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SubjectRe: [PATCH 1/2] libata: get rid of ATA_MAX_QUEUE loop in ata_qc_complete_multiple()
Date
Jens Axboe <jens.axboe@oracle.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@oracle.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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