Re: [PATCH V6 1/1] Softirq:avoid large sched delay from the pending softirqs

[jun qian <qianjun.kernel@gmail.com>]

<peterz@infradead.org> 于2020年9月11日周五 下午11:55写道：
>
> On Wed, Sep 09, 2020 at 05:09:31PM +0800, 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.
>
> But what is all that unreadaable gibberish with pending_new_{flag,bit} ?
>
> Random comments below..
>
>
> > +#define MAX_SOFTIRQ_TIME_NS 2000000
>
>         2*NSEC_PER_MSEC
>
>
> > +DEFINE_PER_CPU(__u32, pending_new_flag);
> > +DEFINE_PER_CPU(__u32, pending_next_bit);
>
> __u32 is for userspace ABI, this is not it, use u32
>
> > +#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;
>
>         __this_cpu_read() / __this_cpu_write()
>
> > +
> > +     pending_left = pending &
> > +             (SOFTIRQ_PENDING_MASK << next_bit);
> > +     pending_new = pending &
> > +             (SOFTIRQ_PENDING_MASK >> (NR_SOFTIRQS - next_bit));
>
> The second mask is the inverse of the first.
>
> > +     /*
> > +      * 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))
>
> This is the _third_ ffs(pending_left), those things are _expensive_ (on
> some archs, see include/asm-generic/bitops/__ffs.h).
>
> > +                     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))
>
> *fourth*...
>
> > +                             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();
>
> This really wants to be a number of separate patches; and I quickly lost
> the plot in your code. Instead of cleaning things up, you're making an
> even bigger mess of things.
>
> That said, I _think_ I've managed to decode what you want. See the
> completely untested patches attached.
>
>

thanks a lot thank for your suggestion. I will rewrite the patch by
following you sugestion, Useing multiple patches to clarify ideas.