Re: [PATCH tip/locking/core v10 5/7] locking/pvqspinlock: Collect slowpath lock statistics

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 09, 2015 at 07:09:25PM -0500, Waiman Long wrote:
> +static ssize_t qstat_read(struct file *file, char __user *user_buf,
> +			  size_t count, loff_t *ppos)
> +{
> +	char buf[64];
> +	int cpu, counter, len;
> +	u64 stat = 0, kicks = 0;
> +
> +	/*
> +	 * Get the counter ID stored in file->f_inode->i_private
> +	 */
> +	if (!file->f_inode) {
> +		WARN_ON_ONCE(1);
> +		return -EBADF;
> +	}
> +	counter = (long)(file->f_inode->i_private);
> +
> +	if (counter >= qstat_num)
> +		return -EBADF;
> +
> +	for_each_possible_cpu(cpu) {
> +		stat += per_cpu(qstats[counter], cpu);
> +		/*
> +		 * Need to sum additional counter for some of them
> +		 */
> +		switch (counter) {
> +
> +		case qstat_pv_latency_kick:
> +		case qstat_pv_hash_hops:
> +			kicks += per_cpu(qstats[qstat_pv_kick_unlock], cpu);
> +			break;
> +
> +		case qstat_pv_latency_wake:
> +			kicks += per_cpu(qstats[qstat_pv_kick_wake], cpu);
> +			break;
> +		}
> +	}
> +
> +	if (counter == qstat_pv_hash_hops) {
> +		/*
> +		 * Return a X.XX decimal number
> +		 */
> +		len = snprintf(buf, sizeof(buf) - 1, "%llu.%02llu\n",
> +			       stat/kicks, ((stat%kicks)*100 + kicks/2)/kicks);
> +	} else {
> +		/*
> +		 * Round to the nearest ns
> +		 */
> +		if ((counter == qstat_pv_latency_kick) ||
> +		    (counter == qstat_pv_latency_wake))
> +			stat = kicks ? (stat + kicks/2)/kicks : 0;
> +		len = snprintf(buf, sizeof(buf) - 1, "%llu\n", stat);
> +	}
> +
> +	return simple_read_from_buffer(user_buf, count, ppos, buf, len);
> +}

That doesn't work on 32bit.

---
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -127,18 +127,25 @@ static ssize_t qstat_read(struct file *f
 	}
 
 	if (counter == qstat_pv_hash_hops) {
+		u64 frac;
+
+		frac = 100ULL * do_div(stat, kicks);
+		frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+
 		/*
 		 * Return a X.XX decimal number
 		 */
-		len = snprintf(buf, sizeof(buf) - 1, "%llu.%02llu\n",
-			       stat/kicks, ((stat%kicks)*100 + kicks/2)/kicks);
+		len = snprintf(buf, sizeof(buf) - 1, "%llu.%02llu\n", stat, frac);
 	} else {
 		/*
 		 * Round to the nearest ns
 		 */
 		if ((counter == qstat_pv_latency_kick) ||
-		    (counter == qstat_pv_latency_wake))
-			stat = kicks ? (stat + kicks/2)/kicks : 0;
+		    (counter == qstat_pv_latency_wake)) {
+			stat = 0;
+			if (kicks)
+				stat = DIV_ROUND_CLOSEST_ULL(stat, kicks);
+		}
 		len = snprintf(buf, sizeof(buf) - 1, "%llu\n", stat);
 	}