blok, bfq: do not plug I/O if all queues are weight-raised
[ Upstream commit c8765de0adfcaaf4ffb2d951e07444f00ffa9453 ] To reduce latency for interactive and soft real-time applications, bfq privileges the bfq_queues containing the I/O of these applications. These privileged queues, referred-to as weight-raised queues, get a much higher share of the device throughput w.r.t. non-privileged queues. To preserve this higher share, the I/O of any non-weight-raised queue must be plugged whenever a sync weight-raised queue, while being served, remains temporarily empty. To attain this goal, bfq simply plugs any I/O (from any queue), if a sync weight-raised queue remains empty while in service. Unfortunately, this plugging typically lowers throughput with random I/O, on devices with internal queueing (because it reduces the filling level of the internal queues of the device). This commit addresses this issue by restricting the cases where plugging is performed: if a sync weight-raised queue remains empty while in service, then I/O plugging is performed only if some of the active bfq_queues are *not* weight-raised (which is actually the only circumstance where plugging is needed to preserve the higher share of the throughput of weight-raised queues). This restriction proved able to boost throughput in really many use cases needing only maximum throughput. Signed-off-by: NPaolo Valente <paolo.valente@linaro.org> Signed-off-by: NJens Axboe <axboe@kernel.dk> Signed-off-by: NSasha Levin <sashal@kernel.org>
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