Messages in this thread Patch in this message | | | Date | Wed, 17 Apr 2019 10:05:49 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation |
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On Tue, Apr 16, 2019 at 02:16:11PM -0400, Waiman Long wrote:
> >> @@ -608,56 +687,63 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) > >> */ > >> waiter.task = current; > >> waiter.type = RWSEM_WAITING_FOR_WRITE; > >> + waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; > >> > >> raw_spin_lock_irq(&sem->wait_lock); > >> > >> /* account for this before adding a new element to the list */ > >> + wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST; > >> > >> list_add_tail(&waiter.list, &sem->wait_list); > >> > >> /* we're now waiting on the lock */ > >> + if (wstate == WRITER_NOT_FIRST) { > >> count = atomic_long_read(&sem->count); > >> > >> /* > >> + * If there were already threads queued before us and: > >> + * 1) there are no no active locks, wake the front > >> + * queued process(es) as the handoff bit might be set. > >> + * 2) there are no active writers and some readers, the lock > >> + * must be read owned; so we try to wake any read lock > >> + * waiters that were queued ahead of us. > >> */ > >> + if (!RWSEM_COUNT_LOCKED(count)) > >> + __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); > >> + else if (!(count & RWSEM_WRITER_MASK) && > >> + (count & RWSEM_READER_MASK)) > >> __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
> > Does the above want to be something like: > > > > if (!(count & RWSEM_WRITER_LOCKED)) { > > __rwsem_mark_wake(sem, (count & RWSEM_READER_MASK) ? > > RWSEM_WAKE_READERS : > > RWSEM_WAKE_ANY, &wake_q); > > } > > Yes. > > >> + else > >> + goto wait; > >> > >> + /* > >> + * The wakeup is normally called _after_ the wait_lock > >> + * is released, but given that we are proactively waking > >> + * readers we can deal with the wake_q overhead as it is > >> + * similar to releasing and taking the wait_lock again > >> + * for attempting rwsem_try_write_lock(). > >> + */ > >> + wake_up_q(&wake_q); > > Hurmph.. the reason we do wake_up_q() outside of wait_lock is such that > > those tasks don't bounce on wait_lock. Also, it removes a great deal of > > hold-time from wait_lock. > > > > So I'm not sure I buy your argument here. > > > > Actually, we don't want to release the wait_lock, do wake_up_q() and > acquire the wait_lock again as the state would have been changed. I > didn't change the comment on this patch, but will reword it to discuss that.
I don't understand, we've queued ourselves, we're on the list, we're not first. How would dropping the lock to try and kick waiters before us be a problem?
Sure, once we re-acquire the lock we have to re-avaluate @wstate to see if we're first now or not, but we need to do that anyway.
So what is wrong with the below?
--- a/include/linux/sched/wake_q.h +++ b/include/linux/sched/wake_q.h @@ -51,6 +51,11 @@ static inline void wake_q_init(struct wa head->lastp = &head->first; } +static inline bool wake_q_empty(struct wake_q_head *head) +{ + return head->first == WAKE_Q_TAIL; +} + extern void wake_q_add(struct wake_q_head *head, struct task_struct *task); extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task); extern void wake_up_q(struct wake_q_head *head); --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -700,25 +700,22 @@ __rwsem_down_write_failed_common(struct * must be read owned; so we try to wake any read lock * waiters that were queued ahead of us. */ - if (!(count & RWSEM_LOCKED_MASK)) - __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); - else if (!(count & RWSEM_WRITER_MASK) && - (count & RWSEM_READER_MASK)) - __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q); - else + if (count & RWSEM_WRITER_LOCKED) goto wait; - /* - * The wakeup is normally called _after_ the wait_lock - * is released, but given that we are proactively waking - * readers we can deal with the wake_q overhead as it is - * similar to releasing and taking the wait_lock again - * for attempting rwsem_try_write_lock(). - */ - wake_up_q(&wake_q); - /* - * Reinitialize wake_q after use. - */ - wake_q_init(&wake_q); + + __rwsem_mark_wake(sem, (count & RWSEM_READER_MASK) ? + RWSEM_WAKE_READERS : + RWSEM_WAKE_ANY, &wake_q); + + if (!wake_q_empty(&wake_q)) { + raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); + /* used again, reinit */ + wake_q_init(&wake_q); + raw_spin_lock_irq(&sem->wait_lock); + if (rwsem_waiter_is_first(sem, &waiter)) + wstate = WRITER_FIRST; + } } else { count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count); }
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