On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote: > 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) > > 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? > +" 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? > + if (!set_mustq && iters >= get_head_spins()) { > + set_mustq = true; > + lock_set_mustq(lock); > + val |= _Q_MUST_Q_VAL; > + } > + } > + > /* If we're the last queued, must clean up the tail. */ > if ((val & _Q_TAIL_CPU_MASK) == tail) { > if (trylock_clear_tail_cpu(lock, val)) > @@ -329,9 +366,26 @@ static int steal_spins_get(void *data, u64 *val) > > DEFINE_SIMPLE_ATTRIBUTE(fops_steal_spins, steal_spins_get, steal_spins_set, > "%llu\n"); > > +static int head_spins_set(void *data, u64 val) > +{ > + HEAD_SPINS = val; > + > + return 0; > +} > + > +static int head_spins_get(void *data, u64 *val) > +{ > + *val = HEAD_SPINS; > + > + return 0; > +} > + > +DEFINE_SIMPLE_ATTRIBUTE(fops_head_spins, head_spins_get, head_spins_set, > "%llu\n"); > + > static __init int spinlock_debugfs_init(void) > { > debugfs_create_file("qspl_steal_spins", 0600, arch_debugfs_dir, NULL, > &fops_steal_spins); > + debugfs_create_file("qspl_head_spins", 0600, arch_debugfs_dir, NULL, > &fops_head_spins); > > return 0; > }