Re: [PATCH 7/8] wbt: add general throttling mechanism

[Jens Axboe <axboe@fb.com>]

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On 04/27/2016 06:06 AM, xiakaixu wrote:
>> +void __wbt_done(struct rq_wb *rwb)
>> +{
>> +	int inflight, limit = rwb->wb_normal;
>> +
>> +	/*
>> +	 * If the device does write back caching, drop further down
>> +	 * before we wake people up.
>> +	 */
>> +	if (rwb->wc && !atomic_read(&rwb->bdi->wb.dirty_sleeping))
>> +		limit = 0;
>> +	else
>> +		limit = rwb->wb_normal;
>> +
>> +	/*
>> +	 * Don't wake anyone up if we are above the normal limit. If
>> +	 * throttling got disabled (limit == 0) with waiters, ensure
>> +	 * that we wake them up.
>> +	 */
>> +	inflight = atomic_dec_return(&rwb->inflight);
>> +	if (limit && inflight >= limit) {
>> +		if (!rwb->wb_max)
>> +			wake_up_all(&rwb->wait);
>> +		return;
>> +	}
>> +
> Hi Jens,
>
> Just a little confused about this. The rwb->wb_max can't be 0 if the variable
> 'limit' does not equal to 0. So the if (!rwb->wb_max) branch maybe does not
> make sense.

You are right, it doesn't make a lot of sense. I think it suffers from 
code shuffling. How about the attached? The important part is that we 
wake up waiters, if wbt got disabled while we had tracked IO in flight.

-- 
Jens Axboe


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diff --git a/lib/wbt.c b/lib/wbt.c
index 650da911f24f..a6b80c135510 100644
--- a/lib/wbt.c
+++ b/lib/wbt.c
@@ -98,18 +98,23 @@ void __wbt_done(struct rq_wb *rwb)
 	else
 		limit = rwb->wb_normal;
 
+	inflight = atomic_dec_return(&rwb->inflight);
+
 	/*
-	 * Don't wake anyone up if we are above the normal limit. If
-	 * throttling got disabled (limit == 0) with waiters, ensure
-	 * that we wake them up.
+	 * wbt got disabled with IO in flight. Wake up any potential
+	 * waiters, we don't have to do more than that.
 	 */
-	inflight = atomic_dec_return(&rwb->inflight);
-	if (limit && inflight >= limit) {
-		if (!rwb->wb_max)
-			wake_up_all(&rwb->wait);
+	if (!rwb_enabled(rwb)) {
+		wake_up_all(&rwb->wait);
 		return;
 	}
 
+	/*
+	 * Don't wake anyone up if we are above the normal limit.
+	 */
+	if (inflight >= limit)
+		return;
+
 	if (waitqueue_active(&rwb->wait)) {
 		int diff = limit - inflight;