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authorLinus Torvalds <torvalds@linux-foundation.org>2019-07-26 10:32:12 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-07-26 10:32:12 -0700
commit04412819652fe30f900d11e96c67b4adfdf17f6b (patch)
treeaed86baef3fd65e6990484a00514f0594d1fdd6c /block/bfq-iosched.c
parent750c930b085ba56cfac3649e8e0dff72a8c5f8a5 (diff)
parent9c0b2596f2ac30967af0b8bb9f038b65926a6f00 (diff)
Merge tag 'for-linus-20190726' of git://git.kernel.dk/linux-block
Pull block fixes from Jens Axboe: - Several io_uring fixes/improvements: - Blocking fix for O_DIRECT (me) - Latter page slowness for registered buffers (me) - Fix poll hang under certain conditions (me) - Defer sequence check fix for wrapped rings (Zhengyuan) - Mismatch in async inc/dec accounting (Zhengyuan) - Memory ordering issue that could cause stall (Zhengyuan) - Track sequential defer in bytes, not pages (Zhengyuan) - NVMe pull request from Christoph - Set of hang fixes for wbt (Josef) - Redundant error message kill for libahci (Ding) - Remove unused blk_mq_sched_started_request() and related ops (Marcos) - drbd dynamic alloc shash descriptor to reduce stack use (Arnd) - blkcg ->pd_stat() non-debug print (Tejun) - bcache memory leak fix (Wei) - Comment fix (Akinobu) - BFQ perf regression fix (Paolo) * tag 'for-linus-20190726' of git://git.kernel.dk/linux-block: (24 commits) io_uring: ensure ->list is initialized for poll commands Revert "nvme-pci: don't create a read hctx mapping without read queues" nvme: fix multipath crash when ANA is deactivated nvme: fix memory leak caused by incorrect subsystem free nvme: ignore subnqn for ADATA SX6000LNP drbd: dynamically allocate shash descriptor block: blk-mq: Remove blk_mq_sched_started_request and started_request bcache: fix possible memory leak in bch_cached_dev_run() io_uring: track io length in async_list based on bytes io_uring: don't use iov_iter_advance() for fixed buffers block: properly handle IOCB_NOWAIT for async O_DIRECT IO blk-mq: allow REQ_NOWAIT to return an error inline io_uring: add a memory barrier before atomic_read rq-qos: use a mb for got_token rq-qos: set ourself TASK_UNINTERRUPTIBLE after we schedule rq-qos: don't reset has_sleepers on spurious wakeups rq-qos: fix missed wake-ups in rq_qos_throttle wait: add wq_has_single_sleeper helper block, bfq: check also in-flight I/O in dispatch plugging block: fix sysfs module parameters directory path in comment ...
Diffstat (limited to 'block/bfq-iosched.c')
-rw-r--r--block/bfq-iosched.c67
1 files changed, 43 insertions, 24 deletions
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 72860325245a..586fcfe227ea 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -3354,38 +3354,57 @@ static void bfq_dispatch_remove(struct request_queue *q, struct request *rq)
* there is no active group, then the primary expectation for
* this device is probably a high throughput.
*
- * We are now left only with explaining the additional
- * compound condition that is checked below for deciding
- * whether the scenario is asymmetric. To explain this
- * compound condition, we need to add that the function
+ * We are now left only with explaining the two sub-conditions in the
+ * additional compound condition that is checked below for deciding
+ * whether the scenario is asymmetric. To explain the first
+ * sub-condition, we need to add that the function
* bfq_asymmetric_scenario checks the weights of only
- * non-weight-raised queues, for efficiency reasons (see
- * comments on bfq_weights_tree_add()). Then the fact that
- * bfqq is weight-raised is checked explicitly here. More
- * precisely, the compound condition below takes into account
- * also the fact that, even if bfqq is being weight-raised,
- * the scenario is still symmetric if all queues with requests
- * waiting for completion happen to be
- * weight-raised. Actually, we should be even more precise
- * here, and differentiate between interactive weight raising
- * and soft real-time weight raising.
+ * non-weight-raised queues, for efficiency reasons (see comments on
+ * bfq_weights_tree_add()). Then the fact that bfqq is weight-raised
+ * is checked explicitly here. More precisely, the compound condition
+ * below takes into account also the fact that, even if bfqq is being
+ * weight-raised, the scenario is still symmetric if all queues with
+ * requests waiting for completion happen to be
+ * weight-raised. Actually, we should be even more precise here, and
+ * differentiate between interactive weight raising and soft real-time
+ * weight raising.
+ *
+ * The second sub-condition checked in the compound condition is
+ * whether there is a fair amount of already in-flight I/O not
+ * belonging to bfqq. If so, I/O dispatching is to be plugged, for the
+ * following reason. The drive may decide to serve in-flight
+ * non-bfqq's I/O requests before bfqq's ones, thereby delaying the
+ * arrival of new I/O requests for bfqq (recall that bfqq is sync). If
+ * I/O-dispatching is not plugged, then, while bfqq remains empty, a
+ * basically uncontrolled amount of I/O from other queues may be
+ * dispatched too, possibly causing the service of bfqq's I/O to be
+ * delayed even longer in the drive. This problem gets more and more
+ * serious as the speed and the queue depth of the drive grow,
+ * because, as these two quantities grow, the probability to find no
+ * queue busy but many requests in flight grows too. By contrast,
+ * plugging I/O dispatching minimizes the delay induced by already
+ * in-flight I/O, and enables bfqq to recover the bandwidth it may
+ * lose because of this delay.
*
* As a side note, it is worth considering that the above
- * device-idling countermeasures may however fail in the
- * following unlucky scenario: if idling is (correctly)
- * disabled in a time period during which all symmetry
- * sub-conditions hold, and hence the device is allowed to
- * enqueue many requests, but at some later point in time some
- * sub-condition stops to hold, then it may become impossible
- * to let requests be served in the desired order until all
- * the requests already queued in the device have been served.
+ * device-idling countermeasures may however fail in the following
+ * unlucky scenario: if I/O-dispatch plugging is (correctly) disabled
+ * in a time period during which all symmetry sub-conditions hold, and
+ * therefore the device is allowed to enqueue many requests, but at
+ * some later point in time some sub-condition stops to hold, then it
+ * may become impossible to make requests be served in the desired
+ * order until all the requests already queued in the device have been
+ * served. The last sub-condition commented above somewhat mitigates
+ * this problem for weight-raised queues.
*/
static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
struct bfq_queue *bfqq)
{
return (bfqq->wr_coeff > 1 &&
- bfqd->wr_busy_queues <
- bfq_tot_busy_queues(bfqd)) ||
+ (bfqd->wr_busy_queues <
+ bfq_tot_busy_queues(bfqd) ||
+ bfqd->rq_in_driver >=
+ bfqq->dispatched + 4)) ||
bfq_asymmetric_scenario(bfqd, bfqq);
}