Re: [PATCH V4] Softirq:avoid large sched delay from the pending softirqs

[jun qian]

On Wed, Jul 29, 2020 at 8:16 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> Qian,
>
> jun qian <qianjun.kernel@xxxxxxxxx> writes:
> > On Mon, Jul 27, 2020 at 11:41 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >> > +                     or_softirq_pending(pending << (vec_nr + 1));
> >>
> >> To or the value interrupts need to be disabled because otherwise you can
> >> lose a bit when an interrupt happens in the middle of the RMW operation
> >> and raises a softirq which is not in @pending and not in the per CPU
> >> local softirq pending storage.
> >>
> > I can't understand the problem described above, could you explain in
> > detail.
>
> Oring a value to a memory location is a Read Modify Write (RMW)
> operation.
>
>         x |= a;
>
> translates to pseudo code:
>
>         R1 =  x      // Load content of memory location x into register R1
>         R1 |= a      // Or value a on R1
>         x = R1       // Write back result
>
> So assume:
>
>    x = 0
>    a = 1
>
>    R1 = x  --> R1 == 0
>    R1 |= a --> R1 == 1
>
> interrupt sets bit 1 in x       --> x == 0x02
>
>    x = R1  --> x == 1
>
> So you lost the set bit 1, right? Not really what you want.
>
wow, thanks a lot, i got it.

> >> There is another problem. Assume bit 0 and 1 are pending when the
> >> processing starts. Now it breaks out after bit 0 has been handled and
> >> stores back bit 1 as pending. Before ksoftirqd runs bit 0 gets raised
> >> again. ksoftirqd runs and handles bit 0, which takes more than the
> >> timeout. As a result the bit 0 processing can starve all other softirqs.
> >>
> > May I use a percpu val to record the order of processing softirq, by the order
> > val, when it is in ksoftirqd we can process the pending softirq in order. In the
> > scenario you described above, before ksoftirqd runs, bit 0 gets raised again,
> > ksoftirqd runs and handles bit 1 by the percpu val. just like a ring.
>
> Yes, you need something to save information about the not-processed
> bits. Keeping track of which bit to process next works, but whether
> that's going to result in efficient and simple code is a different
> question.
>
ok, i will modify it in the next version.

> Thanks,
>
>         tglx
>