Re: [PATCHv1] arm: reduce power use by contented spin locks with WFE/SEV

[David Vrabel <david.vrabel@citrix.com>]

On 31/07/15 11:45, David Vrabel wrote:
> Instead of cpu_relax() while spinning and observing the ticket head,
> introduce spin_relax() which executes a WFE instruction.  After the
> ticket head is changed call spin_signal() to execute an SVE
> instruction to wake any spinners.
> 
> This should improve power consumption when locks are contented and
> spinning.
> 
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
> I've not tested this but it looks straight-forward...
> ---
>  xen/common/spinlock.c          | 5 +++--
>  xen/include/asm-arm/spinlock.h | 3 ++-
>  xen/include/asm-x86/spinlock.h | 3 +++
>  3 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
> index 29149d1..fc3e8e7 100644
> --- a/xen/common/spinlock.c
> +++ b/xen/common/spinlock.c
> @@ -141,7 +141,7 @@ void _spin_lock(spinlock_t *lock)
>      while ( tickets.tail != observe_head(&lock->tickets) )
>      {
>          LOCK_PROFILE_BLOCK;
> -        cpu_relax();
> +        spin_relax();
>      }
>      LOCK_PROFILE_GOT;
>      preempt_disable();
> @@ -170,6 +170,7 @@ void _spin_unlock(spinlock_t *lock)
>      preempt_enable();
>      LOCK_PROFILE_REL;
>      add_sized(&lock->tickets.head, 1);
> +    spin_signal();

It occurs to me that perhaps there should be a barrier between the
add_sized() and the spin_signal() so the update value is visible before
we signal (otherwise the spinner may be woken and observe the old value
and WFE again).

spin_relax() and spin_signal() might be better named arch_lock_relax()
and arch_lock_signal() to match the naming of existing arch_lock_*() hooks.

I think someone with more arm experience (and the means to test) should
take this patch on.

David