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SubjectRe: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing
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Hi Alexander!

Apologies for the long delay on this follow-up.. Comments below.

On Fri, 2013-09-20 at 17:19 +0200, Alexander Gordeev wrote:
> On Thu, Aug 15, 2013 at 07:19:29PM -0700, Nicholas A. Bellinger wrote:
> > Ok, here's a bit better idea of what is going on now..
> >
> > The problem is that blkdev_issue_flush() -> blk_mq_make_request() ->
> > __blk_mq_alloc_request() allocates the first tag, which calls
> > blk_insert_flush() -> blk_flush_complete_seq() -> blk_flush_kick() ->
> > mq_flush_work() -> blk_mq_alloc_request() to allocate a second tag for
> > the struct request that actually gets dispatched into scsi-mq as a
> > SYCHRONIZE_CACHE command..
> >
> > I'm not exactly sure why this double tag usage of struct request is
> > occurring, but AFAICT it does happen for every flush, and is not
> > specific to the blkdev_issue_flush() codepath.. I'm sure that Jens can
> > fill us in on that bit. ;)
>
> I also played with the double tag using a reserved tag (below).
>
> While it fixes 'fdisk /dev/sda' issue when trying to 'mount /dev/sda1 /mnt'
> what appears to be a call to bio->bi_end_io() from the free'd bio hits in.
>
> Not sure if I should pursue the root cause until the whole double-tag
> thingy is confirmed.
>
> Jens?
>
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 6fc1df3..81794dc 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -874,14 +874,14 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
> hctx = q->mq_ops->map_queue(q, ctx->cpu);
>
> trace_block_getrq(q, bio, rw);
> - rq = __blk_mq_alloc_request(hctx, GFP_ATOMIC, false);
> + rq = __blk_mq_alloc_request(hctx, GFP_ATOMIC, is_flush_fua);
> if (likely(rq))
> blk_mq_rq_ctx_init(ctx, rq, rw);
> else {
> blk_mq_put_ctx(ctx);
> trace_block_sleeprq(q, bio, rw);
> rq = blk_mq_alloc_request_pinned(q, rw, __GFP_WAIT|GFP_ATOMIC,
> - false);
> + is_flush_fua);
> ctx = rq->mq_ctx;
> hctx = q->mq_ops->map_queue(q, ctx->cpu);
> }

So this is what I ended up doing as well, and does address the specific
bug with queue_depth=1.

> @@ -1317,6 +1317,9 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_reg *reg,
> reg->queue_depth = BLK_MQ_MAX_DEPTH;
> }
>
> + reg->queue_depth++;
> + reg->reserved_tags++;
> +
> ctx = alloc_percpu(struct blk_mq_ctx);
> if (!ctx)
> return ERR_PTR(-ENOMEM);
>

I was actually setting this within scsi_mq_alloc_queue(), but given that
the queue_depth=1 issue is independent of scsi-mq, this does make more
sense.

Also, these extra increments should probably happen only when the passed
queue_depth == 1 && reserved_tags == 0.

Other than that minor nit.

Reviewed-by: Nicholas Bellinger <nab@linux-iscsi.org>



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