On Thu Nov 10, 2022 at 10:40 AM AEST, Jordan Niethe wrote:
> On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
> [resend as utf-8, not utf-7]
> > Give the queue head the ability to stop stealers. After a number of
> > spins without sucessfully acquiring the lock, the queue head employs
> > this, which will assure it is the next owner.
> > ---
> > arch/powerpc/include/asm/qspinlock_types.h | 10 +++-
> > arch/powerpc/lib/qspinlock.c | 56 +++++++++++++++++++++-
> > 2 files changed, 63 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/qspinlock_types.h
> > b/arch/powerpc/include/asm/qspinlock_types.h
> > index 210adf05b235..8b20f5e22bba 100644
> > --- a/arch/powerpc/include/asm/qspinlock_types.h
> > +++ b/arch/powerpc/include/asm/qspinlock_types.h
> > @@ -29,7 +29,8 @@ typedef struct qspinlock {
> > * Bitfields in the lock word:
> > *
> > * 0: locked bit
> > - * 16-31: tail cpu (+1)
> > + * 16: must queue bit
> > + * 17-31: tail cpu (+1)
> > */
> > #define _Q_SET_MASK(type) (((1U << _Q_ ## type ## _BITS) - 1)\
> > << _Q_ ## type ## _OFFSET)
> > @@ -38,7 +39,12 @@ typedef struct qspinlock {
> > #define _Q_LOCKED_MASK _Q_SET_MASK(LOCKED)
> > #define _Q_LOCKED_VAL (1U << _Q_LOCKED_OFFSET)
> >
> > -#define _Q_TAIL_CPU_OFFSET 16
> > +#define _Q_MUST_Q_OFFSET 16
> > +#define _Q_MUST_Q_BITS 1
> > +#define _Q_MUST_Q_MASK _Q_SET_MASK(MUST_Q)
> > +#define _Q_MUST_Q_VAL (1U << _Q_MUST_Q_OFFSET)
> > +
> > +#define _Q_TAIL_CPU_OFFSET 17
> > #define _Q_TAIL_CPU_BITS (32 - _Q_TAIL_CPU_OFFSET)
> > #define _Q_TAIL_CPU_MASK _Q_SET_MASK(TAIL_CPU)
>
> Not a big deal but some of these values could be calculated like in the
> generic version. e.g.
>
> #define _Q_PENDING_OFFSET (_Q_LOCKED_OFFSET +_Q_LOCKED_BITS)
Yeah, we don't *really* have more than one locked bit though. Haven't
made up my mind about these defines yet.
> > diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> > index 1625cce714b2..a906cc8f15fa 100644
> > --- a/arch/powerpc/lib/qspinlock.c
> > +++ b/arch/powerpc/lib/qspinlock.c
> > @@ -22,6 +22,7 @@ struct qnodes {
> > /* Tuning parameters */
> > static int STEAL_SPINS __read_mostly = (1<<5);
> > static bool MAYBE_STEALERS __read_mostly = true;
> > +static int HEAD_SPINS __read_mostly = (1<<8);
> >
> > static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
> >
> > @@ -30,6 +31,11 @@ static __always_inline int get_steal_spins(void)
> > return STEAL_SPINS;
> > }
> >
> > +static __always_inline int get_head_spins(void)
> > +{
> > + return HEAD_SPINS;
> > +}
> > +
> > static inline u32 encode_tail_cpu(void)
> > {
> > return (smp_processor_id() + 1) << _Q_TAIL_CPU_OFFSET;
> > @@ -142,6 +148,23 @@ static __always_inline u32 publish_tail_cpu(struct
> > qspinlock *lock, u32 tail)
> > return prev;
> > }
> >
> > +static __always_inline u32 lock_set_mustq(struct qspinlock *lock)
> > +{
> > + u32 new = _Q_MUST_Q_VAL;
> > + u32 prev;
> > +
> > + asm volatile(
> > +"1: lwarx %0,0,%1 # lock_set_mustq
> > \n"
>
> Is the EH bit not set because we don't hold the lock here?
Right, we're still waiting for it.
> > +" or %0,%0,%2 \n"
> > +" stwcx. %0,0,%1 \n"
> > +" bne- 1b \n"
> > + : "=&r" (prev)
> > + : "r" (&lock->val), "r" (new)
> > + : "cr0", "memory");
>
> This is another usage close to the DEFINE_TESTOP() pattern.
>
> > +
> > + return prev;
> > +}
> > +
> > static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
> > {
> > int cpu = get_tail_cpu(val);
> > @@ -165,6 +188,9 @@ static inline bool try_to_steal_lock(struct qspinlock
> > *lock)
> > for (;;) {
> > u32 val = READ_ONCE(lock->val);
> >
> > + if (val & _Q_MUST_Q_VAL)
> > + break;
> > +
> > if (unlikely(!(val & _Q_LOCKED_VAL))) {
> > if (trylock_with_tail_cpu(lock, val))
> > return true;
> > @@ -246,11 +272,22 @@ static inline void queued_spin_lock_mcs_queue(struct
> > qspinlock *lock)
> > /* We must be the owner, just set the lock bit and acquire */
> > lock_set_locked(lock);
> > } else {
> > + int iters = 0;
> > + bool set_mustq = false;
> > +
> > again:
> > /* We're at the head of the waitqueue, wait for the lock. */
> > - while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
> > + while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
> > cpu_relax();
> >
> > + iters++;
>
> It seems instead of using set_mustq, (val & _Q_MUST_Q_VAL) could be checked?
I wanted to give the reader (and compiler for what that's worth) the
idea that it won't change concurrently after we set it.
Thanks,
Nick
