rq-qos sits in the io path so we want to take locks as sparingly as possible. To accomplish this we try not to take the waitqueue head lock unless we are sure we need to go to sleep, and we have an optimization to make sure that we don't starve out existing waiters. Since we check if there are existing waiters locklessly we need to be able to update our view of the waitqueue list after we've added ourselves to the waitqueue. Accomplish this by adding this helper to see if there is more than just ourselves on the list.
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| @@ -126,6 +126,19 @@ static inline int waitqueue_active(struct wait_queue_head *wq_head) · 125 unmodified lines | ||
| 126 | 126 | return !list_empty(&wq_head->head); |
| 127 | 127 | } |
| 128 | 128 | |
| 129 | +/** | |
| 130 | + * wq_has_single_sleeper - check if there is only one sleeper | |
| 131 | + * @wq_head: wait queue head | |
| 132 | + * | |
| 133 | + * Returns true of wq_head has only one sleeper on the list. | |
| 134 | + * | |
| 135 | + * Please refer to the comment for waitqueue_active. | |
| 136 | + */ | |
| 137 | +static inline bool wq_has_single_sleeper(struct wait_queue_head *wq_head) | |
| 138 | +{ | |
| 139 | + return list_is_singular(&wq_head->head); | |
| 140 | +} | |
| 141 | + | |
| 129 | 142 | /** |
| 130 | 143 | * wq_has_sleeper - check if there are any waiting processes |
| 131 | 144 | * @wq_head: wait queue head |