On 01/30/18 06:24, Mike Snitzer wrote:
+ *
+ * If driver returns BLK_STS_RESOURCE and SCHED_RESTART
+ * bit is set, run queue after a delay to avoid IO stalls
+ * that could otherwise occur if the queue is idle.
*/
- if (!blk_mq_sched_needs_restart(hctx) ||
+ needs_restart = blk_mq_sched_needs_restart(hctx);
+ if (!needs_restart ||
(no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))
blk_mq_run_hw_queue(hctx, true);
+ else if (needs_restart && (ret == BLK_STS_RESOURCE))
+ blk_mq_delay_run_hw_queue(hctx, BLK_MQ_QUEUE_DELAY);
}
If a request completes concurrently with execution of the above code
then the request completion will trigger a call of
blk_mq_sched_restart_hctx() and that call will clear the
BLK_MQ_S_SCHED_RESTART bit. If that bit is cleared before the above code
tests it then the above code will schedule an asynchronous call of
__blk_mq_run_hw_queue(). If the .queue_rq() call triggered by the new
queue run returns again BLK_STS_RESOURCE then the above code will be
executed again. In other words, a loop occurs. That loop will repeat as
long as the described race occurs. The current (kernel v4.15) block
layer behavior is simpler: only block drivers call
blk_mq_delay_run_hw_queue() and the block layer core never calls that
function. Hence that loop cannot occur with the v4.15 block layer core
and block drivers. A motivation of why that loop is preferred compared
to the current behavior (no loop) is missing. Does this mean that that
loop is a needless complication of the block layer core?
Sorry but I still prefer the v4.15 block layer approach because this
patch has in my view the following disadvantages:
- It involves a blk-mq API change. API changes are always risky and need
some time to stabilize.
- The delay after which to rerun the queue is moved from block layer
drivers into the block layer core. I think that's wrong because only
the block driver authors can make a good choice for this constant.
- This patch makes block drivers harder to understand. Anyone who sees
return BLK_STS_RESOURCE / return BLK_STS_DEV_RESOURCE for the first
time will have to look up the meaning of these constants. An explicit
blk_mq_delay_run_hw_queue() call is easier to understand.
- This patch makes the blk-mq core harder to understand because of the
loop mentioned above.
- This patch does not fix any bugs nor makes block drivers easier to
read or to implement. So why is this patch considered useful?
Thanks,
Bart.
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