Hi Sebastian,
On 09/09/2016 07:46 AM, Grygorii Strashko wrote:
> On 09/08/2016 07:00 PM, Sebastian Andrzej Siewior wrote:
>> On 2016-08-19 17:29:16 [+0300], Grygorii Strashko wrote:
>>> I've collected trace before first occurrence of "NOHZ: local_softirq_pending 80"
>>>
>
>>
>>> irq/354-4848400-85 [000] 90.642393: softirq_exit: vec=3 [action=NET_RX]
>>> irq/354-4848400-85 [000] 90.642419: sched_switch: irq/354-4848400:85 [49] S ==> rcuc/0:11 [98]
>>
>> We don't serve TIMER & SCHED because those two are pushed to the
>> ksoftirq thread(s). So we keep mostly doing NET_RX and now we switch to
>> the next best thing which is RCU.
>>
>>> rcuc/0-11 [000] 90.642430: irq_handler_entry: irq=354 name=48484000.ethernet
>> but not for long.
>>
>>> rcuc/0-11 [000] 90.642432: irq_handler_exit: irq=354 ret=handled
>>> rcuc/0-11 [000] 90.642435: sched_waking: comm=irq/354-4848400 pid=85 prio=49 target_cpu=000
>>> rcuc/0-11 [000] 90.642442: sched_migrate_task: comm=irq/354-4848400 pid=85 prio=49 orig_cpu=0 dest_cpu=1
>>> rcuc/0-11 [000] 90.642453: sched_wakeup: irq/354-4848400:85 [49] success=1 CPU:001
>>> iperf-1284 [001] 90.642462: sched_stat_runtime: comm=iperf pid=1284 runtime=113053 [ns] vruntime=2106997666 [ns]
>>> rcuc/0-11 [000] 90.642464: irq_handler_entry: irq=355 name=48484000.ethernet
>>> rcuc/0-11 [000] 90.642466: irq_handler_exit: irq=355 ret=handled
>>> rcuc/0-11 [000] 90.642469: sched_waking: comm=irq/355-4848400 pid=86 prio=49 target_cpu=001
>>> iperf-1284 [001] 90.642473: sched_switch: iperf:1284 [120] R ==> irq/354-4848400:85 [49]
>>> irq/354-4848400-85 [001] 90.642481: softirq_raise: vec=3 [action=NET_RX]
>>> rcuc/0-11 [000] 90.642483: sched_wakeup: irq/355-4848400:86 [49] success=1 CPU:001
>>> irq/354-4848400-85 [001] 90.642493: softirq_entry: vec=3 [action=NET_RX]
>>> rcuc/0-11 [000] 90.642497: sched_migrate_task: comm=irq/355-4848400 pid=86 prio=49 orig_cpu=1 dest_cpu=0
>> ach that IRQ thread no pinned. Good. We migrate.
>>
>
> It looks like scheduler playing ping-pong between CPUs with threaded irqs irq/354-355.
> And seems this might be the case - if I pin both threaded IRQ handlers to CPU0
> I can see better latency and netperf improvement
> cyclictest -m -Sp98 -q -D4m
> T: 0 ( 1318) P:98 I:1000 C: 240000 Min: 9 Act: 14 Avg: 15 Max: 42
> T: 1 ( 1319) P:98 I:1500 C: 159909 Min: 9 Act: 14 Avg: 16 Max: 39
>
> if I arrange hwirqs and pin pin both threaded IRQ handlers on CPU1
> I can observe more less similar results as with this patch.
>
>
>>> rcuc/0-11 [000] 90.642515: irq_handler_entry: irq=354 name=48484000.ethernet
>>> rcuc/0-11 [000] 90.642516: irq_handler_exit: irq=354 ret=handled
>
>>
>> As you see ksoftirqd left the CPU with a D so I would assume it is
>> blocked on a lock and waits.
>> NET_RX is in progress but scheduled out due to RCUC which is also
>> scheduled out.
>>
>> I assume we got to softirq because nothing else can run. It will see
>> that NET_RX is pending and tries it but blocks on the lock
>> (lock_softirq()). It schedules out. Nothing left -> idle.
>>
>> The idle code checks to see if a softirq is pending and in fact there is
>> SCHED on the list and ksoftirq was about to handle it but due to
>> ordering complication (NET_RX before SCHED) it can't. And we have the
>> warning.
>>
>> This
>>
>> --- a/kernel/softirq.c
>> +++ b/kernel/softirq.c
>> @@ -105,6 +105,7 @@ void softirq_check_pending_idle(void)
>> {
>> static int rate_limit;
>> struct softirq_runner *sr = this_cpu_ptr(&softirq_runners);
>> + struct task_struct *ksoft_tsk = __this_cpu_read(ksoftirqd);
>> u32 warnpending;
>> int i;
>>
>> @@ -112,7 +113,7 @@ void softirq_check_pending_idle(void)
>> return;
>>
>> warnpending = local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK;
>> - for (i = 0; i < NR_SOFTIRQS; i++) {
>> + for (i = 0; (i < NR_SOFTIRQS) && warnpending; i++) {
>> struct task_struct *tsk = sr->runner[i];
>>
>> /*
>> @@ -132,6 +133,15 @@ void softirq_check_pending_idle(void)
>> }
>> }
>>
>> + if (warnpending && ksoft_tsk) {
>> + raw_spin_lock(&ksoft_tsk->pi_lock);
>> + if (ksoft_tsk->pi_blocked_on || ksoft_tsk->state == TASK_RUNNING) {
>> + /* Clear all bits pending in that task */
>> + warnpending &= ~(ksoft_tsk->softirqs_raised);
>> + }
>> + raw_spin_unlock(&ksoft_tsk->pi_lock);
>> + }
>> +
>> if (warnpending) {
>> printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
>> warnpending);
>>
>>
>> should avoid the warning if the softirq thread is blocked.
>
>
> with this change i do not see "NOHZ: local_softirq_pending 80" any more
> Tested-by: Grygorii Strashko <grygorii.strashko@xxxxxx>
>
Above change is not present in K4.9 and I can still see NOHZ messages if
i run iperf on am57xx-evm, I'd like to know if you have any plans regarding this?
Copy pasting here your comments from another e-mail
--
okay. So I need to think what I do about this. Either this or trying to
run the "higher" softirq first but this could break things.
Thanks for the confirmation.
((c) Sebastian Andrzej Siewior)
--
root@am57xx-evm:~# iperf -c 192.168.1.1 -w128K -d -i5 -t120 & cyclictest -n -m -Sp97 -q -D2m
[1] 1078
# /dev/cpu_dma_latency set to 0us
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 256 KByte (WARNING: requested 128 KByte)
------------------------------------------------------------
------------------------------------------------------------
Client connecting to 192.168.1.1, TCP port 5001
TCP window size: 256 KByte (WARNING: requested 128 KByte)
------------------------------------------------------------
[ 5] local 192.168.1.2 port 39346 connected with 192.168.1.1 port 5001
[ 4] local 192.168.1.2 port 5001 connected with 192.168.1.1 port 50594
[ 90.829952] NOHZ: local_softirq_pending 80
[ 90.830001] NOHZ: local_softirq_pending 80
[ 91.529926] NOHZ: local_softirq_pending 80
[ 93.299956] NOHZ: local_softirq_pending 80
[ 93.680078] NOHZ: local_softirq_pending 80
[ 93.680128] NOHZ: local_softirq_pending 80
[ ID] Interval Transfer Bandwidth
[ 5] 0.0- 5.0 sec 73.2 MBytes 123 Mbits/sec
[ 4] 0.0- 5.0 sec 47.5 MBytes 79.7 Mbits/sec
[ 94.719951] NOHZ: local_softirq_pending 80
[ 96.439923] NOHZ: local_softirq_pending 80
[ 96.569922] NOHZ: local_softirq_pending 80
[ 96.569951] NOHZ: local_softirq_pending 80
[ 5] 5.0-10.0 sec 73.5 MBytes 123 Mbits/sec
PS: if i set net irqs to run on cpu0 (or CPU1) only it improve net throughput and
eliminates this NOHZ messages.
--
regards,
-grygorii
--
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