linux-kernel - Re: [PATCH V5 1/7] blk-mq: issue rq directly in blk_mq_request_bypass

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Date:   Tue, 3 Oct 2017 01:58:50 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Laurence Oberman <loberman@...hat.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Tom Nguyen <tom81094@...il.com>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Omar Sandoval <osandov@...com>
Subject: Re: [PATCH V5 1/7] blk-mq: issue rq directly in
 blk_mq_request_bypass_insert()

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.

> +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?

>  /*
>   * 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.

> +/*
> + * '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.