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

[Frederic Weisbecker]

On Fri, Jan 12, 2018 at 06:35:54AM +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)) {

Ah I see the problem. Say in do_softirq() we had pending VECTOR 1 and 2.
And we had overrun only VECTOR 1 so VECTOR 1 is enqueued to workqueue.
Right after that we go back to the restart loop in do_softirq in order to
handle pending VECTOR 2 but we erase the local_softirqs_pending state. So
when the workqueue runs, it doesn't see anymore VECTOR 1 pending and we lose
it.

So I need to remove the above condition and make the vector work
unconditionally execute the vector callback.

Now I can go to sleep...


> +		struct softirq_action *sa = &softirq_vec[vec];
> +
> +		kstat_incr_softirqs_this_cpu(vec);
> +		trace_softirq_entry(vec);
> +		sa->action(sa);
> +		trace_softirq_exit(vec);
> +	}