cfq-iosched: limit coop preemption

CFQ has an optimization for cooperated applications. if several
io-context have close requests, they will get boost. But the
optimization get abused. Considering thread a, b, which work on one
file. a reads sectors s, s+2, s+4, ...; b reads sectors s+1, s+3, s
+5, ... Both a and b are sequential read, so they can open idle window.
a reads a sector s and goes to idle window and wakeup b. b reads sector
s+1, since in current implementation, cfq_should_preempt() thinks a and
b are cooperators, b will preempt a. b then reads sector s+1 and goes to
idle window and wakeup a. for the same reason, a will preempt b and
reads s+2. a and b will continue the circle. The circle will be very
long, and a and b will occupy whole disk queue. Other applications will
nearly have no chance to run.

Fix this limiting coop preempt until a queue is scheduled normally
again.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -196,6 +196,7 @@ enum cfqq_state_flags {
 	CFQ_CFQQ_FLAG_slice_new,	/* no requests dispatched in slice */
 	CFQ_CFQQ_FLAG_sync,		/* synchronous queue */
 	CFQ_CFQQ_FLAG_coop,		/* has done a coop jump of the queue */
+	CFQ_CFQQ_FLAG_coop_preempt,	/* coop preempt */
 };
 
 #define CFQ_CFQQ_FNS(name)						\
@@ -222,6 +223,7 @@ CFQ_CFQQ_FNS(prio_changed);
 CFQ_CFQQ_FNS(slice_new);
 CFQ_CFQQ_FNS(sync);
 CFQ_CFQQ_FNS(coop);
+CFQ_CFQQ_FNS(coop_preempt);
 #undef CFQ_CFQQ_FNS
 
 #define cfq_log_cfqq(cfqd, cfqq, fmt, args...)	\
@@ -945,10 +947,13 @@ static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd,
 {
 	if (!cfqq) {
 		cfqq = cfq_get_next_queue(cfqd);
-		if (cfqq)
+		if (cfqq && !cfq_cfqq_coop_preempt(cfqq))
 			cfq_clear_cfqq_coop(cfqq);
 	}
 
+	if (cfqq)
+		cfq_clear_cfqq_coop_preempt(cfqq);
+
 	__cfq_set_active_queue(cfqd, cfqq);
 	return cfqq;
 }
@@ -2066,8 +2071,16 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
 	 * if this request is as-good as one we would expect from the
 	 * current cfqq, let it preempt
 	 */
-	if (cfq_rq_close(cfqd, rq))
+	if (cfq_rq_close(cfqd, rq) && (!cfq_cfqq_coop(new_cfqq) ||
+	    cfqd->busy_queues == 1)) {
+		/*
+		 * Mark new queue coop_preempt, so its coop flag will not be
+		 * cleared when new queue gets scheduled at the very first time
+		 */
+		cfq_mark_cfqq_coop_preempt(new_cfqq);
+		cfq_mark_cfqq_coop(new_cfqq);
 		return true;
+	}
 
 	return false;
 }