mlx4_wq_overflow question

mlx4_wq_overflow question

[Eli Cohen]

Hi Roland,

referring to the function below, I can't find a strong reason to check
again after acquiring the cq spinlock. Any idea?

static int mlx4_wq_overflow(struct mlx4_ib_wq *wq, int nreq, struct
ib_cq *ib_cq)
{
        unsigned cur;
        struct mlx4_ib_cq *cq;

        cur = wq->head - wq->tail;
        if (likely(cur + nreq < wq->max_post))
                return 0;

        cq = to_mcq(ib_cq);
        spin_lock(&cq->lock);
        cur = wq->head - wq->tail;
        spin_unlock(&cq->lock);

        return cur + nreq >= wq->max_post;
}
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Re: mlx4_wq_overflow question

[Roland Dreier]

On Wed, Feb 8, 2012 at 2:45 PM, Eli Cohen <eli-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org> wrote:
> referring to the function below, I can't find a strong reason to check
> again after acquiring the cq spinlock. Any idea?
>
> static int mlx4_wq_overflow(struct mlx4_ib_wq *wq, int nreq, struct
> ib_cq *ib_cq)
> {
>        unsigned cur;
>        struct mlx4_ib_cq *cq;
>
>        cur = wq->head - wq->tail;
>        if (likely(cur + nreq < wq->max_post))
>                return 0;
>
>        cq = to_mcq(ib_cq);
>        spin_lock(&cq->lock);
>        cur = wq->head - wq->tail;
>        spin_unlock(&cq->lock);
>
>        return cur + nreq >= wq->max_post;
> }

This is the typical optimization of doing a cheaper imprecise check
first, and then only if we need to (ie the QP was close to full) take
the lock and synchronize with completions being polled to get an
exact answer.  Without the second check we might have spurious
"WQ full" indications.

If I recall correctly, MST first proposed this optimization for mthca
years ago.  If you search the archives you can probably find his
detailed explanation.

 - R.
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