Re: [PATCH 06/17] powerpc/qspinlock: theft prevention to control latency

[Jordan Niethe <jniethe5@gmail.com>]

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;
>  }