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

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Jul 24, 2020 at 10:31:23AM -0400, qianjun.kernel@gmail.com wrote:
> From: jun qian <qianjun.kernel@gmail.com>
> 
> When get the pending softirqs, it need to process all the pending
> softirqs in the while loop. If the processing time of each pending
> softirq is need more than 2 msec in this loop, or one of the softirq
> will running a long time, according to the original code logic, it
> will process all the pending softirqs without wakeuping ksoftirqd,
> which will cause a relatively large scheduling delay on the
> corresponding CPU, which we do not wish to see. The patch will check
> the total time to process pending softirq, if the time exceeds 2 ms
> we need to wakeup the ksofirqd to aviod large sched delay.
> 
> Signed-off-by: jun qian <qianjun.kernel@gmail.com>
> ---
>  kernel/softirq.c | 28 ++++++++++++++++++++--------
>  1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index c4201b7f..d572ce4 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -25,6 +25,7 @@
>  #include <linux/smpboot.h>
>  #include <linux/tick.h>
>  #include <linux/irq.h>
> +#include <linux/sched/clock.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/irq.h>
> @@ -200,17 +201,15 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
>  /*
>   * 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.
>   *
>   * These limits have been established via experimentation.
>   * The two things to balance is latency against fairness -
>   * 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_NS 2000000
>  #define MAX_SOFTIRQ_RESTART 10
>  
>  #ifdef CONFIG_TRACE_IRQFLAGS
> @@ -248,7 +247,7 @@ static inline void lockdep_softirq_end(bool in_hardirq) { }
>  
>  asmlinkage __visible void __softirq_entry __do_softirq(void)
>  {
> -	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
> +	u64 end = sched_clock() + MAX_SOFTIRQ_TIME_NS;
>  	unsigned long old_flags = current->flags;
>  	int max_restart = MAX_SOFTIRQ_RESTART;
>  	struct softirq_action *h;
> @@ -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));
> +			break;
> +		}
>  	}
>  
>  	if (__this_cpu_read(ksoftirqd) == current)
> @@ -307,8 +319,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>  
>  	pending = local_softirq_pending();
>  	if (pending) {
> -		if (time_before(jiffies, end) && !need_resched() &&
> -		    --max_restart)
> +		if (!need_resched() && --max_restart &&
> +		    sched_clock() <= end)
>  			goto restart;
>  
>  		wakeup_softirqd();
> 
To me it looks OKr.

Thank you for fixing the "2 msec resolution case".

--
Vlad Rezki