| 1 | // SPDX-License-Identifier: GPL-3.0-or-later |
| 2 | |
| 3 | #ifndef NETDATA_WAITQ_H |
| 4 | #define NETDATA_WAITQ_H |
| 5 | |
| 6 | #include "libnetdata/libnetdata.h" |
| 7 | |
| 8 | /* |
| 9 | * WAITING QUEUE |
| 10 | * Like a spinlock, but: |
| 11 | * |
| 12 | * 1. Waiters get a sequence number (FIFO) |
| 13 | * 2. FIFO is respected within each priority |
| 14 | * 3. Higher priority threads get in first |
| 15 | * |
| 16 | * This is equivalent to 3 atomic operations for lock, and 1 for unlock. |
| 17 | * |
| 18 | * As lightweight and fast as it can be. |
| 19 | * About 3M thread switches/s per WAITING QUEUE, on modern hardware. |
| 20 | * |
| 21 | * Be careful: higher priority threads can starve the rest! |
| 22 | * |
| 23 | */ |
| 24 | |
| 25 | typedef enum __attribute__((packed)) { |
| 26 | WAITQ_PRIO_URGENT = 0, // will be first |
| 27 | WAITQ_PRIO_HIGH, // will be second |
| 28 | WAITQ_PRIO_NORMAL, // will be third |
| 29 | WAITQ_PRIO_LOW, // will be last |
| 30 | |
| 31 | // terminator |
| 32 | WAITQ_PRIO_MAX, |
| 33 | } WAITQ_PRIORITY; |
| 34 | |
| 35 | typedef struct waiting_queue { |
| 36 | SPINLOCK spinlock; // protects the actual resource |
| 37 | pid_t writer; // the pid the thread currently holding the lock |
| 38 | uint64_t current_priority; // current highest priority attempting to acquire |
| 39 | uint32_t last_seqno; // for FIFO ordering within same priority |
| 40 | } WAITQ; |
| 41 | |
| 42 | #define WAITQ_INITIALIZER (WAITQ){ .spinlock = SPINLOCK_INITIALIZER, .current_priority = 0, .last_seqno = 0, } |
| 43 | |
| 44 | // Initialize a waiting queue |
| 45 | void waitq_init(WAITQ *waitq); |
| 46 | |
| 47 | // Destroy a waiting queue - must be empty |
| 48 | void waitq_destroy(WAITQ *wq); |
| 49 | |
| 50 | // Returns true when the queue is acquired |
| 51 | bool waitq_try_acquire_with_trace(WAITQ *waitq, WAITQ_PRIORITY priority, const char *func); |
| 52 | #define waitq_try_acquire(waitq, priority) waitq_try_acquire_with_trace(waitq, priority, __FUNCTION__) |
| 53 | |
| 54 | // Returns when it is our turn to run |
| 55 | // Returns time spent waiting in microseconds |
| 56 | void waitq_acquire_with_trace(WAITQ *waitq, WAITQ_PRIORITY priority, const char *func); |
| 57 | #define waitq_acquire(waitq, priority) waitq_acquire_with_trace(waitq, priority, __FUNCTION__) |
| 58 | |
| 59 | // Mark that we are done - wakes up the next in line |
| 60 | void waitq_release(WAITQ *waitq); |
| 61 | |
| 62 | int unittest_waiting_queue(void); |
| 63 | |
| 64 | #endif // NETDATA_WAITQ_H |