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Date: Fri, 11 Sep 2020 17:46:45 +0100
From: Qais Yousef <qais.yousef@....com>
To: qianjun.kernel@...il.com
Cc: tglx@...utronix.de, peterz@...radead.org, will@...nel.org,
luto@...nel.org, linux-kernel@...r.kernel.org,
laoar.shao@...il.com, urezki@...il.com,
John Dias <joaodias@...gle.com>, Wei Wang <wvw@...gle.com>,
Quentin Perret <qperret@...gle.com>
Subject: Re: [PATCH V6 1/1] Softirq:avoid large sched delay from the pending
softirqs
On 09/09/20 17:09, qianjun.kernel@...il.com wrote:
> From: jun qian <qianjun.kernel@...il.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@...il.com>
In Android there's a patch that tries to avoid schedling an RT task on a cpu
that is running softirqs. I wonder if this patch helps with this case.
https://android.googlesource.com/kernel/msm/+/5c3f54c34acf4d9ed01530288d4a98acff815d79%5E%21/#F0
John, Wei, is this something of interest to you?
IIUC this patch will make sure the total softirq duration is 2ms rather than
each call is 2ms.
I persume if there's a single handler that takes a lot of time then this won't
help. But in that case, one can argue there's a potential bug with this
handler.
Cheers
--
Qais Yousef
> ---
> kernel/softirq.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 69 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index c4201b7f..1f696c8 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>
> @@ -199,18 +200,17 @@ 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.
> + * but break the loop if need_resched() is set or after MAX_SOFTIRQ_TIME_NS
> + * ns. In the loop, if the processing time of the softirq has exceeded
> + * MAX_SOFTIRQ_TIME_NS ns, 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
> @@ -246,15 +246,20 @@ static inline void lockdep_softirq_end(bool in_hardirq)
> static inline void lockdep_softirq_end(bool in_hardirq) { }
> #endif
>
> +DEFINE_PER_CPU(__u32, pending_new_flag);
> +DEFINE_PER_CPU(__u32, pending_next_bit);
> +#define SOFTIRQ_PENDING_MASK ((1UL << NR_SOFTIRQS) - 1)
> +
> 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;
> bool in_hardirq;
> - __u32 pending;
> - int softirq_bit;
> + __u32 pending, pending_left, pending_new;
> + int softirq_bit, next_bit;
> + unsigned long flags;
>
> /*
> * Mask out PF_MEMALLOC as the current task context is borrowed for the
> @@ -277,10 +282,33 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>
> h = softirq_vec;
>
> - while ((softirq_bit = ffs(pending))) {
> - unsigned int vec_nr;
> + next_bit = per_cpu(pending_next_bit, smp_processor_id());
> + per_cpu(pending_new_flag, smp_processor_id()) = 0;
> +
> + pending_left = pending &
> + (SOFTIRQ_PENDING_MASK << next_bit);
> + pending_new = pending &
> + (SOFTIRQ_PENDING_MASK >> (NR_SOFTIRQS - next_bit));
> +
> + /*
> + * In order to be fair, we shold process the pengding bits by the
> + * last processing order.
> + */
> + while ((softirq_bit = ffs(pending_left)) ||
> + (softirq_bit = ffs(pending_new))) {
> int prev_count;
> + unsigned int vec_nr = 0;
>
> + /*
> + * when the left pengding bits have been handled, we should
> + * to reset the h to softirq_vec.
> + */
> + if (!ffs(pending_left)) {
> + if (per_cpu(pending_new_flag, smp_processor_id()) == 0) {
> + h = softirq_vec;
> + per_cpu(pending_new_flag, smp_processor_id()) = 1;
> + }
> + }
> h += softirq_bit - 1;
>
> vec_nr = h - softirq_vec;
> @@ -298,17 +326,44 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
> preempt_count_set(prev_count);
> }
> h++;
> - pending >>= softirq_bit;
> +
> + if (ffs(pending_left))
> + pending_left >>= softirq_bit;
> + else
> + pending_new >>= softirq_bit;
> +
> + /*
> + * the softirq's action has been run too much time,
> + * so it may need to wakeup the ksoftirqd
> + */
> + if (need_resched() && sched_clock() > end) {
> + /*
> + * Ensure that the remaining pending bits will be
> + * handled.
> + */
> + local_irq_save(flags);
> + if (ffs(pending_left))
> + or_softirq_pending((pending_left << (vec_nr + 1)) |
> + pending_new);
> + else
> + or_softirq_pending(pending_new << (vec_nr + 1));
> + local_irq_restore(flags);
> + per_cpu(pending_next_bit, smp_processor_id()) = vec_nr + 1;
> + break;
> + }
> }
>
> + /* reset the pending_next_bit */
> + per_cpu(pending_next_bit, smp_processor_id()) = 0;
> +
> if (__this_cpu_read(ksoftirqd) == current)
> rcu_softirq_qs();
> local_irq_disable();
>
> 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();
> --
> 1.8.3.1
>
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