On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
[resend as utf-8, not utf-7]
> If the head of queue is preventing stealing but it finds the owner vCPU
> is preempted, it will yield its cycles to the owner which could cause it
> to become preempted. Add an option to re-allow stealers before yielding,
> and disallow them again after returning from the yield.
>
> Disable this option by default for now, i.e., no logical change.
> ---
> arch/powerpc/lib/qspinlock.c | 56 ++++++++++++++++++++++++++++++++++--
> 1 file changed, 53 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> index b39f8c5b329c..94f007f66942 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_allow_steal __read_mostly = false;
To me this one does read as a boolean, but if you go with those other changes
I'd make it pv_yield_steal_enable to be consistent.
> static bool pv_yield_prev __read_mostly = true;
>
> static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
> @@ -173,6 +174,23 @@ static __always_inline u32 lock_set_mustq(struct
> qspinlock *lock)
> return prev;
> }
>
> +static __always_inline u32 lock_clear_mustq(struct qspinlock *lock)
> +{
> + u32 new = _Q_MUST_Q_VAL;
> + u32 prev;
> +
> + asm volatile(
> +"1: lwarx %0,0,%1 # lock_clear_mustq \n"
> +" andc %0,%0,%2 \n"
> +" stwcx. %0,0,%1 \n"
> +" bne- 1b \n"
> + : "=&r" (prev)
> + : "r" (&lock->val), "r" (new)
> + : "cr0", "memory");
> +
This is pretty similar to the DEFINE_TESTOP() pattern again with the same llong
caveat.
> + return prev;
> +}
> +
> static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
> {
> int cpu = get_tail_cpu(val);
> @@ -188,7 +206,7 @@ static struct qnode *get_tail_qnode(struct qspinlock
> *lock, u32 val)
> BUG();
> }
>
> -static __always_inline void yield_to_locked_owner(struct qspinlock *lock,
> u32 val, bool paravirt)
> +static __always_inline void __yield_to_locked_owner(struct qspinlock *lock,
> u32 val, bool paravirt, bool clear_mustq)
/* See yield_to_locked_owner comment */ comment needs to be updated now.
> {
> int owner;
> u32 yield_count;
> @@ -217,7 +235,11 @@ static __always_inline void yield_to_locked_owner(struct
> qspinlock *lock, u32 va
> smp_rmb();
>
> if (READ_ONCE(lock->val) == val) {
> + if (clear_mustq)
> + lock_clear_mustq(lock);
> yield_to_preempted(owner, yield_count);
> + if (clear_mustq)
> + lock_set_mustq(lock);
> /* Don't relax if we yielded. Maybe we should? */
> return;
> }
> @@ -225,6 +247,16 @@ static __always_inline void yield_to_locked_owner(struct
> qspinlock *lock, u32 va
> cpu_relax();
> }
>
> +static __always_inline void yield_to_locked_owner(struct qspinlock *lock,
> u32 val, bool paravirt)
> +{
> + __yield_to_locked_owner(lock, val, paravirt, false);
> +}
> +
> +static __always_inline void yield_head_to_locked_owner(struct qspinlock
> *lock, u32 val, bool paravirt, bool clear_mustq)
> +{
The check for pv_yield_allow_steal seems like it could go here instead of
being done by the caller.
__yield_to_locked_owner() checks for pv_yield_owner so it seems more
consistent.
> + __yield_to_locked_owner(lock, val, paravirt, clear_mustq);
> +}
> +
> static __always_inline void yield_to_prev(struct qspinlock *lock, struct
> qnode *node, int prev_cpu, bool paravirt)
> {
> u32 yield_count;
> @@ -332,7 +364,7 @@ static __always_inline void
> queued_spin_lock_mcs_queue(struct qspinlock *lock, b
> if (!MAYBE_STEALERS) {
> /* We're at the head of the waitqueue, wait for the lock. */
> while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
> - yield_to_locked_owner(lock, val, paravirt);
> + yield_head_to_locked_owner(lock, val, paravirt, false);
>
> /* If we're the last queued, must clean up the tail. */
> if ((val & _Q_TAIL_CPU_MASK) == tail) {
> @@ -350,7 +382,8 @@ static __always_inline void
> queued_spin_lock_mcs_queue(struct qspinlock *lock, b
> again:
> /* We're at the head of the waitqueue, wait for the lock. */
> while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
> - yield_to_locked_owner(lock, val, paravirt);
> + yield_head_to_locked_owner(lock, val, paravirt,
> + pv_yield_allow_steal && set_mustq);
>
> iters++;
> if (!set_mustq && iters >= get_head_spins(paravirt)) {
> @@ -475,6 +508,22 @@ static int pv_yield_owner_get(void *data, u64 *val)
>
> DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_owner, pv_yield_owner_get,
> pv_yield_owner_set, "%llu\n");
>
> +static int pv_yield_allow_steal_set(void *data, u64 val)
> +{
> + pv_yield_allow_steal = !!val;
> +
> + return 0;
> +}
> +
> +static int pv_yield_allow_steal_get(void *data, u64 *val)
> +{
> + *val = pv_yield_allow_steal;
> +
> + return 0;
> +}
> +
> +DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_allow_steal, pv_yield_allow_steal_get,
> pv_yield_allow_steal_set, "%llu\n");
> +
> static int pv_yield_prev_set(void *data, u64 val)
> {
> pv_yield_prev = !!val;
> @@ -497,6 +546,7 @@ static __init int spinlock_debugfs_init(void)
> debugfs_create_file("qspl_head_spins", 0600, arch_debugfs_dir, NULL,
> &fops_head_spins);
> if (is_shared_processor()) {
> debugfs_create_file("qspl_pv_yield_owner", 0600,
> arch_debugfs_dir, NULL, &fops_pv_yield_owner);
> + debugfs_create_file("qspl_pv_yield_allow_steal", 0600,
> arch_debugfs_dir, NULL, &fops_pv_yield_allow_steal);
> debugfs_create_file("qspl_pv_yield_prev", 0600,
> arch_debugfs_dir, NULL, &fops_pv_yield_prev);
> }
>
