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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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