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SubjectRe: [PATCH V7 4/4] softirq: Allow early break the softirq processing loop
On Sat, Sep 26, 2020 at 12:42:25AM +0200, Thomas Gleixner wrote:
> On Fri, Sep 25 2020 at 02:42, Frederic Weisbecker wrote:
>
> > On Thu, Sep 24, 2020 at 05:37:42PM +0200, Thomas Gleixner wrote:
> >> Subject: softirq; Prevent starvation of higher softirq vectors
> > [...]
> >> + /*
> >> + * Word swap pending to move the not yet handled bits of the previous
> >> + * run first and then clear the duplicates in the newly raised ones.
> >> + */
> >> + swahw32s(&cur_pending);
> >> + pending = cur_pending & ~(cur_pending << SIRQ_PREV_SHIFT);
> >> +
> >> for_each_set_bit(vec_nr, &pending, NR_SOFTIRQS) {
> >> int prev_count;
> >>
> >> + vec_nr &= SIRQ_VECTOR_MASK;
> >
> > Shouldn't NR_SOFTIRQS above protect from that?
>
> It does, but that's wrong. The bitmap size in that for_each() loop must
> obviously be SIRQ_PREV_SHIFT + NR_SOFTIRQS for this to work.

Ah! I see, I thought you were ignoring the high bits on
purpose, hence my questions after about pending.

>
> >> + } else {
> >> + /*
> >> + * Retain the unprocessed bits and swap @cur_pending back
> >> + * into normal ordering
> >> + */
> >> + cur_pending = (u32)pending;
> >> + swahw32s(&cur_pending);
> >> + /*
> >> + * If the previous bits are done move the low word of
> >> + * @pending into the high word so it's processed first.
> >> + */
> >> + if (!(cur_pending & SIRQ_PREV_MASK))
> >> + cur_pending <<= SIRQ_PREV_SHIFT;
> >
> > If the previous bits are done and there is no timeout, should
> > we consider to restart a loop?
>
> We only enter this code path if there was a timeout. Otherwise pending
> would be 0.

Right with SIRQ_PREV_SHIFT + NR_SOFTIRQS now that whole makes sense!

Thanks!

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