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SubjectRe: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation
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On 04/17/2019 04:05 AM, Peter Zijlstra wrote:
> 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);
> }

Yes, we can certainly do that. My point is that I haven't changed the
existing logic regarding that wakeup, I only move it around in the
patch. As it is not related to lock handoff, we can do it as a separate
patch.

Cheers,
Longman

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