This commit fixes a bug that causes bfq to fail to guarantee a high responsiveness on some drives, if there is heavy random read+write I/O in the background. More precisely, such a failure allowed this bug to be found [1], but the bug may well cause other yet unreported anomalies.
BFQ raises the weight of the bfq_queues associated with soft real-time applications, to privilege the I/O, and thus reduce latency, for these applications. This mechanism is named soft-real-time weight raising in BFQ. A soft real-time period may happen to be nested into an interactive weight raising period, i.e., it may happen that, when a bfq_queue switches to a soft real-time weight-raised state, the bfq_queue is already being weight-raised because deemed interactive too. In this case, BFQ saves in a special variable wr_start_at_switch_to_srt, the time instant when the interactive weight-raising period started for the bfq_queue, i.e., the time instant when BFQ started to deem the bfq_queue interactive. This value is then used to check whether the interactive weight-raising period would still be in progress when the soft real-time weight-raising period ends. If so, interactive weight raising is restored for the bfq_queue. This restore is useful, in particular, because it prevents bfq_queues from losing their interactive weight raising prematurely, as a consequence of spurious, short-lived soft real-time weight-raising periods caused by wrong detections as soft real-time. If, instead, a bfq_queue switches to soft-real-time weight raising while it *is not* already in an interactive weight-raising period, then the variable wr_start_at_switch_to_srt has no meaning during the following soft real-time weight-raising period. Unfortunately the handling of this case is wrong in BFQ: not only the variable is not flagged somehow as meaningless, but it is also set to the time when the switch to soft real-time weight-raising occurs. This may cause an interactive weight-raising period to be considered mistakenly as still in progress, and thus a spurious interactive weight-raising period to start for the bfq_queue, at the end of the soft-real-time weight-raising period. In particular the spurious interactive weight-raising period will be considered as still in progress, if the soft-real-time weight-raising period does not last very long. The bfq_queue will then be wrongly privileged and, if I/O bound, will unjustly steal bandwidth to truly interactive or soft real-time bfq_queues, harming responsiveness and low latency. This commit fixes this issue by just setting wr_start_at_switch_to_srt to minus infinity (farthest past time instant according to jiffies macros): when the soft-real-time weight-raising period ends, certainly no interactive weight-raising period will be considered as still in progress. [1] Background I/O Type: Random - Background I/O mix: Reads and writes - Application to start: LibreOffice Writer in http://www.phoronix.com/scan.php?page=news_item&px=Linux-4.13-IO-Laptop Signed-off-by: Paolo Valente <paolo.vale...@linaro.org> Signed-off-by: Angelo Ruocco <angeloruocc...@gmail.com> Tested-by: Oleksandr Natalenko <oleksa...@natalenko.name> Tested-by: Lee Tibbert <lee.tibb...@gmail.com> Tested-by: Mirko Montanari <mirkomontanar...@gmail.com> --- block/bfq-iosched.c | 50 +++++++++++++++++++++++++++++++------------------- 1 file changed, 31 insertions(+), 19 deletions(-) diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index a4783da..c25955c 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -1202,6 +1202,24 @@ static unsigned int bfq_wr_duration(struct bfq_data *bfqd) return dur; } +/* + * 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. + */ +static unsigned long bfq_smallest_from_now(void) +{ + return jiffies - MAX_JIFFY_OFFSET; +} + static void bfq_update_bfqq_wr_on_rq_arrival(struct bfq_data *bfqd, struct bfq_queue *bfqq, unsigned int old_wr_coeff, @@ -1216,7 +1234,19 @@ static void bfq_update_bfqq_wr_on_rq_arrival(struct bfq_data *bfqd, bfqq->wr_coeff = bfqd->bfq_wr_coeff; bfqq->wr_cur_max_time = bfq_wr_duration(bfqd); } else { - bfqq->wr_start_at_switch_to_srt = jiffies; + /* + * No interactive weight raising in progress + * here: assign minus infinity to + * wr_start_at_switch_to_srt, to make sure + * that, at the end of the soft-real-time + * weight raising periods that is starting + * now, no interactive weight-raising period + * may be wrongly considered as still in + * progress (and thus actually started by + * mistake). + */ + bfqq->wr_start_at_switch_to_srt = + bfq_smallest_from_now(); bfqq->wr_coeff = bfqd->bfq_wr_coeff * BFQ_SOFTRT_WEIGHT_FACTOR; bfqq->wr_cur_max_time = @@ -2897,24 +2927,6 @@ static unsigned long bfq_bfqq_softrt_next_start(struct bfq_data *bfqd, jiffies + nsecs_to_jiffies(bfqq->bfqd->bfq_slice_idle) + 4); } -/* - * 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. - */ -static unsigned long bfq_smallest_from_now(void) -{ - return jiffies - MAX_JIFFY_OFFSET; -} - /** * bfq_bfqq_expire - expire a queue. * @bfqd: device owning the queue. -- 2.10.0
