Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH V4] Softirq:avoid large sched delay from the pending softirqs | Date | Mon, 27 Jul 2020 17:41:04 +0200 |
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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.
So this needs more thought.
Thanks,
tglx
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