The following commit has been merged into the locking/core branch of tip: Commit-ID: 992caf7f17243d736fc996770bac6566103778f6 Gitweb: https://git.kernel.org/tip/992caf7f17243d736fc996770bac6566103778f6 Author: Steven Rostedt <rostedt@xxxxxxxxxxx> AuthorDate: Sun, 15 Aug 2021 23:29:25 +02:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Tue, 17 Aug 2021 19:06:11 +02:00 locking/rtmutex: Add adaptive spinwait mechanism Going to sleep when locks are contended can be quite inefficient when the contention time is short and the lock owner is running on a different CPU. The MCS mechanism cannot be used because MCS is strictly FIFO ordered while for rtmutex based locks the waiter ordering is priority based. Provide a simple adaptive spinwait mechanism which currently restricts the spinning to the top priority waiter. [ tglx: Provide a contemporary changelog, extended it to all rtmutex based locks and updated it to match the other spin on owner implementations ] Originally-by: Gregory Haskins <ghaskins@xxxxxxxxxx> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Link: https://lore.kernel.org/r/20210815211305.912050691@xxxxxxxxxxxxx --- kernel/locking/rtmutex.c | 67 +++++++++++++++++++++++++++++++++++++-- 1 file changed, 65 insertions(+), 2 deletions(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 3eaf636..8aaa352 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -8,6 +8,11 @@ * Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@xxxxxxxxxxx> * Copyright (C) 2005 Kihon Technologies Inc., Steven Rostedt * Copyright (C) 2006 Esben Nielsen + * Adaptive Spinlocks: + * Copyright (C) 2008 Novell, Inc., Gregory Haskins, Sven Dietrich, + * and Peter Morreale, + * Adaptive Spinlocks simplification: + * Copyright (C) 2008 Red Hat, Inc., Steven Rostedt <srostedt@xxxxxxxxxx> * * See Documentation/locking/rt-mutex-design.rst for details. */ @@ -1297,6 +1302,52 @@ static __always_inline void __rt_mutex_unlock(struct rt_mutex_base *lock) rt_mutex_slowunlock(lock); } +#ifdef CONFIG_SMP +static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock, + struct rt_mutex_waiter *waiter, + struct task_struct *owner) +{ + bool res = true; + + rcu_read_lock(); + for (;;) { + /* If owner changed, trylock again. */ + if (owner != rt_mutex_owner(lock)) + break; + /* + * Ensure that @owner is dereferenced after checking that + * the lock owner still matches @owner. If that fails, + * @owner might point to freed memory. If it still matches, + * the rcu_read_lock() ensures the memory stays valid. + */ + barrier(); + /* + * Stop spinning when: + * - the lock owner has been scheduled out + * - current is not longer the top waiter + * - current is requested to reschedule (redundant + * for CONFIG_PREEMPT_RCU=y) + * - the VCPU on which owner runs is preempted + */ + if (!owner->on_cpu || waiter != rt_mutex_top_waiter(lock) || + need_resched() || vcpu_is_preempted(task_cpu(owner))) { + res = false; + break; + } + cpu_relax(); + } + rcu_read_unlock(); + return res; +} +#else +static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock, + struct rt_mutex_waiter *waiter, + struct task_struct *owner) +{ + return false; +} +#endif + #ifdef RT_MUTEX_BUILD_MUTEX /* * Functions required for: @@ -1381,6 +1432,7 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter) { struct rt_mutex *rtm = container_of(lock, struct rt_mutex, rtmutex); + struct task_struct *owner; int ret = 0; for (;;) { @@ -1403,9 +1455,14 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock, break; } + if (waiter == rt_mutex_top_waiter(lock)) + owner = rt_mutex_owner(lock); + else + owner = NULL; raw_spin_unlock_irq(&lock->wait_lock); - schedule(); + if (!owner || !rtmutex_spin_on_owner(lock, waiter, owner)) + schedule(); raw_spin_lock_irq(&lock->wait_lock); set_current_state(state); @@ -1561,6 +1618,7 @@ static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock, static void __sched rtlock_slowlock_locked(struct rt_mutex_base *lock) { struct rt_mutex_waiter waiter; + struct task_struct *owner; lockdep_assert_held(&lock->wait_lock); @@ -1579,9 +1637,14 @@ static void __sched rtlock_slowlock_locked(struct rt_mutex_base *lock) if (try_to_take_rt_mutex(lock, current, &waiter)) break; + if (&waiter == rt_mutex_top_waiter(lock)) + owner = rt_mutex_owner(lock); + else + owner = NULL; raw_spin_unlock_irq(&lock->wait_lock); - schedule_rtlock(); + if (!owner || !rtmutex_spin_on_owner(lock, &waiter, owner)) + schedule_rtlock(); raw_spin_lock_irq(&lock->wait_lock); set_current_state(TASK_RTLOCK_WAIT);
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