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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 31 Jul 2015, Julien Grall wrote:
> Hi David,
> 
> On 31/07/15 11:52, David Vrabel wrote:
> > 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@xxxxxxxxxx>
> >> ---
> >> 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).
> 
> sev is usually precede by dsb to ensure that all the instructions before
> as completed before executing the sev.

Yes, a dsb() is required. This being common code, we could use wmb().

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