Re: Pin-pointing the root of unusual application latencies

[John Sigler <linux.kernel@free.fr>]

Ingo Molnar wrote:

> John Sigler wrote:
>
> > Ingo Molnar wrote:
> >
> > > does your test-app have higher priority than softirq--4 ?
> >
> > PID 4 is [softirq-timer/0] and has priority 50 in SCHED_FIFO. My 
> > process has priority 80 in SCHED_RR. It is waiting for IRQ10.
> >
> > My user-space app has higher priority than everything except PID 2 
> > which is [posix_cpu_timer]
>
> well what priority does the IRQ 10 kernel thread have? It should be prio 
> 80 too if it's in your critical path.

Is there some form of priority inheritance? Does the IRQ handler get a 
priority boost if a high priority task is waiting for it?

I didn't dare mess with the default kernel thread priorities.

# ps -eo pid,class,rtprio,ni,pri,stat,comm
  PID CLS RTPRIO  NI PRI STAT COMMAND
    1 TS       -   0  24 Ss   init
    2 FF      99   - 139 S    posix_cpu_timer
    3 FF      50   -  90 S    softirq-high/0
    4 FF      50   -  90 S    softirq-timer/0
    5 FF      50   -  90 S    softirq-net-tx/
    6 FF      50   -  90 S    softirq-net-rx/
    7 FF      50   -  90 S    softirq-block/0
    8 FF      50   -  90 S    softirq-tasklet
    9 FF      50   -  90 S    softirq-sched/0
   10 FF      50   -  90 S    softirq-hrtimer
   11 FF      50   -  90 S    softirq-rcu/0
   12 TS       - -10  34 S<   desched/0
   13 FF       1   -  41 S<   events/0
   14 TS       -  -5  19 S<   khelper
   15 TS       -  -5  19 S<   kthread
   34 TS       -  -5  29 S<   kblockd/0
   35 TS       -  -5  19 S<   kacpid
   36 FF      50   -  90 S<   IRQ-7
   82 TS       -  -5  29 S<   kseriod
  101 TS       -   0  16 S    pdflush
  102 TS       -   0  24 S    pdflush
  103 TS       -  -5  21 S<   kswapd0
  104 TS       -  -5  29 S<   flush_filesd/0
  105 TS       -  -5  21 S<   aio/0
  704 FF      50   -  90 S<   IRQ-14
  720 FF      50   -  90 S<   IRQ-12
  721 FF      50   -  90 S<   IRQ-1
  848 FF      50   -  90 S<   IRQ-11
  913 FF      50   -  90 S<   IRQ-10
  922 TS       -   0  21 Ss   sshd
  925 TS       -   0  24 Ss+  agetty
  926 TS       -   0  21 Ss+  agetty
  931 TS       -   0  24 Rs   sshd
  933 TS       -   0  24 Rs   bash

# cat /proc/interrupts
           CPU0
  0:         37    XT-PIC-XT        timer
  1:          2    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  7:          0    XT-PIC-XT        acpi
 10:        175    XT-PIC-XT        eth2, Dta1xx
 11:       1129    XT-PIC-XT        eth0
 12:          4    XT-PIC-XT        eth1
 14:      21482    XT-PIC-XT        ide0
NMI:          0
LOC:     161632
ERR:          0
MIS:          0

IRQ 10 is shared between a NIC and an I/O board.

For eth2, the kernel said:
ACPI: PCI Interrupt 0000:00:0a.0[A] -> Link [LNKC]
  -> GSI 10 (level, low) -> IRQ 10

For Dta1xx, the kernel said:
ACPI: PCI Interrupt 0000:02:0e.0[A] -> Link [LNKC]
  -> GSI 10 (level, low) -> IRQ 10

Is it possible to avoid the two boards sharing IRQ 10?

Regards.
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