linux-kernel - Re: [PATCH 1/2] libata: get rid of ATA_MAX_QUEUE loop in ata_qc_complete

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Date:	Wed, 22 Oct 2008 20:23:50 +0200
From:	Elias Oltmanns <eo@...ensachen.de>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	tj@...nel.org, jeff@...zik.org
Subject: Re: [PATCH 1/2] libata: get rid of ATA_MAX_QUEUE loop in ata_qc_complete_multiple()

Jens Axboe <jens.axboe@...cle.com> wrote:
> We very rarely (if ever) complete more than one command in the
> sactive mask at the time, even for extremely high IO rates. So
> looping over the entire range of possible tags is pointless,
> instead use __ffs() to just find the completed tags directly.
>
> Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> ---
>  drivers/ata/libata-core.c |   15 +++++++++------
>  1 files changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
> index 1ee9499..c3c53e7 100644
> --- a/drivers/ata/libata-core.c
> +++ b/drivers/ata/libata-core.c
> @@ -4799,9 +4799,9 @@ void ata_qc_complete(struct ata_queued_cmd *qc)
>   */
>  int ata_qc_complete_multiple(struct ata_port *ap, u32 qc_active)
>  {
> +	unsigned int i = 0;
>  	int nr_done = 0;
>  	u32 done_mask;
> -	int i;
>  
>  	done_mask = ap->qc_active ^ qc_active;
>  
> @@ -4811,16 +4811,19 @@ int ata_qc_complete_multiple(struct ata_port *ap, u32 qc_active)
>  		return -EINVAL;
>  	}
>  
> -	for (i = 0; i < ATA_MAX_QUEUE; i++) {
> +	while (done_mask) {
>  		struct ata_queued_cmd *qc;
> +		unsigned int next = __ffs(done_mask);
>  
> -		if (!(done_mask & (1 << i)))
> -			continue;
> -
> -		if ((qc = ata_qc_from_tag(ap, i))) {
> +		qc = ata_qc_from_tag(ap, i + next);
> +		if (qc) {
>  			ata_qc_complete(qc);
>  			nr_done++;
>  		}
> +		if (++next >= ATA_MAX_QUEUE)
> +			break;

If you think about it, this statement is equivalent to

	if (ap->qc_active ^ qc_active == (1 << (ATA_MAX_QUEUE - 1)))

To fix this, you could say

	if (++next + i >= ATA_MAX_QUEUE)

but perhaps it would be even more efficient (or not much worse) to skip
this check entirely.

Regards,

Elias
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