Message-ID: <YsYZeMJHsHwrOAe7@boqun-archlinux>
Date:   Wed, 6 Jul 2022 16:23:36 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/13] locking/qspinlock: remove pv_node abstraction

On Tue, Jul 05, 2022 at 12:38:08AM +1000, Nicholas Piggin wrote:
> There isn't much point trying to separate struct qnode from struct pv_node
> when struct qnode has to know about pv_node anyway.
> 
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
> ---
>  kernel/locking/qspinlock.c          |  3 ++-
>  kernel/locking/qspinlock_paravirt.h | 34 ++++++++++++-----------------
>  2 files changed, 16 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 65a9a10caa6f..a0fc21d99199 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -82,7 +82,8 @@
>  struct qnode {
>  	struct mcs_spinlock mcs;
>  #ifdef CONFIG_PARAVIRT_SPINLOCKS
> -	long reserved[2];
> +	int			cpu;
> +	u8			state;
>  #endif
>  };
>  
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index e84d21aa0722..b6a175155f36 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -47,12 +47,6 @@ enum vcpu_state {
>  	vcpu_hashed,		/* = pv_hash'ed + vcpu_halted */
>  };
>  
> -struct pv_node {
> -	struct mcs_spinlock	mcs;
> -	int			cpu;
> -	u8			state;
> -};
> -
>  /*
>   * Hybrid PV queued/unfair lock
>   *
> @@ -170,7 +164,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
>   */
>  struct pv_hash_entry {
>  	struct qspinlock *lock;
> -	struct pv_node   *node;
> +	struct qnode   *node;
>  };
>  
>  #define PV_HE_PER_LINE	(SMP_CACHE_BYTES / sizeof(struct pv_hash_entry))
> @@ -209,7 +203,7 @@ void __init __pv_init_lock_hash(void)
>  	     offset < (1 << pv_lock_hash_bits);						\
>  	     offset++, he = &pv_lock_hash[(hash + offset) & ((1 << pv_lock_hash_bits) - 1)])
>  
> -static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
> +static struct qspinlock **pv_hash(struct qspinlock *lock, struct qnode *node)
>  {
>  	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
>  	struct pv_hash_entry *he;
> @@ -236,11 +230,11 @@ static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
>  	BUG();
>  }
>  
> -static struct pv_node *pv_unhash(struct qspinlock *lock)
> +static struct qnode *pv_unhash(struct qspinlock *lock)
>  {
>  	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
>  	struct pv_hash_entry *he;
> -	struct pv_node *node;
> +	struct qnode *node;
>  
>  	for_each_hash_entry(he, offset, hash) {
>  		if (READ_ONCE(he->lock) == lock) {
> @@ -264,7 +258,7 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
>   * in a running state.
>   */
>  static inline bool
> -pv_wait_early(struct pv_node *prev, int loop)
> +pv_wait_early(struct qnode *prev, int loop)
>  {
>  	if ((loop & PV_PREV_CHECK_MASK) != 0)
>  		return false;
> @@ -277,9 +271,9 @@ pv_wait_early(struct pv_node *prev, int loop)
>   */
>  static void pv_init_node(struct mcs_spinlock *node)
>  {
> -	struct pv_node *pn = (struct pv_node *)node;
> +	struct qnode *pn = (struct qnode *)node;
>  
> -	BUILD_BUG_ON(sizeof(struct pv_node) > sizeof(struct qnode));
> +	BUILD_BUG_ON(sizeof(struct qnode) > sizeof(struct qnode));

This line can actually be removed ;-)

Other part looks good to me.

Acked-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

>  
>  	pn->cpu = smp_processor_id();
>  	pn->state = vcpu_running;
> @@ -292,8 +286,8 @@ static void pv_init_node(struct mcs_spinlock *node)
>   */
>  static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
>  {
> -	struct pv_node *pn = (struct pv_node *)node;
> -	struct pv_node *pp = (struct pv_node *)prev;
> +	struct qnode *pn = (struct qnode *)node;
> +	struct qnode *pp = (struct qnode *)prev;
>  	int loop;
>  	bool wait_early;
>  
> @@ -359,7 +353,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
>   */
>  static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
>  {
> -	struct pv_node *pn = (struct pv_node *)node;
> +	struct qnode *pn = (struct qnode *)node;
>  
>  	/*
>  	 * If the vCPU is indeed halted, advance its state to match that of
> @@ -402,7 +396,7 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
>  static u32
>  pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
>  {
> -	struct pv_node *pn = (struct pv_node *)node;
> +	struct qnode *pn = (struct qnode *)node;
>  	struct qspinlock **lp = NULL;
>  	int waitcnt = 0;
>  	int loop;
> @@ -492,7 +486,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
>  __visible void
>  __pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked)
>  {
> -	struct pv_node *node;
> +	struct qnode *node;
>  
>  	if (unlikely(locked != _Q_SLOW_VAL)) {
>  		WARN(!debug_locks_silent,
> @@ -517,14 +511,14 @@ __pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked)
>  	node = pv_unhash(lock);
>  
>  	/*
> -	 * Now that we have a reference to the (likely) blocked pv_node,
> +	 * Now that we have a reference to the (likely) blocked qnode,
>  	 * release the lock.
>  	 */
>  	smp_store_release(&lock->locked, 0);
>  
>  	/*
>  	 * At this point the memory pointed at by lock can be freed/reused,
> -	 * however we can still use the pv_node to kick the CPU.
> +	 * however we can still use the qnode to kick the CPU.
>  	 * The other vCPU may not really be halted, but kicking an active
>  	 * vCPU is harmless other than the additional latency in completing
>  	 * the unlock.
> -- 
> 2.35.1
> 

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