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SubjectRe: [PATCH V4] Softirq:avoid large sched delay from the pending softirqs
On Mon, Jul 27, 2020 at 11:41 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Qian,
>
> qianjun.kernel@gmail.com writes:
> > /*
> > * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
> > * but break the loop if need_resched() is set or after 2 ms.
> > - * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
> > - * certain cases, such as stop_machine(), jiffies may cease to
> > - * increment and so we need the MAX_SOFTIRQ_RESTART limit as
> > - * well to make sure we eventually return from this method.
> > + * In the loop, if the processing time of the softirq has exceeded 2
> > + * milliseconds, we also need to break the loop to wakeup the
> > ksofirqd.
>
> You are removing the MAX_SOFTIRQ_RESTART limit explanation and I rather
> have MAX_SOFTIRQ_TIME_NS there than '2 milliseconds' in case the value
> gets adjusted later on. Also while sched_clock() is granular on many
> systems it still can be jiffies based and then the above problem
> persists.
>
> > @@ -299,6 +298,19 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
> > }
> > h++;
> > pending >>= softirq_bit;
> > +
> > + /*
> > + * the softirq's action has been running for too much time
> > + * so it may need to wakeup the ksoftirqd
> > + */
> > + if (need_resched() && sched_clock() > end) {
> > + /*
> > + * Ensure that the remaining pending bits are
> > + * handled.
> > + */
> > + 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.
>
> 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.
>
I got it. I will try to slove this problem. Thanks.

> So this needs more thought.
>
> Thanks,
>
> tglx

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