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SubjectRe: [PATCH V5 1/7] blk-mq: issue rq directly in blk_mq_request_bypass_insert()
On Tue, Oct 03, 2017 at 01:58:50AM -0700, Christoph Hellwig wrote:
> This patch does two many things at once and needs a split. I also
> don't really understand why it's in this series and not your dm-mpath
> performance one.

Because the following patches only set hctx as busy after
BLK_STS_RESOURCE is returned from .queue_rq(), then add the
rq into hctx->dispatch.

But commit 157f377beb71(block: directly insert blk-mq request from
blk_insert_cloned_request()) just inserts rq into hctx->dispatch
directly, then we can't think hctx as busy any more if there are
requests in hctx->dispatch. That said the commit(157f377beb71)
makes the busy detection approach not working any more.

>
> > +static void blk_mq_request_direct_insert(struct blk_mq_hw_ctx *hctx,
> > + struct request *rq)
> > +{
> > + spin_lock(&hctx->lock);
> > + list_add_tail(&rq->queuelist, &hctx->dispatch);
> > + spin_unlock(&hctx->lock);
> > +
> > + blk_mq_run_hw_queue(hctx, false);
> > +}
>
> Why doesn't this share code with blk_mq_sched_bypass_insert?

It actually shares the code as this function is called
by blk_mq_request_bypass_insert().

>
> > /*
> > * Should only be used carefully, when the caller knows we want to
> > * bypass a potential IO scheduler on the target device.
> > */
> > -void blk_mq_request_bypass_insert(struct request *rq)
> > +blk_status_t blk_mq_request_bypass_insert(struct request *rq)
> > {
> > struct blk_mq_ctx *ctx = rq->mq_ctx;
> > struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(rq->q, ctx->cpu);
> > + blk_qc_t cookie;
> > + blk_status_t ret;
> >
> > - spin_lock(&hctx->lock);
> > - list_add_tail(&rq->queuelist, &hctx->dispatch);
> > - spin_unlock(&hctx->lock);
> > -
> > - blk_mq_run_hw_queue(hctx, false);
> > + ret = blk_mq_try_issue_directly(hctx, rq, &cookie, true);
> > + if (ret == BLK_STS_RESOURCE)
> > + blk_mq_request_direct_insert(hctx, rq);
> > + return ret;
>
> If you actually insert the request on BLK_STS_RESOURCE why do you
> pass the error on? In general BLK_STS_RESOURCE indicates a failure
> to issue.

OK, I will change it into BLK_STS_OK and switch it back in
the dm-rq patches.

>
> > +/*
> > + * 'dispatch_only' means we only try to dispatch it out, and
> > + * don't deal with dispatch failure if BLK_STS_RESOURCE or
> > + * BLK_STS_IOERR happens.
> > + */
> > +static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> > + struct request *rq, blk_qc_t *cookie, bool may_sleep,
> > + bool dispatch_only)
>
> This dispatch_only argument that completely changes behavior is a
> nightmare. Try to find a way to have a low-level helper that
> always behaves as if dispatch_only is set, and then build another
> helper that actually issues/completes around it.

OK, I will try to work towards that way.

--
Ming

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