Subject: Re: [PATCH 09/17] powerpc/qspinlock: implement option to yield to previous node

From: "Nicholas Piggin" <[email-masked]>
Date: Thu, 10 Nov 2022 21:14:26 +1000

On Thu Nov 10, 2022 at 10:41 AM AEST, Jordan Niethe wrote:
> On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
> [resend as utf-8, not utf-7]
> > Queued waiters which are not at the head of the queue don't spin on
> > the lock word but their qnode lock word, waiting for the previous queued
> > CPU to release them. Add an option which allows these waiters to yield
> > to the previous CPU if its vCPU is preempted.
> > 
> > Disable this option by default for now, i.e., no logical change.
> > ---
> >  arch/powerpc/lib/qspinlock.c | 46 +++++++++++++++++++++++++++++++++++-
> >  1 file changed, 45 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> > index 55286ac91da5..b39f8c5b329c 100644
> > --- a/arch/powerpc/lib/qspinlock.c
> > +++ b/arch/powerpc/lib/qspinlock.c
> > @@ -26,6 +26,7 @@ static bool MAYBE_STEALERS __read_mostly = true;
> >  static int HEAD_SPINS __read_mostly = (1<<8);
> >  
> >  static bool pv_yield_owner __read_mostly = true;
> > +static bool pv_yield_prev __read_mostly = true;
>
> Similiar suggestion, maybe pv_yield_prev_enabled would read better.
>
> Isn't this enabled by default contrary to the commit message?

Yeah a few of those changelogs got out of synch.

>
> >  
> >  static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
> >  
> > @@ -224,6 +225,31 @@ static __always_inline void yield_to_locked_owner(struct qspinlock *lock, u32 va
> >  	cpu_relax();
> >  }
> >  
> > +static __always_inline void yield_to_prev(struct qspinlock *lock, struct qnode *node, int prev_cpu, bool paravirt)
>
> yield_to_locked_owner() takes a raw val and works out the cpu to yield to.
> I think for consistency have yield_to_prev() take the raw val and work it out too.

Good thinking.

> > +{
> > +	u32 yield_count;
> > +
> > +	if (!paravirt)
> > +		goto relax;
> > +
> > +	if (!pv_yield_prev)
> > +		goto relax;
> > +
> > +	yield_count = yield_count_of(prev_cpu);
> > +	if ((yield_count & 1) == 0)
> > +		goto relax; /* owner vcpu is running */
> > +
> > +	smp_rmb(); /* See yield_to_locked_owner comment */
> > +
> > +	if (!node->locked) {
> > +		yield_to_preempted(prev_cpu, yield_count);
> > +		return;
> > +	}
> > +
> > +relax:
> > +	cpu_relax();
> > +}
> > +
> >  
> >  static __always_inline bool try_to_steal_lock(struct qspinlock *lock, bool paravirt)
> >  {
> > @@ -291,13 +317,14 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
> >  	 */
> >  	if (old & _Q_TAIL_CPU_MASK) {
> >  		struct qnode *prev = get_tail_qnode(lock, old);
> > +		int prev_cpu = get_tail_cpu(old);
>
> This could then be removed.
>
> >  
> >  		/* Link @node into the waitqueue. */
> >  		WRITE_ONCE(prev->next, node);
> >  
> >  		/* Wait for mcs node lock to be released */
> >  		while (!node->locked)
> > -			cpu_relax();
> > +			yield_to_prev(lock, node, prev_cpu, paravirt);
>
> And would have this as:
> 			yield_to_prev(lock, node, old, paravirt);

Yep.

Thanks,
Nick