blk-mq-sched: support request batch dispatching for sq elevator#55
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Introduce a new spinlock in elevator_queue, and switch dd->lock to use the new lock. There are no functional changes. Signed-off-by: Yu Kuai <[email protected]>
Currently issue io can grab queue_lock three times from bfq_bio_merge(), bfq_limit_depth() and bfq_prepare_request(), the queue_lock is not necessary if icq is already created: - queue_usage_counter is already grabbed and queue won't exist; - current thread won't exist; - if other thread is allocating and inserting new icq to ioc->icq_tree, rcu can be used to protect lookup icq from the raidx tree, it's safe to use extracted icq until queue or current thread exit; If ioc or icq is not created, then bfq_prepare_request() will create it, which means the task is issuing io to queue the first time, this can consider a slow path and queue_lock will still be held to protect inserting allocated icq to ioc->icq_tree. Signed-off-by: Yu Kuai <[email protected]>
Replace the internal spinlock bfqd->lock with the new spinlock in elevator_queue. There are no functional changes. Signed-off-by: Yu Kuai <[email protected]> Reviewed-by: Damien Le Moal <[email protected]>
Currently, both mq-deadline and bfq have global spin lock that will be grabbed inside elevator methods like dispatch_request, insert_requests, and bio_merge. And the global lock is the main reason mq-deadline and bfq can't scale very well. For dispatch_request method, current behavior is dispatching one request at a time. In the case of multiple dispatching contexts, this behavior will cause huge lock contention and messing up the requests dispatching order. And folloiwng patches will support requests batch dispatching to fix thoses problems. While dispatching request, blk_mq_get_disatpch_budget() and blk_mq_get_driver_tag() must be called, and they are not ready to be called inside elevator methods, hence introduce a new method like dispatch_requests is not possible. In conclusion, this patch factor the global lock out of dispatch_request method, and following patches will support request batch dispatch by calling the methods multiple time while holding the lock. Signed-off-by: Yu Kuai <[email protected]>
Introduce struct sched_dispatch_ctx, and split the helper into elevator_dispatch_one_request() and elevator_finish_dispatch(). Also and comments about the non-error return value. Make code cleaner, and make it easier to add a new branch to dispatch a batch of requests at a time in the next patch. Signed-off-by: Yu Kuai <[email protected]>
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Upstream branch: 25fae0b |
For dispatch_request method, current behavior is dispatching one request at
a time. In the case of multiple dispatching contexts, This behavior, on the
one hand, introduce intense lock contention:
t1: t2: t3:
lock lock lock
// grab lock
ops.dispatch_request
unlock
// grab lock
ops.dispatch_request
unlock
// grab lock
ops.dispatch_request
unlock
on the other hand, messing up the requests dispatching order:
t1:
lock
rq1 = ops.dispatch_request
unlock
t2:
lock
rq2 = ops.dispatch_request
unlock
lock
rq3 = ops.dispatch_request
unlock
lock
rq4 = ops.dispatch_request
unlock
//rq1,rq3 issue to disk
// rq2, rq4 issue to disk
In this case, the elevator dispatch order is rq 1-2-3-4, however,
such order in disk is rq 1-3-2-4, the order for rq2 and rq3 is inversed.
Fix those problems by introducing elevator_dispatch_requests(), this
helper will grab the lock and dispatch a batch of requests while holding
the lock.
Signed-off-by: Yu Kuai <[email protected]>
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Pull request for series with
subject: blk-mq-sched: support request batch dispatching for sq elevator
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=984570