Re: [Xen-devel] [PATCHv3 3/4] xen: use ticket locks for spin locks

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> David Vrabel <david.vrabel@xxxxxxxxxx> 04/28/15 6:16 PM >>>
>On 23/04/15 12:58, Jan Beulich wrote:
>>> +typedef union {
>>> +    u32 head_tail;
>>> +    struct {
>>> +        u16 head;
>>> +        u16 tail;
>>> +    };
>>> +} spinlock_tickets_t;
>>> +
>>>  typedef struct spinlock {
>>> -    raw_spinlock_t raw;
>>> +    spinlock_tickets_t tickets;
>> 
>> At least for x86 this means a growth of this and hence various
>> other structures - did you examine the effects thereof? Of
>> course otoh getting the lock size uniform across architectures
>> is a good thing.
>
>I've not looked.
>
>Are there any structures whose size you're particularly concerned about?

No specific ones (but of course structures with an inherent size constraint
- like struct domain and struct vcpu - are, with all of their sub-structures,
primary candidates). I just recall that in some cases (and this may no longer
apply due to later additions) structures got arranged specifically taking in
mind the 2-byte size of the locks, and the growth here may thus mean a
structure size growth of more than just two bytes.

Jan


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