在Linux内核代码里当需要限制printk打印频率时会用到__ratelimit或printk_ratelimit（封装了__ratelimit）。

/*                                                                                                     
 * printk rate limiting, lifted from the networking subsystem.                                         
 *                                                                                                     
 * This enforces a rate limit: not more than 10 kernel messages                                        
 * every 5s to make a denial-of-service attack impossible.                                             
 */                                                                                                    
DEFINE_RATELIMIT_STATE(printk_ratelimit_state, 5 * HZ, 10);                                            
                                                                                                       
int printk_ratelimit(void)                                                                             
{                                                                                                      
    return __ratelimit(&printk_ratelimit_state);                                                       
}                                                                                                      
EXPORT_SYMBOL(printk_ratelimit);

/var/log/messages里一个可能的输出如下：

kernel: end_request: I/O error, dev sdr, sector 2048                            
kernel: end_request: I/O error, dev sdr, sector 2048                            
kernel: end_request: I/O error, dev sdr, sector 2104                            
kernel: end_request: I/O error, dev sdr, sector 2048                            
kernel: end_request: I/O error, dev sdr, sector 2048                            
kernel: __ratelimit: 125 callbacks suppressed                                   
kernel: sd 21:0:0:0: [sdr] Unhandled error code                                 
kernel: sd 21:0:0:0: [sdr] Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK 
kernel: sd 21:0:0:0: [sdr] CDB: Read(10): 28 00 00 00 00 00 00 00 20 00         
kernel: __ratelimit: 125 callbacks suppressed                                   
kernel: __ratelimit: 128 callbacks suppressed                                   
kernel: Buffer I/O error on device sdr, logical block 0                         
kernel: Buffer I/O error on device sdr, logical block 1                         
kernel: Buffer I/O error on device sdr, logical block 2                         
kernel: Buffer I/O error on device sdr, logical block 3                         

    printk_ratelimit默认允许在5s内最多打印10条消息出来。可通过 /proc/sys/kernel/printk_ratelimit （多长时间）和 /proc/sys/kernel/printk_ratelimit_burst （在printk_ratelimit时间段内最多允许的消息数量）。当然如果内核代码里没有调用printk_ratelimit就不受这两个值的限制。

    内核块设备代码中的一个例子如下：

bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
{
	int total_bytes, bio_nbytes, next_idx = 0;
	struct bio *bio;

	if (!req->bio)
		return false;

	trace_block_rq_complete(req->q, req);

	/*
	 * For fs requests, rq is just carrier of independent bio's
	 * and each partial completion should be handled separately.
	 * Reset per-request error on each partial completion.
	 *
	 * TODO: tj: This is too subtle.  It would be better to let
	 * low level drivers do what they see fit.
	 */
	if (req->cmd_type == REQ_TYPE_FS)
		req->errors = 0;

	if (error && req->cmd_type == REQ_TYPE_FS &&
	    !(req->cmd_flags & REQ_QUIET)) {
		char *error_type;

		switch (error) {
		case -ENOLINK:
			error_type = "recoverable transport";
			break;
		case -EREMOTEIO:
			error_type = "critical target";
			break;
		case -EBADE:
			error_type = "critical nexus";
			break;
		case -EIO:
		default:
			error_type = "I/O";
			break;
		}
		printk_ratelimited(KERN_ERR "end_request: %s error, dev %s, "
				   "sector %llu\n", error_type, req->rq_disk ?
				   req->rq_disk->disk_name : "?",
				   (unsigned long long)blk_rq_pos(req));
	}

	blk_account_io_completion(req, nr_bytes);
...
}

    printk_ratelimited是一个宏，并直接调用了 __ratelimit

#define printk_ratelimited(fmt, ...)  ({        \                                                      
    static struct ratelimit_state _rs = {       \                                                      
        .interval = DEFAULT_RATELIMIT_INTERVAL, \                                                      
        .burst = DEFAULT_RATELIMIT_BURST,       \
    };                                              \                                                  
                            \
    if (!__ratelimit(&_rs))                         \                                                  
        printk(fmt, ##__VA_ARGS__);     \                                                              
})