Re: Softirq priority inversion from "softirq: reduce latencies"

Peter Hurley
2016-02-29T18:05:36Z

On 02/29/2016 08:21 AM, Eric Dumazet wrote:
> On lun., 2016-02-29 at 07:54 -0800, Peter Hurley wrote:
>
>> The current kernel is HZ=250 but this would occur on HZ=1000 as well.
>
> Right. But the problem with HZ=100 and HZ=250 is that the detection can
> happens because jiffy granularity is too coarse, since
>
> msecs_to_jiffies(2) -> 1
>
> Following patch might reduce the probability, but wont really fix your
> problem.
>
> Fact that ksoftirqd prio is not what you want is completely orthogonal.
>
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 479e443..f7cc594 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -180,7 +180,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
>
> /*
> * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
> - * but break the loop if need_resched() is set or after 2 ms.
> + * but break the loop if need_resched() is set or after 2 ms/ticks.
> * 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
> @@ -191,7 +191,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
> * we want to handle softirqs as soon as possible, but they
> * should not be able to lock up the box.
> */
> -#define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
> +#define MAX_SOFTIRQ_TIME (1 + msecs_to_jiffies(2))
> #define MAX_SOFTIRQ_RESTART 10
>
> #ifdef CONFIG_TRACE_IRQFLAGS

While I appreciate the attempt, that's not the problem.

Just to be clear

if (time_before(jiffies, end) && !need_resched() &&
--max_restart)
goto restart;

aborts softirq *even if 0ns have elapsed*, if NET_RX has woken a process.