On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
[resend as utf-8, not utf-7]
> Allow new waiters a number of spins on the lock word before queueing,
> which particularly helps paravirt performance when physical CPUs are
> oversubscribed.
> ---
> arch/powerpc/lib/qspinlock.c | 152 ++++++++++++++++++++++++++++++++---
> 1 file changed, 141 insertions(+), 11 deletions(-)
>
> diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> index 7c71e5e287df..1625cce714b2 100644
> --- a/arch/powerpc/lib/qspinlock.c
> +++ b/arch/powerpc/lib/qspinlock.c
> @@ -19,8 +19,17 @@ struct qnodes {
> struct qnode nodes[MAX_NODES];
> };
>
> +/* Tuning parameters */
> +static int STEAL_SPINS __read_mostly = (1<<5);
> +static bool MAYBE_STEALERS __read_mostly = true;
I can understand why, but macro case variables can be a bit confusing.
> +
> static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
>
> +static __always_inline int get_steal_spins(void)
> +{
> + return STEAL_SPINS;
> +}
> +
> static inline u32 encode_tail_cpu(void)
> {
> return (smp_processor_id() + 1) << _Q_TAIL_CPU_OFFSET;
> @@ -76,6 +85,39 @@ static __always_inline int trylock_clear_tail_cpu(struct
> qspinlock *lock, u32 ol
> return 0;
> }
>
> +static __always_inline u32 __trylock_cmpxchg(struct qspinlock *lock, u32
> old, u32 new)
> +{
> + u32 prev;
> +
> + BUG_ON(old & _Q_LOCKED_VAL);
> +
> + asm volatile(
> +"1: lwarx %0,0,%1,%4 # queued_spin_trylock_cmpxchg \n"
s/queued_spin_trylock_cmpxchg/__trylock_cmpxchg/
btw what is the format you using for the '\n's in the inline asm?
> +" cmpw 0,%0,%2 \n"
> +" bne- 2f \n"
> +" stwcx. %3,0,%1 \n"
> +" bne- 1b \n"
> +"\t" PPC_ACQUIRE_BARRIER " \n"
> +"2: \n"
> + : "=&r" (prev)
> + : "r" (&lock->val), "r"(old), "r" (new),
> + "i" (IS_ENABLED(CONFIG_PPC64) ? 1 : 0)
> + : "cr0", "memory");
This is very similar to trylock_clear_tail_cpu(). So maybe it is worth having
some form of "test and set" primitive helper.
> +
> + return prev;
> +}
> +
> +/* Take lock, preserving tail, cmpxchg with val (which must not be locked) */
> +static __always_inline int trylock_with_tail_cpu(struct qspinlock *lock, u32
> val)
> +{
> + u32 newval = _Q_LOCKED_VAL | (val & _Q_TAIL_CPU_MASK);
> +
> + if (__trylock_cmpxchg(lock, val, newval) == val)
> + return 1;
> + else
> + return 0;
same optional style nit: return __trylock_cmpxchg(lock, val, newval) == val
> +}
> +
> /*
> * Publish our tail, replacing previous tail. Return previous value.
> *
> @@ -115,6 +157,31 @@ static struct qnode *get_tail_qnode(struct qspinlock
> *lock, u32 val)
> BUG();
> }
>
> +static inline bool try_to_steal_lock(struct qspinlock *lock)
> +{
> + int iters;
> +
> + /* Attempt to steal the lock */
> + for (;;) {
> + u32 val = READ_ONCE(lock->val);
> +
> + if (unlikely(!(val & _Q_LOCKED_VAL))) {
> + if (trylock_with_tail_cpu(lock, val))
> + return true;
> + continue;
> + }
The continue would bypass iters++/cpu_relax but the next time around
if (unlikely(!(val & _Q_LOCKED_VAL))) {
should fail so everything should be fine?
> +
> + cpu_relax();
> +
> + iters++;
> +
> + if (iters >= get_steal_spins())
> + break;
> + }
> +
> + return false;
> +}
> +
> static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
> {
> struct qnodes *qnodesp;
> @@ -164,20 +231,39 @@ static inline void queued_spin_lock_mcs_queue(struct
> qspinlock *lock)
> smp_rmb(); /* acquire barrier for the mcs lock */
> }
>
> - /* We're at the head of the waitqueue, wait for the lock. */
> - while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
> - cpu_relax();
> + if (!MAYBE_STEALERS) {
> + /* We're at the head of the waitqueue, wait for the lock. */
> + while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
> + cpu_relax();
>
> - /* 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))
> - goto release;
> - /* Another waiter must have enqueued */
> - }
> + /* 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))
> + goto release;
> + /* Another waiter must have enqueued. */
> + }
> +
> + /* We must be the owner, just set the lock bit and acquire */
> + lock_set_locked(lock);
> + } else {
> +again:
> + /* We're at the head of the waitqueue, wait for the lock. */
> + while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
> + cpu_relax();
>
> - /* We must be the owner, just set the lock bit and acquire */
> - lock_set_locked(lock);
> + /* 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))
> + goto release;
> + /* Another waiter must have enqueued, or lock stolen. */
> + } else {
> + if (trylock_with_tail_cpu(lock, val))
> + goto unlock_next;
> + }
> + goto again;
> + }
>
> +unlock_next:
> /* contended path; must wait for next != NULL (MCS protocol) */
> while (!(next = READ_ONCE(node->next)))
> cpu_relax();
> @@ -197,6 +283,9 @@ static inline void queued_spin_lock_mcs_queue(struct
> qspinlock *lock)
>
> void queued_spin_lock_slowpath(struct qspinlock *lock)
> {
> + if (try_to_steal_lock(lock))
> + return;
> +
> queued_spin_lock_mcs_queue(lock);
> }
> EXPORT_SYMBOL(queued_spin_lock_slowpath);
> @@ -207,3 +296,44 @@ void pv_spinlocks_init(void)
> }
> #endif
>
> +#include <linux/debugfs.h>
> +static int steal_spins_set(void *data, u64 val)
> +{
> + static DEFINE_MUTEX(lock);
I just want to check if it would be possible to get rid of the MAYBE_STEALERS
variable completely and do something like:
bool maybe_stealers() { return STEAL_SPINS > 0; }
I guess based on the below code it wouldn't work, but I'm still not quite sure
why that is.
> +
> + mutex_lock(&lock);
> + if (val && !STEAL_SPINS) {
> + MAYBE_STEALERS = true;
> + /* wait for waiter to go away */
> + synchronize_rcu();
> + STEAL_SPINS = val;
> + } else if (!val && STEAL_SPINS) {
> + STEAL_SPINS = val;
> + /* wait for all possible stealers to go away */
> + synchronize_rcu();
> + MAYBE_STEALERS = false;
> + } else {
> + STEAL_SPINS = val;
> + }
> + mutex_unlock(&lock);
STEAL_SPINS is an int not a u64.
> +
> + return 0;
> +}
> +
> +static int steal_spins_get(void *data, u64 *val)
> +{
> + *val = STEAL_SPINS;
> +
> + return 0;
> +}
> +
> +DEFINE_SIMPLE_ATTRIBUTE(fops_steal_spins, steal_spins_get, steal_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);
> +
> + return 0;
> +}
> +device_initcall(spinlock_debugfs_init);
> +
