On 09/09/20 17:09, 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>
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 >
