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Re: [RFC PATCH 2/2] softirq: Per vector thread deferment




On Fri, Jan 12, 2018 at 10:07:25AM +0100, Paolo Abeni wrote:
> 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@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Cc: Dmitry Safonov <dima@xxxxxxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: David Miller <davem@xxxxxxxxxxxxx>
> > Cc: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Levin Alexander <alexander.levin@xxxxxxxxxxx>
> > Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Cc: Radu Rendec <rrendec@xxxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > Cc: Stanislaw Gruszka <sgruszka@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > ---
> >  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.

Yes that could be possible indeed. I guess having workqueues serializing
vector works is not much different that what ksoftirqd does.

I can try that.

> 
> > +
> > +	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 ?

Yes they could have similar treatment. If need_resched() we schedule
everything that is still pending: do_softirq_overrun(pending, pending),
otherwise we take the other branch and still do a goto restart.

In fact it can even be simplified this way:


      if (need_resched())
          overrun = pending;
      pending = do_softirq_overrun(overrun, pending);
      if (pending)
          goto restart;

Thanks.