Messages in this thread | | | Subject | Re: [RFC PATCH 2/2] softirq: Per vector thread deferment | From | Paolo Abeni <> | Date | Fri, 12 Jan 2018 10:07:25 +0100 |
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On Fri, 2018-01-12 at 06:35 +0100, Frederic Weisbecker wrote: > Some softirq vectors can be more CPU hungry than others. Especially > networking may sometimes deal with packet storm and need more CPU than > IRQ tail can offer without inducing scheduler latencies. In this case > the current code defers to ksoftirqd that behaves nicer. Now this nice > behaviour can be bad for other IRQ vectors that usually need quick > processing. > > To solve this we only defer to threading the vectors that outreached the > time limit on IRQ tail processing and leave the others inline on real > Soft-IRQs service. This is achieved using workqueues with > per-CPU/per-vector worklets. > > Note ksoftirqd is not removed as it is still needed for threaded IRQs > mode. > > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > Cc: Dmitry Safonov <dima@arista.com> > Cc: Eric Dumazet <edumazet@google.com> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: David Miller <davem@davemloft.net> > Cc: Hannes Frederic Sowa <hannes@stressinduktion.org> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Levin Alexander <alexander.levin@verizon.com> > Cc: Paolo Abeni <pabeni@redhat.com> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Cc: Radu Rendec <rrendec@arista.com> > Cc: Rik van Riel <riel@redhat.com> > Cc: Stanislaw Gruszka <sgruszka@redhat.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Wanpeng Li <wanpeng.li@hotmail.com> > --- > kernel/softirq.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-- > 1 file changed, 87 insertions(+), 3 deletions(-) > > diff --git a/kernel/softirq.c b/kernel/softirq.c > index fa267f7..0c817ec6 100644 > --- a/kernel/softirq.c > +++ b/kernel/softirq.c > @@ -74,6 +74,13 @@ struct softirq_stat { > > static DEFINE_PER_CPU(struct softirq_stat, softirq_stat_cpu); > > +struct vector_work { > + int vec; > + struct work_struct work; > +}; > + > +static DEFINE_PER_CPU(struct vector_work[NR_SOFTIRQS], vector_work_cpu); > + > /* > * we cannot loop indefinitely here to avoid userspace starvation, > * but we also don't want to introduce a worst case 1/HZ latency > @@ -251,6 +258,70 @@ static inline bool lockdep_softirq_start(void) { return false; } > static inline void lockdep_softirq_end(bool in_hardirq) { } > #endif > > +static void vector_work_func(struct work_struct *work) > +{ > + struct vector_work *vector_work; > + u32 pending; > + int vec; > + > + vector_work = container_of(work, struct vector_work, work); > + vec = vector_work->vec; > + > + local_irq_disable(); > + pending = local_softirq_pending(); > + account_irq_enter_time(current); > + __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET); > + lockdep_softirq_enter(); > + set_softirq_pending(pending & ~(1 << vec)); > + local_irq_enable(); > + > + if (pending & (1 << vec)) { > + struct softirq_action *sa = &softirq_vec[vec]; > + > + kstat_incr_softirqs_this_cpu(vec); > + trace_softirq_entry(vec); > + sa->action(sa); > + trace_softirq_exit(vec); > + } > + > + local_irq_disable(); > + > + pending = local_softirq_pending(); > + if (pending & (1 << vec)) > + schedule_work_on(smp_processor_id(), work);
If we check for the overrun condition here, as done in the __do_softirq() main loop, we could avoid ksoftirqd completely and probably have less code duplication.
> + > + lockdep_softirq_exit(); > + account_irq_exit_time(current); > + __local_bh_enable(SOFTIRQ_OFFSET); > + local_irq_enable(); > +} > + > +static int do_softirq_overrun(u32 overrun, u32 pending) > +{ > + struct softirq_action *h = softirq_vec; > + int softirq_bit; > + > + if (!overrun) > + return pending; > + > + overrun &= pending; > + pending &= ~overrun; > + > + while ((softirq_bit = ffs(overrun))) { > + struct vector_work *work; > + unsigned int vec_nr; > + > + h += softirq_bit - 1; > + vec_nr = h - softirq_vec; > + work = this_cpu_ptr(&vector_work_cpu[vec_nr]); > + schedule_work_on(smp_processor_id(), &work->work); > + h++; > + overrun >>= softirq_bit; > + } > + > + return pending; > +} > + > asmlinkage __visible void __softirq_entry __do_softirq(void) > { > struct softirq_stat *sstat = this_cpu_ptr(&softirq_stat_cpu); > @@ -321,10 +392,13 @@ asmlinkage __visible void __softirq_entry __do_softirq(void) > > pending = local_softirq_pending(); > if (pending) { > - if (overrun || need_resched()) > + if (need_resched()) { > wakeup_softirqd(); > - else > - goto restart; > + } else { > + pending = do_softirq_overrun(overrun, pending); > + if (pending) > + goto restart; > + } > } > > lockdep_softirq_end(in_hardirq);
This way the 'overrun' branch is not triggered if we (also) need resched, should we test for overrun first ?
Cheers,
Paolo
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