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With the use of wake_q, we can do task wakeups without holding the wait_lock. There is one exception in the rwsem code, though. It is when the writer in the slowpath detects that there are waiters ahead but the rwsem is not held by a writer. This can lead to a long wait_lock hold time especially when a large number of readers are to be woken up. Remediate this situation by releasing the wait_lock before waking up tasks and re-acquiring it afterward. The rwsem_try_write_lock() function is also modified to read the rwsem count directly to avoid stale count value. Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Waiman Long <longman@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Will Deacon <will.deacon@arm.com> Cc: huang ying <huang.ying.caritas@gmail.com> Link: https://lkml.kernel.org/r/20190520205918.22251-9-longman@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
63 lines
2.1 KiB
C
63 lines
2.1 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_SCHED_WAKE_Q_H
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#define _LINUX_SCHED_WAKE_Q_H
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/*
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* Wake-queues are lists of tasks with a pending wakeup, whose
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* callers have already marked the task as woken internally,
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* and can thus carry on. A common use case is being able to
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* do the wakeups once the corresponding user lock as been
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* released.
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*
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* We hold reference to each task in the list across the wakeup,
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* thus guaranteeing that the memory is still valid by the time
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* the actual wakeups are performed in wake_up_q().
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*
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* One per task suffices, because there's never a need for a task to be
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* in two wake queues simultaneously; it is forbidden to abandon a task
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* in a wake queue (a call to wake_up_q() _must_ follow), so if a task is
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* already in a wake queue, the wakeup will happen soon and the second
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* waker can just skip it.
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*
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* The DEFINE_WAKE_Q macro declares and initializes the list head.
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* wake_up_q() does NOT reinitialize the list; it's expected to be
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* called near the end of a function. Otherwise, the list can be
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* re-initialized for later re-use by wake_q_init().
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*
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* NOTE that this can cause spurious wakeups. schedule() callers
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* must ensure the call is done inside a loop, confirming that the
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* wakeup condition has in fact occurred.
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*
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* NOTE that there is no guarantee the wakeup will happen any later than the
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* wake_q_add() location. Therefore task must be ready to be woken at the
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* location of the wake_q_add().
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*/
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#include <linux/sched.h>
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struct wake_q_head {
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struct wake_q_node *first;
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struct wake_q_node **lastp;
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};
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#define WAKE_Q_TAIL ((struct wake_q_node *) 0x01)
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#define DEFINE_WAKE_Q(name) \
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struct wake_q_head name = { WAKE_Q_TAIL, &name.first }
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static inline void wake_q_init(struct wake_q_head *head)
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{
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head->first = WAKE_Q_TAIL;
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head->lastp = &head->first;
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}
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static inline bool wake_q_empty(struct wake_q_head *head)
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{
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return head->first == WAKE_Q_TAIL;
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}
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extern void wake_q_add(struct wake_q_head *head, struct task_struct *task);
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extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task);
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extern void wake_up_q(struct wake_q_head *head);
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#endif /* _LINUX_SCHED_WAKE_Q_H */
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