On 02/06/2017 12:53 PM, Omar Sandoval wrote: > On Mon, Feb 06, 2017 at 12:39:57PM -0700, Jens Axboe wrote: >> On 02/06/2017 12:24 PM, Omar Sandoval wrote: >>> From: Omar Sandoval <[email protected]> >>> >>> In blk_mq_sched_dispatch_requests(), we call blk_mq_sched_mark_restart() >>> after we dispatch requests left over on our hardware queue dispatch >>> list. This is so we'll go back and dispatch requests from the scheduler. >>> In this case, it's only necessary to restart the hardware queue that we >>> are running; there's no reason to run other hardware queues just because >>> we are using shared tags. >>> >>> So, split out blk_mq_sched_mark_restart() into two operations, one for >>> just the hardware queue and one for the whole request queue. The core >>> code uses both, and I/O schedulers may also want to use them. >>> >>> Signed-off-by: Omar Sandoval <[email protected]> >>> --- >>> Patch based on block/for-next. >>> >>> block/blk-mq-sched.c | 2 +- >>> block/blk-mq-sched.h | 25 ++++++++++++++++++------- >>> block/blk-mq.c | 5 ++++- >>> 3 files changed, 23 insertions(+), 9 deletions(-) >>> >>> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c >>> index ee455e7cf9d8..7538565359ea 100644 >>> --- a/block/blk-mq-sched.c >>> +++ b/block/blk-mq-sched.c >>> @@ -201,7 +201,7 @@ void blk_mq_sched_dispatch_requests(struct >>> blk_mq_hw_ctx *hctx) >>> * needing a restart in that case. >>> */ >>> if (!list_empty(&rq_list)) { >>> - blk_mq_sched_mark_restart(hctx); >>> + blk_mq_sched_mark_restart_hctx(hctx); >>> blk_mq_dispatch_rq_list(hctx, &rq_list); >> >> What if we dispatched nothing on this hardware queue, and it currently >> doesn't have any IO pending? > > Hm, so there are two ways that could happen. If it's because > ->queue_rq() returned BLK_MQ_RQ_QUEUE_BUSY, then the driver is supposed > to kick I/O off again, right? > > If it's because we failed to get a driver tag, then we'll call > blk_mq_sched_mark_restart_queue() in the shared case. I just realized > that there's a bug there, though. Since we already set the hctx restart > bit, we won't set the queue restart bit. The below should work, and > makes more sense in general. > > Or were you thinking of something else?
No, I think that covers it, I had not read far enough either to see that you handle the shared tag case for tag starvation in the caller. -- Jens Axboe
