[PATCH 08/17] powerpc/qspinlock: paravirt yield to lock owner
Jordan Niethe
jniethe5 at gmail.com
Fri Aug 12 12:01:15 AEST 2022
On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
> Waiters spinning on the lock word should yield to the lock owner if the
> vCPU is preempted. This improves performance when the hypervisor has
> oversubscribed physical CPUs.
> ---
> arch/powerpc/lib/qspinlock.c | 97 ++++++++++++++++++++++++++++++------
> 1 file changed, 83 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> index aa26cfe21f18..55286ac91da5 100644
> --- a/arch/powerpc/lib/qspinlock.c
> +++ b/arch/powerpc/lib/qspinlock.c
> @@ -5,6 +5,7 @@
> #include <linux/percpu.h>
> #include <linux/smp.h>
> #include <asm/qspinlock.h>
> +#include <asm/paravirt.h>
>
> #define MAX_NODES 4
>
> @@ -24,14 +25,16 @@ static int STEAL_SPINS __read_mostly = (1<<5);
> static bool MAYBE_STEALERS __read_mostly = true;
> static int HEAD_SPINS __read_mostly = (1<<8);
>
> +static bool pv_yield_owner __read_mostly = true;
Not macro case for these globals? To me name does not make it super clear this
is a boolean. What about pv_yield_owner_enabled?
> +
> static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
>
> -static __always_inline int get_steal_spins(void)
> +static __always_inline int get_steal_spins(bool paravirt)
> {
> return STEAL_SPINS;
> }
>
> -static __always_inline int get_head_spins(void)
> +static __always_inline int get_head_spins(bool paravirt)
> {
> return HEAD_SPINS;
> }
> @@ -46,7 +49,11 @@ static inline int get_tail_cpu(u32 val)
> return (val >> _Q_TAIL_CPU_OFFSET) - 1;
> }
>
> -/* Take the lock by setting the bit, no other CPUs may concurrently lock it. */
> +static inline int get_owner_cpu(u32 val)
> +{
> + return (val & _Q_OWNER_CPU_MASK) >> _Q_OWNER_CPU_OFFSET;
> +}
> +
> /* Take the lock by setting the lock bit, no other CPUs will touch it. */
> static __always_inline void lock_set_locked(struct qspinlock *lock)
> {
> @@ -180,7 +187,45 @@ static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
> BUG();
> }
>
> -static inline bool try_to_steal_lock(struct qspinlock *lock)
> +static __always_inline void yield_to_locked_owner(struct qspinlock *lock, u32 val, bool paravirt)
This name doesn't seem correct for the non paravirt case.
> +{
> + int owner;
> + u32 yield_count;
> +
> + BUG_ON(!(val & _Q_LOCKED_VAL));
> +
> + if (!paravirt)
> + goto relax;
> +
> + if (!pv_yield_owner)
> + goto relax;
> +
> + owner = get_owner_cpu(val);
> + yield_count = yield_count_of(owner);
> +
> + if ((yield_count & 1) == 0)
> + goto relax; /* owner vcpu is running */
I wonder why not use vcpu_is_preempted()?
> +
> + /*
> + * Read the lock word after sampling the yield count. On the other side
> + * there may a wmb because the yield count update is done by the
> + * hypervisor preemption and the value update by the OS, however this
> + * ordering might reduce the chance of out of order accesses and
> + * improve the heuristic.
> + */
> + smp_rmb();
> +
> + if (READ_ONCE(lock->val) == val) {
> + yield_to_preempted(owner, yield_count);
> + /* Don't relax if we yielded. Maybe we should? */
> + return;
> + }
> +relax:
> + cpu_relax();
> +}
> +
> +
> +static __always_inline bool try_to_steal_lock(struct qspinlock *lock, bool paravirt)
> {
> int iters;
>
> @@ -197,18 +242,18 @@ static inline bool try_to_steal_lock(struct qspinlock *lock)
> continue;
> }
>
> - cpu_relax();
> + yield_to_locked_owner(lock, val, paravirt);
>
> iters++;
>
> - if (iters >= get_steal_spins())
> + if (iters >= get_steal_spins(paravirt))
> break;
> }
>
> return false;
> }
>
> -static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
> +static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, bool paravirt)
> {
> struct qnodes *qnodesp;
> struct qnode *next, *node;
> @@ -260,7 +305,7 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
> 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();
> + yield_to_locked_owner(lock, val, paravirt);
>
> /* If we're the last queued, must clean up the tail. */
> if ((val & _Q_TAIL_CPU_MASK) == tail) {
> @@ -278,10 +323,10 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
> again:
> /* We're at the head of the waitqueue, wait for the lock. */
> while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
> - cpu_relax();
> + yield_to_locked_owner(lock, val, paravirt);
>
> iters++;
> - if (!set_mustq && iters >= get_head_spins()) {
> + if (!set_mustq && iters >= get_head_spins(paravirt)) {
> set_mustq = true;
> lock_set_mustq(lock);
> val |= _Q_MUST_Q_VAL;
> @@ -320,10 +365,15 @@ 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);
> + if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) && is_shared_processor()) {
> + if (try_to_steal_lock(lock, true))
> + return;
> + queued_spin_lock_mcs_queue(lock, true);
> + } else {
> + if (try_to_steal_lock(lock, false))
> + return;
> + queued_spin_lock_mcs_queue(lock, false);
> + }
> }
There is not really a need for a conditional:
bool paravirt = IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
is_shared_processor();
if (try_to_steal_lock(lock, paravirt))
return;
queued_spin_lock_mcs_queue(lock, paravirt);
The paravirt parameter used by the various functions seems always to be
equivalent to (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) && is_shared_processor()).
I wonder if it would be simpler testing (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) && is_shared_processor())
(using a helper function) in those functions instead passing it as a parameter?
> EXPORT_SYMBOL(queued_spin_lock_slowpath);
>
> @@ -382,10 +432,29 @@ static int head_spins_get(void *data, u64 *val)
>
> DEFINE_SIMPLE_ATTRIBUTE(fops_head_spins, head_spins_get, head_spins_set, "%llu\n");
>
> +static int pv_yield_owner_set(void *data, u64 val)
> +{
> + pv_yield_owner = !!val;
> +
> + return 0;
> +}
> +
> +static int pv_yield_owner_get(void *data, u64 *val)
> +{
> + *val = pv_yield_owner;
> +
> + return 0;
> +}
> +
> +DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_owner, pv_yield_owner_get, pv_yield_owner_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);
> + if (is_shared_processor()) {
> + debugfs_create_file("qspl_pv_yield_owner", 0600, arch_debugfs_dir, NULL, &fops_pv_yield_owner);
> + }
>
> return 0;
> }
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