Re: [PATCH] locking/pvqspinlock: Add lock holder CPU argument to pv_wait()

Pan Xinhui
2016-04-14T09:37:29Z

On 2016年04月14日 04:59, Waiman Long wrote:
> Pan Xinhui was asking for a lock holder cpu argument in pv_wait()
> to help the porting of pvqspinlock to PPC. The new argument will can
> potentially help hypervisor expediate the execution of the critical
> section so that the lock holder vCPU can release the lock sooner.
>
> This patch does just that by storing the previous node vCPU number.
> In pv_wait_head_or_lock(), pv_wait() will be called with that vCPU
> number as it is likely to be the lock holder. In pv_wait_node(),
> -1 will be passed to pv_wait() instead to indicate that it doesn't
> know what the current lock holder is.
>
> This patch introduces negligible overhead to the current pvqspinlock
> code. The extra lockcpu argument isn't currently used in x86
> architecture.
>
> Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
> ---
> arch/x86/include/asm/paravirt.h | 4 ++--
> arch/x86/include/asm/paravirt_types.h | 2 +-
> arch/x86/kernel/kvm.c | 2 +-
> arch/x86/xen/spinlock.c | 2 +-
> kernel/locking/qspinlock_paravirt.h | 19 +++++++++++++++----
> kernel/locking/qspinlock_stat.h | 8 ++++----
> 6 files changed, 24 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index 601f1b8..b89eccf 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -676,9 +676,9 @@ static __always_inline void pv_queued_spin_unlock(struct qspinlock *lock)
> PVOP_VCALLEE1(pv_lock_ops.queued_spin_unlock, lock);
> }
>
> -static __always_inline void pv_wait(u8 *ptr, u8 val)
> +static __always_inline void pv_wait(u8 *ptr, u8 val, int lockcpu)
> {
> - PVOP_VCALL2(pv_lock_ops.wait, ptr, val);
> + PVOP_VCALL3(pv_lock_ops.wait, ptr, val, lockcpu);
> }
>
> static __always_inline void pv_kick(int cpu)
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index e8c2326..2fc26c1 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -312,7 +312,7 @@ struct pv_lock_ops {
> void (*queued_spin_lock_slowpath)(struct qspinlock *lock, u32 val);
> struct paravirt_callee_save queued_spin_unlock;
>
> - void (*wait)(u8 *ptr, u8 val);
> + void (*wait)(u8 *ptr, u8 val, int lockcpu);
> void (*kick)(int cpu);
> #else /* !CONFIG_QUEUED_SPINLOCKS */
> struct paravirt_callee_save lock_spinning;
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index dc1207e..47ab4e1 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -590,7 +590,7 @@ static void kvm_kick_cpu(int cpu)
>
> #include <asm/qspinlock.h>
>
> -static void kvm_wait(u8 *ptr, u8 val)
> +static void kvm_wait(u8 *ptr, u8 val, int lockcpu)
> {
> unsigned long flags;
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 9e2ba5c..6f78c41 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -33,7 +33,7 @@ static void xen_qlock_kick(int cpu)
> /*
> * Halt the current CPU & release it back to the host
> */
> -static void xen_qlock_wait(u8 *byte, u8 val)
> +static void xen_qlock_wait(u8 *byte, u8 val, int lockcpu)
> {
> int irq = __this_cpu_read(lock_kicker_irq);
>
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index 21ede57..4bec98b 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -51,6 +51,7 @@ struct pv_node {
> struct mcs_spinlock __res[3];
>
> int cpu;
> + int prev_cpu; /* Previous node cpu */
> u8 state;
> };
>
> @@ -156,8 +157,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
> * 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page.
> *
> * Since we should not be holding locks from NMI context (very rare indeed) the
> - * max load factor is 0.75, which is around the point where open addressing
> - * breaks down.
> + * max load factor is 0.75.
> *
> */
> struct pv_hash_entry {
> @@ -275,6 +275,7 @@ static void pv_init_node(struct mcs_spinlock *node)
>
> pn->cpu = smp_processor_id();
> pn->state = vcpu_running;
> + pn->prev_cpu = -1;
> }
>
> /*
> @@ -290,6 +291,8 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> int loop;
> bool wait_early;
>
> + pn->prev_cpu = pp->cpu; /* Save previous node vCPU */
> +
> /* waitcnt processing will be compiled out if !QUEUED_LOCK_STAT */
> for (;; waitcnt++) {
> for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
> @@ -317,7 +320,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> qstat_inc(qstat_pv_wait_node, true);
> qstat_inc(qstat_pv_wait_again, waitcnt);
> qstat_inc(qstat_pv_wait_early, wait_early);
> - pv_wait(&pn->state, vcpu_halted);
> + pv_wait(&pn->state, vcpu_halted, -1);
If the contention is high, we might run here. And we indeed need the lock holder on such scenario.
how about that, we store the lock into pv_node, then search the lock in hashtable,
code might look like,
node = pv_hash_lookup(pn->lock);
pv_wait(...,node->holder);
thanks
xinhui
> }
>
> /*
> @@ -453,7 +456,15 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
> WRITE_ONCE(pn->state, vcpu_halted);
> qstat_inc(qstat_pv_wait_head, true);
> qstat_inc(qstat_pv_wait_again, waitcnt);
> - pv_wait(&l->locked, _Q_SLOW_VAL);
> +
> + /*
> + * Pass in the previous node vCPU nmber which is likely to be
> + * the lock holder vCPU. This additional information may help
> + * the hypervisor to give more resource to that vCPU so that
> + * it can release the lock faster. With lock stealing,
> + * however, that vCPU may not be the actual lock holder.
> + */
> + pv_wait(&l->locked, _Q_SLOW_VAL, pn->prev_cpu);
>
> /*
> * The unlocker should have freed the lock before kicking the
> diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
> index eb2a2c9..8728348 100644
> --- a/kernel/locking/qspinlock_stat.h
> +++ b/kernel/locking/qspinlock_stat.h
> @@ -266,12 +266,12 @@ static inline void __pv_kick(int cpu)
> /*
> * Replacement function for pv_wait()
> */
> -static inline void __pv_wait(u8 *ptr, u8 val)
> +static inline void __pv_wait(u8 *ptr, u8 val, int lockcpu)
> {
> u64 *pkick_time = this_cpu_ptr(&pv_kick_time);
>
> *pkick_time = 0;
> - pv_wait(ptr, val);
> + pv_wait(ptr, val, lockcpu);
> if (*pkick_time) {
> this_cpu_add(qstats[qstat_pv_latency_wake],
> sched_clock() - *pkick_time);
> @@ -279,8 +279,8 @@ static inline void __pv_wait(u8 *ptr, u8 val)
> }
> }
>
> -#define pv_kick(c) __pv_kick(c)
> -#define pv_wait(p, v) __pv_wait(p, v)
> +#define pv_kick(c) __pv_kick(c)
> +#define pv_wait(p, v, c) __pv_wait(p, v, c)
>
> #else /* CONFIG_QUEUED_LOCK_STAT */
>