On 11/10/2017 10:34 AM, Christoph Hellwig wrote: > 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. Ah, that makes more sense. Since the function isn't necessarily that straight forward if open coded (to read, I mean), I'd prefer to keep it separate. But yeah, cosmetic, can always be revisited. -- Jens Axboe
