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

[Paolo Abeni <pabeni@redhat.com>]

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