On 05/02/2015 06:31 PM, Shaohua Li wrote:
Normally if driver is busy to dispatch a request the logic is like below:
block layer: driver:
__blk_mq_run_hw_queue
a. blk_mq_stop_hw_queue
b. rq add to ctx->dispatch
later:
1. blk_mq_start_hw_queue
2. __blk_mq_run_hw_queue
But it's possible step 1-2 runs between a and b. And since rq isn't in
ctx->dispatch yet, step 2 will not run rq. The rq might get lost if
there are no subsequent requests kick in.
Good catch! But the patch introduces a potentially never ending loop in
__blk_mq_run_hw_queue(). Not sure how we can fully close it, but it
might be better to punt the re-run after adding the requests back to the
worker. That would turn a potential busy loop (until requests complete)
into something with nicer behavior, at least. Ala
if (!test_bit(BLK_MQ_S_STOPPED, &hctx->state))
kblockd_schedule_delayed_work_on(blk_mq_hctx_next_cpu(hctx),
&hctx->run_work, 0);
--
Jens Axboe
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