On Fri, Nov 10, 2017 at 10:31:29AM -0700, Jens Axboe wrote: > On 11/10/2017 10:29 AM, Christoph Hellwig wrote: > > On Fri, Nov 10, 2017 at 09:12:18AM -0700, Jens Axboe wrote: > >> Currently we are inconsistent in when we decide to run the queue. Using > >> blk_mq_run_hw_queues() we check if the hctx has pending IO before > >> running it, but we don't do that from the individual queue run function, > >> blk_mq_run_hw_queue(). This results in a lot of extra and pointless > >> queue runs, potentially, on flush requests and (much worse) on tag > >> starvation situations. This is observable just looking at the top > >> output, with lots of kworkers active. For the !async runs, it just adds > >> to the CPU overhead of blk-mq. > >> > >> Move the has-pending check into the run function instead of having > >> callers do it. > >> > >> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> > > > > Do we even still need the blk_mq_hctx_has_pending helper at all? > > We do, otherwise we end up running the queue for cases where we don't > need to. The caller doesn't always know if it's necessary. And even > if the caller can figure it out, it'd add complicated logic for some > of those cases (like flush). It's easier/cleaner and more efficient > to keep it. I didn't mean the logic - just wondering if we need the separate helper given it has a single caller left. But it's really just a cosmetic point.
