Re: [Xen-devel] [PATCH] xen: Send spinlock IPI to all waiters

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2013-02-15 at 15:46 +0000, Jan Beulich wrote:
> >>> On 15.02.13 at 16:14, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > On Fri, Feb 15, 2013 at 11:31:23AM +0000, Ian Campbell wrote:
> >> On Fri, 2013-02-15 at 11:26 +0000, Jan Beulich wrote:
> >> > >>> On 15.02.13 at 12:10, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> >> > > On Fri, 2013-02-15 at 10:52 +0000, Stefan Bader wrote:
> >> > >> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> >> > >> index 83e866d..f7a080e 100644
> >> > >> --- a/arch/x86/xen/spinlock.c
> >> > >> +++ b/arch/x86/xen/spinlock.c
> >> > >> @@ -328,7 +328,6 @@ static noinline void xen_spin_unlock_slow(struct 
> >> > > xen_spinlock *xl)
> >> > >>               if (per_cpu(lock_spinners, cpu) == xl) {
> >> > >>                       ADD_STATS(released_slow_kicked, 1);
> >> > >>                       xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
> >> > >> -                     break;
> >> > > 
> >> > > It would be more efficient to build a mask and use xen_send_IPI_mask().
> >> > 
> >> > In order for __xen_send_IPI_mask() to then take the list apart
> >> > again and call xen_send_IPI_one()? There's no batching
> >> > implemented currently...
> >> 
> >> Oh, I simply assumed it must obviously do that!
> > 
> > Perhaps if it was done via a multicall? We could batch then things up. But 
> > I 
> > recall you saying that for homogeneous calls it is silly to use multicalls.
> 
> Aren't you mixing this up with it being pointless to use multicalls
> for things that already allow for batching, like grant table ops?

Right, it is only silly to use multicalls for homogeneous calls which
already natively support batching.

Ian.



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