diff options
author | Paolo Valente <paolo.valente@linaro.org> | 2021-01-22 21:19:47 +0300 |
---|---|---|
committer | Jens Axboe <axboe@kernel.dk> | 2021-01-25 04:18:24 +0300 |
commit | 3c337690d2ebb7a01fa13bfa59ce4911f358df42 (patch) | |
tree | 98bdad23b303bb1679fb2dcdbca0694bfabac712 /block/bfq-iosched.c | |
parent | 91b896f65d32610d6d58af02170b15f8d37a7702 (diff) | |
download | linux-3c337690d2ebb7a01fa13bfa59ce4911f358df42.tar.xz |
block, bfq: avoid spurious switches to soft_rt of interactive queues
BFQ tags some bfq_queues as interactive or soft_rt if it deems that
these bfq_queues contain the I/O of, respectively, interactive or soft
real-time applications. BFQ privileges both these special types of
bfq_queues over normal bfq_queues. To privilege a bfq_queue, BFQ
mainly raises the weight of the bfq_queue. In particular, soft_rt
bfq_queues get a higher weight than interactive bfq_queues.
A bfq_queue may turn from interactive to soft_rt. And this leads to a
tricky issue. Soft real-time applications usually start with an
I/O-bound, interactive phase, in which they load themselves into main
memory. BFQ correctly detects this phase, and keeps the bfq_queues
associated with the application in interactive mode for a
while. Problems arise when the I/O pattern of the application finally
switches to soft real-time. One of the conditions for a bfq_queue to
be deemed as soft_rt is that the bfq_queue does not consume too much
bandwidth. But the bfq_queues associated with a soft real-time
application consume as much bandwidth as they can in the loading phase
of the application. So, after the application becomes truly soft
real-time, a lot of time should pass before the average bandwidth
consumed by its bfq_queues finally drops to a value acceptable for
soft_rt bfq_queues. As a consequence, there might be a time gap during
which the application is not privileged at all, because its bfq_queues
are not interactive any longer, but cannot be deemed as soft_rt yet.
To avoid this problem, BFQ pretends that an interactive bfq_queue
consumes zero bandwidth, and allows an interactive bfq_queue to switch
to soft_rt. Yet, this fake zero-bandwidth consumption easily causes
the bfq_queue to often switch to soft_rt deceptively, during its
loading phase. As in soft_rt mode, the bfq_queue gets its bandwidth
correctly computed, and therefore soon switches back to
interactive. Then it switches again to soft_rt, and so on. These
spurious fluctuations usually cause losses of throughput, because they
deceive BFQ's mechanisms for boosting throughput (injection,
I/O-plugging avoidance, ...).
This commit addresses this issue as follows:
1) It does compute actual bandwidth consumption also for interactive
bfq_queues. This avoids the above false positives.
2) When a bfq_queue switches from interactive to normal mode, the
consumed bandwidth is reset (forgotten). This allows the
bfq_queue to enjoy soft_rt very quickly. In particular, two
alternatives are possible in this switch:
- the bfq_queue still has backlog, and therefore there is a budget
already scheduled to serve the bfq_queue; in this case, the
scheduling of the current budget of the bfq_queue is not
hindered, because only the scheduling of the next budget will
be affected by the weight drop. After that, if the bfq_queue is
actually in a soft_rt phase, and becomes empty during the
service of its current budget, which is the natural behavior of
a soft_rt bfq_queue, then the bfq_queue will be considered as
soft_rt when its next I/O arrives. If, in contrast, the
bfq_queue remains constantly non-empty, then its next budget
will be scheduled with a low weight, which is the natural
treatment for an I/O-bound (non soft_rt) bfq_queue.
- the bfq_queue is empty; in this case, the bfq_queue may be
considered unjustly soft_rt when its new I/O arrives. Yet
the problem is now much smaller than before, because it is
unlikely that more than one spurious fluctuation occurs.
Tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/bfq-iosched.c')
-rw-r--r-- | block/bfq-iosched.c | 57 |
1 files changed, 37 insertions, 20 deletions
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 161badb744d6..003c96fa01ad 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -2356,6 +2356,24 @@ static void bfq_requests_merged(struct request_queue *q, struct request *rq, /* Must be called with bfqq != NULL */ static void bfq_bfqq_end_wr(struct bfq_queue *bfqq) { + /* + * If bfqq has been enjoying interactive weight-raising, then + * reset soft_rt_next_start. We do it for the following + * reason. bfqq may have been conveying the I/O needed to load + * a soft real-time application. Such an application actually + * exhibits a soft real-time I/O pattern after it finishes + * loading, and finally starts doing its job. But, if bfqq has + * been receiving a lot of bandwidth so far (likely to happen + * on a fast device), then soft_rt_next_start now contains a + * high value that. So, without this reset, bfqq would be + * prevented from being possibly considered as soft_rt for a + * very long time. + */ + + if (bfqq->wr_cur_max_time != + bfqq->bfqd->bfq_wr_rt_max_time) + bfqq->soft_rt_next_start = jiffies; + if (bfq_bfqq_busy(bfqq)) bfqq->bfqd->wr_busy_queues--; bfqq->wr_coeff = 1; @@ -3956,30 +3974,15 @@ void bfq_bfqq_expire(struct bfq_data *bfqd, * If we get here, and there are no outstanding * requests, then the request pattern is isochronous * (see the comments on the function - * bfq_bfqq_softrt_next_start()). Thus we can compute - * soft_rt_next_start. And we do it, unless bfqq is in - * interactive weight raising. We do not do it in the - * latter subcase, for the following reason. bfqq may - * be conveying the I/O needed to load a soft - * real-time application. Such an application will - * actually exhibit a soft real-time I/O pattern after - * it finally starts doing its job. But, if - * soft_rt_next_start is computed here for an - * interactive bfqq, and bfqq had received a lot of - * service before remaining with no outstanding - * request (likely to happen on a fast device), then - * soft_rt_next_start would be assigned such a high - * value that, for a very long time, bfqq would be - * prevented from being possibly considered as soft - * real time. + * bfq_bfqq_softrt_next_start()). Therefore we can + * compute soft_rt_next_start. * * If, instead, the queue still has outstanding * requests, then we have to wait for the completion * of all the outstanding requests to discover whether * the request pattern is actually isochronous. */ - if (bfqq->dispatched == 0 && - bfqq->wr_coeff != bfqd->bfq_wr_coeff) + if (bfqq->dispatched == 0) bfqq->soft_rt_next_start = bfq_bfqq_softrt_next_start(bfqd, bfqq); else if (bfqq->dispatched > 0) { @@ -4563,9 +4566,21 @@ static void bfq_update_wr_data(struct bfq_data *bfqd, struct bfq_queue *bfqq) bfqq->wr_cur_max_time)) { if (bfqq->wr_cur_max_time != bfqd->bfq_wr_rt_max_time || time_is_before_jiffies(bfqq->wr_start_at_switch_to_srt + - bfq_wr_duration(bfqd))) + bfq_wr_duration(bfqd))) { + /* + * Either in interactive weight + * raising, or in soft_rt weight + * raising with the + * interactive-weight-raising period + * elapsed (so no switch back to + * interactive weight raising). + */ bfq_bfqq_end_wr(bfqq); - else { + } else { /* + * soft_rt finishing while still in + * interactive period, switch back to + * interactive weight raising + */ switch_back_to_interactive_wr(bfqq, bfqd); bfqq->entity.prio_changed = 1; } @@ -5016,6 +5031,8 @@ bfq_set_next_ioprio_data(struct bfq_queue *bfqq, struct bfq_io_cq *bic) } bfqq->entity.new_weight = bfq_ioprio_to_weight(bfqq->new_ioprio); + bfq_log_bfqq(bfqd, bfqq, "new_ioprio %d new_weight %d", + bfqq->new_ioprio, bfqq->entity.new_weight); bfqq->entity.prio_changed = 1; } |