On Mon, 14 Nov 2022 at 16:29, Christian Löhle <CLoehle@xxxxxxxxxxxxxx> wrote: > > In case the driver set RQF_DONTPREP flag, requeue the request as head as > it is likely that the backing storage already had a request to an > adjacent region, so getting the requeued request out as soon as possible > may give us some performance benefit. > > The are various reasons a driver may requeue a request > (and therefore RQF_DONTPREP being set). > One reason may be that the driver or the hardware cannot satisfy > the block layer alignment/sizes. > > This for example is the case with mmcblk with a host driver that > cannot deal with multiple block transfers. > Consider a request for lba 42 for one page so 4K or 8 blocks. > mmcblk will do a single block request for lba 42 and requeue for > the rest if the host can only perform single block transfers. > In this case the device only delivered 512 bytes of data at lba 42, > but may have performed a readahead itself for the following lbas, > or do an 'implicit' readahead as it only deals with larger block > size mappings itself. > In that case it would be beneificial if the request for lba 43 and > the rest of the remaining blocks follow immediately after the initial > lba 42 request. > Requeueing already partially processed requests as head preserves the > intended request order (unless the driver has a queue of its own) > and therefore mitigates this problem . > > Signed-off-by: Christian Loehle <cloehle@xxxxxxxxxxxxxx> Thanks for updating the commit message! Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx> As I said for v1, this seems reasonable to me, but I am deferring to Jens to get his opinion. Beyond that, I will have a look at pacth2 and patch3 too. Kind regards Uffe > --- > -v2: Extended commit message with example use case, no code change > > block/blk-mq.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 33292c01875d..d863c826fb23 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -1429,7 +1429,7 @@ static void blk_mq_requeue_work(struct work_struct *work) > * merge. > */ > if (rq->rq_flags & RQF_DONTPREP) > - blk_mq_request_bypass_insert(rq, false, false); > + blk_mq_request_bypass_insert(rq, true, false); > else > blk_mq_sched_insert_request(rq, true, false, false); > } > -- > 2.37.3 > Hyperstone GmbH | Reichenaustr. 39a | 78467 Konstanz > Managing Director: Dr. Jan Peter Berns. > Commercial register of local courts: Freiburg HRB381782 >
