From: Davide Sapienza <sapienza.dav@gmail.com>
Date: Thu, 31 May 2018 16:45:08 +0200
Subject: [PATCH] block, bfq: prevent soft_rt_next_start from being stuck at
infinity
Git-commit: f6c3ca0e58446e8de6c6f1a2a73f037fb900c372
Patch-mainline: v4.18-rc1
References: bsc#1104967,FATE#325924
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>
Acked-by: Hannes Reinecke <hare@suse.com>
---
block/bfq-iosched.c | 29 ++---------------------------
1 file changed, 2 insertions(+), 27 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 128b3be49463..495b9ddb3355 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -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.
--
2.16.4