On Fri, Jul 24, 2020 at 10:31:23AM -0400, qianjun.ker...@gmail.com wrote:
> From: jun qian <qianjun.ker...@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.ker...@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
