Commit f6c3ca0e authored May 31, 2018 by Davide Sapienza Committed by Jens Axboe May 31, 2018

block, bfq: prevent soft_rt_next_start from being stuck at infinity



BFQ can deem a bfq_queue as soft real-time only if the queue
- periodically becomes completely idle, i.e., empty and with
  no still-outstanding I/O request;
- after becoming idle, gets new I/O only after a special reference
  time soft_rt_next_start.

In this respect, after commit "block, bfq: consider also past I/O in
soft real-time detection", the value of soft_rt_next_start can never
decrease. This causes a problem with the following special updating
case for soft_rt_next_start: to prevent queues that are not completely
idle to be wrongly detected as soft real-time (when they become
non-empty again), soft_rt_next_start is temporarily set to infinity
for empty queues with still outstanding I/O requests. But, if such an
update is actually performed, then, because of the above commit,
soft_rt_next_start will be stuck at infinity forever, and the queue
will have no more chance to be considered soft real-time.

On slow systems, this problem does cause actual soft real-time
applications to be occasionally not detected as such.

This commit addresses this issue by eliminating the pushing of
soft_rt_next_start to infinity, and by changing the way non-empty
queues are prevented from being wrongly detected as soft
real-time. Simply, a queue that becomes non-empty again can now be
detected as soft real-time only if it has no outstanding I/O request.

Signed-off-by: Davide Sapienza <sapienza.dav@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent d450542e

block/bfq-iosched.c

+2 −27

Original line number	Diff line number	Diff line
		@@ -1362,15 +1362,6 @@ static bool bfq_bfqq_update_budg_for_activation(struct bfq_data *bfqd,
		return wr_or_deserves_wr;
		}

		/*
		* Return the farthest future time instant according to jiffies
		* macros.
		*/
		static unsigned long bfq_greatest_from_now(void)
		{
		return jiffies + MAX_JIFFY_OFFSET;
		}

		/*
		* Return the farthest past time instant according to jiffies
		* macros.
		@@ -1515,7 +1506,8 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
		in_burst = bfq_bfqq_in_large_burst(bfqq);
		soft_rt = bfqd->bfq_wr_max_softrt_rate > 0 &&
		!in_burst &&
		time_is_before_jiffies(bfqq->soft_rt_next_start);
		time_is_before_jiffies(bfqq->soft_rt_next_start) &&
		bfqq->dispatched == 0;
		*interactive = !in_burst && idle_for_long_time;
		wr_or_deserves_wr = bfqd->low_latency &&
		(bfqq->wr_coeff > 1 \|\|
		@@ -3221,23 +3213,6 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
		bfqq->soft_rt_next_start =
		bfq_bfqq_softrt_next_start(bfqd, bfqq);
		else {
		/*
		* The application is still waiting for the
		* completion of one or more requests:
		* prevent it from possibly being incorrectly
		* deemed as soft real-time by setting its
		* soft_rt_next_start to infinity. In fact,
		* without this assignment, the application
		* would be incorrectly deemed as soft
		* real-time if:
		* 1) it issued a new request before the
		* completion of all its in-flight
		* requests, and
		* 2) at that time, its soft_rt_next_start
		* happened to be in the past.
		*/
		bfqq->soft_rt_next_start =
		bfq_greatest_from_now();
		/*
		* Schedule an update of soft_rt_next_start to when
		* the task may be discovered to be isochronous.