Re: [RFC PATCH 0/2] net: threadable napi poll loop

From: Eric Dumazet
Date: Wed May 11 2016 - 13:55:29 EST

On Tue, 2016-05-10 at 14:53 -0700, Eric Dumazet wrote:
> On Tue, 2016-05-10 at 17:35 -0400, Rik van Riel wrote:
>
> > You might need another one of these in invoke_softirq()
> >
>
> Excellent.
>
> I gave it a quick try (without your suggestion), and host seems to
> survive a stress test.

Well, we instantly trigger rcu issues.

How to reproduce :

netserver &
for i in `seq 1 100`
do
netperf -H 127.0.0.1 -t TCP_RR -l 1000 &
done
# local hack to enable the new behavior
# without having to add a new sysctl, but hacking an existing one
echo 1001 >/proc/sys/net/core/netdev_max_backlog

<bang :>



[ 236.977511] INFO: rcu_sched self-detected stall on CPU
[ 236.977512] INFO: rcu_sched self-detected stall on CPU
[ 236.977515] INFO: rcu_sched self-detected stall on CPU
[ 236.977518] INFO: rcu_sched self-detected stall on CPU
[ 236.977519] INFO: rcu_sched self-detected stall on CPU
[ 236.977521] INFO: rcu_sched self-detected stall on CPU
[ 236.977522] INFO: rcu_sched self-detected stall on CPU
[ 236.977523] INFO: rcu_sched self-detected stall on CPU
[ 236.977525] INFO: rcu_sched self-detected stall on CPU
[ 236.977526] INFO: rcu_sched self-detected stall on CPU
[ 236.977527] INFO: rcu_sched self-detected stall on CPU
[ 236.977529] INFO: rcu_sched self-detected stall on CPU
[ 236.977530] INFO: rcu_sched self-detected stall on CPU
[ 236.977532] INFO: rcu_sched self-detected stall on CPU
[ 236.977532] 47-...: (1 GPs behind) idle=8d1/1/0 softirq=2500/2506 fqs=1
[ 236.977535] INFO: rcu_sched self-detected stall on CPU
[ 236.977536] INFO: rcu_sched self-detected stall on CPU
[ 236.977540] 36-...: (1 GPs behind) idle=d05/1/0 softirq=2637/2644 fqs=1
[ 236.977546]
[ 236.977546] 38-...: (1 GPs behind) idle=a5b/1/0 softirq=2612/2618 fqs=1
[ 236.977549] 0-...: (1 GPs behind) idle=c39/1/0 softirq=15315/15321 fqs=1
[ 236.977551] 24-...: (1 GPs behind) idle=ea3/1/0 softirq=2455/2461 fqs=1
[ 236.977554] 18-...: (20995 ticks this GP) idle=ef5/1/0 softirq=8530/8530 fqs=1
[ 236.977556] 39-...: (1 GPs behind) idle=f9d/1/0 softirq=2144/2150 fqs=1
[ 236.977558]
[ 236.977558] 22-...: (1 GPs behind) idle=5a7/1/0 softirq=10238/10244 fqs=1
[ 236.977561] 7-...: (1 GPs behind) idle=323/1/0 softirq=5279/5285 fqs=1
[ 236.977563] 31-...: (1 GPs behind) idle=47d/1/0 softirq=2526/2532 fqs=1
[ 236.977565] 33-...: (1 GPs behind) idle=175/1/0 softirq=2060/2066 fqs=1
[ 236.977568] 10-...: (1 GPs behind) idle=c3d/1/0 softirq=4864/4870 fqs=1
[ 236.977570] 34-...: (20995 ticks this GP) idle=dd5/1/0 softirq=2243/2243 fqs=1
[ 236.977574]
[ 236.977574] 37-...: (1 GPs behind) idle=aef/1/0 softirq=2660/2666 fqs=1
[ 236.977576] 13-...: (1 GPs behind) idle=a2b/1/0 softirq=9928/9934 fqs=1
[ 236.977578]
[ 236.977578]
[ 236.977579]
[ 236.977580]
[ 236.977582]
[ 236.977583]
[ 236.977583]
[ 236.977584]
[ 236.977584]
[ 236.977586]
[ 236.977587] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977588]
[ 236.977589]
[ 236.977595] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977603] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977607] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977609] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977610] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977612] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977614] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977616] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977618] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977619] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977620] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977622] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977626] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.977627] rcu_sched kthread starved for 20997 jiffies! g33049 c33048 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 236.978512] INFO: rcu_sched self-detected stall on CPU
[ 236.978512] INFO: rcu_sched self-detected stall on CPU
[ 236.978514] INFO: rcu_sched self-detected stall on CPU
[ 236.978516] INFO: rcu_sched self-detected stall on CPU
[ 236.978517] INFO: rcu_sched self-detected stall on CPU
[ 236.978518] INFO: rcu_sched self-detected stall on CPU
[ 236.978519] INFO: rcu_sched self-detected stall on CPU
[ 236.978520] INFO: rcu_sched self-detected stall on CPU
[ 236.978521] INFO: rcu_sched self-detected stall on CPU
[ 236.978522] INFO: rcu_sched self-detected stall on CPU
[ 236.978523] INFO: rcu_sched self-detected stall on CPU
[ 236.978524] INFO: rcu_sched self-detected stall on CPU
[ 236.978532] 45-...: (1 GPs behind) idle=8ed/1/0 softirq=3047/3053 fqs=1
[ 236.978534] 19-...: (20996 ticks this GP) idle=b5d/1/0 softirq=8157/8157 fqs=1
[ 236.978538] 17-...: (1 GPs behind) idle=5ad/1/0 softirq=7839/7845 fqs=1
[ 236.978539] 41-...: (1 GPs behind) idle=f4f/1/0 softirq=2345/2351 fqs=1
[ 236.978542] 6-...: (1 GPs behind) idle=a39/1/0 softirq=5492/5498 fqs=1
[ 236.978544] 30-...: (1 GPs behind) idle=c51/1/0 softirq=2499/2505 fqs=1
[ 236.978546] 5-...: (1 GPs behind) idle=917/1/0 softirq=5196/5202 fqs=1
[ 236.978548] 26-...: (20996 ticks this GP) idle=c61/1/0 softirq=2863/2863 fqs=1
[ 236.978550] 32-...: (1 GPs behind) idle=8db/1/0 softirq=2588/2594 fqs=1
[ 236.978552] 35-...: (1 GPs behind) idle=351/1/0 softirq=1869/1875 fqs=1
[ 236.978554] 8-...: (1 GPs behind) idle=221/1/0 softirq=5192/5198 fqs=1
[ 236.978556] 11-...: (1 GPs behind) idle=485/1/0 softirq=4480/4486 fqs=1
[ 236.978557]
[ 236.978558]
[ 236.978559]
[ 236.978560]
[ 236.978561]


Tentative proto / patch (not including Peter suggestions yet)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 17caf4b63342..be94e0241a70 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -56,6 +56,14 @@ EXPORT_SYMBOL(irq_stat);
static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;

DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
+DEFINE_PER_CPU(bool, ksoftirqd_scheduled);
+
+static inline bool ksoftirqd_running(void)
+{
+ extern int netdev_max_backlog; /* temp hack */
+
+ return (netdev_max_backlog & 1) && __this_cpu_read(ksoftirqd_scheduled);
+}

const char * const softirq_to_name[NR_SOFTIRQS] = {
"HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "BLOCK_IOPOLL",
@@ -73,8 +81,10 @@ static void wakeup_softirqd(void)
/* Interrupts are disabled: no need to stop preemption */
struct task_struct *tsk = __this_cpu_read(ksoftirqd);

- if (tsk && tsk->state != TASK_RUNNING)
+ if (tsk && tsk->state != TASK_RUNNING) {
+ __this_cpu_write(ksoftirqd_scheduled, true);
wake_up_process(tsk);
+ }
}

/*
@@ -313,7 +323,7 @@ asmlinkage __visible void do_softirq(void)

pending = local_softirq_pending();

- if (pending)
+ if (pending && !ksoftirqd_running())
do_softirq_own_stack();

local_irq_restore(flags);
@@ -340,6 +350,9 @@ void irq_enter(void)

static inline void invoke_softirq(void)
{
+ if (ksoftirqd_running())
+ return;
+
if (!force_irqthreads) {
#ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
/*
@@ -660,6 +673,8 @@ static void run_ksoftirqd(unsigned int cpu)
* in the task stack here.
*/
__do_softirq();
+ if (!local_softirq_pending())
+ __this_cpu_write(ksoftirqd_scheduled, false);
local_irq_enable();
cond_resched_rcu_qs();
return;