are keeping xen_nopvspin (even as deprecated) it should
behave as designed, so locktype_queued is correct.


Juergen