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Date: Fri, 12 Jan 2018 10:07:25 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Cc: Levin Alexander <alexander.levin@...izon.com>,
Peter Zijlstra <peterz@...radead.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Hannes Frederic Sowa <hannes@...essinduktion.org>,
"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
Wanpeng Li <wanpeng.li@...mail.com>,
Dmitry Safonov <dima@...sta.com>,
Thomas Gleixner <tglx@...utronix.de>,
Eric Dumazet <edumazet@...gle.com>,
Radu Rendec <rrendec@...sta.com>,
Ingo Molnar <mingo@...nel.org>,
Stanislaw Gruszka <sgruszka@...hat.com>,
Rik van Riel <riel@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH 2/2] softirq: Per vector thread deferment
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@...ux-foundation.org>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Dmitry Safonov <dima@...sta.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Miller <davem@...emloft.net>
> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Levin Alexander <alexander.levin@...izon.com>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Radu Rendec <rrendec@...sta.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Stanislaw Gruszka <sgruszka@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Wanpeng Li <wanpeng.li@...mail.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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