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SubjectRe: Soft IRQ statistics under /proc/stat
> I've been observing some oddities in the statistics produced by mpstat 
> with respect to soft IRQs (for example, considerable soft IRQ time on
> processors sending UDP packets on dummy NICs).

Here's some data to support my claims.
The first experiment consists of sending UDP packets on a dummy network interface. No
interrupts are generated, so there should be no soft IRQs. Nevertheless, /proc/stat shows
that a considerable share of CPU time is taken by soft IRQs:

CPU %user %nice %sys %iowait %irq %soft %steal %guest %idle
0 4.52 0.00 67.84 0.00 0.00 27.64 0.00 0.00 0.00
0 4.00 0.00 70.00 0.00 0.00 26.00 0.00 0.00 0.00
0 4.98 0.00 68.16 0.00 0.00 26.87 0.00 0.00 0.00
0 4.02 0.00 69.85 0.00 0.00 26.13 0.00 0.00 0.00

In a second experiment, UDP packets are sent over a real NIC by a process pinned to CPU 0,
while the respective network interrupts are pinned to CPU 2. Here, you can see that CPU 0
is executing soft IRQs, despite the interrupt affinity:

CPU %user %nice %sys %iowait %irq %soft %steal %guest %idle
0 4.02 0.00 63.82 0.00 0.00 32.16 0.00 0.00 0.00
2 0.00 0.00 0.00 0.00 6.47 40.30 0.00 0.00 53.23
0 2.48 0.00 67.33 0.00 0.00 30.20 0.00 0.00 0.00
2 0.00 0.00 0.00 0.00 6.47 41.79 0.00 0.00 51.74

I have verified, from /proc/interrupts, that in both cases the number of interrupts per
second on CPU 0 is negligible.

Next, I modified the kernel code to raise a per-CPU flag at the beginning of
__do_softirq(), and clear it at the end. Using this flag, account_system_time() can
differentiate between a "true" soft IRQ, and code running under local_bh_disable(). The
results of the first experiment (dummy NIC) are as follows:

CPU %user %nice %sys %iowait %irq %soft %steal %guest %bh_dis %idle
0 4.50 0.00 71.00 0.00 0.00 0.00 0.00 0.00 24.50 0.00
0 4.00 0.00 67.00 0.00 0.00 0.00 0.00 0.00 29.00 0.00
0 3.98 0.00 69.15 0.00 0.00 0.00 0.00 0.00 26.87 0.00
0 3.50 0.00 69.00 0.00 0.00 0.00 0.00 0.00 27.50 0.00

where bh_dis refers to code executing with softirq_count() greater than 0, but with the
soft IRQ flag cleared. The results for the second experiment (real NIC):

CPU %user %nice %sys %iowait %irq %soft %steal %guest %bh_dis %idle
0 3.00 0.00 67.50 0.00 0.00 0.00 0.00 0.00 29.50 0.00
2 0.00 0.00 0.00 0.00 3.48 40.30 0.00 0.00 0.00 56.22
0 1.99 0.00 66.67 0.00 0.00 0.00 0.00 0.00 31.34 0.00
2 0.00 0.00 0.00 0.49 3.45 39.41 0.00 0.00 0.00 56.65

These results make much more sense.

Elad

P.S.,

For reference, here's the patch I applied to the kernel in order to differentiate between
"true" and "false" soft IRQs (I am not suggesting it as a permanent patch, it's just for
testing and verification purposes):

diff -u -r --exclude='*.o' --exclude='*.cmd' linux-2.6.26.5/fs/proc/proc_misc.c
linux-2.6.26.5-bh/fs/proc/proc_misc.c
--- linux-2.6.26.5/fs/proc/proc_misc.c 2008-09-08 13:40:20.000000000 -0400
+++ linux-2.6.26.5-bh/fs/proc/proc_misc.c 2008-09-11 19:39:52.000000000 -0400
@@ -478,6 +478,7 @@
unsigned long jif;
cputime64_t user, nice, system, idle, iowait, irq, softirq, steal;
cputime64_t guest;
+ cputime64_t bh_disabled;
u64 sum = 0;
struct timespec boottime;
unsigned int *per_irq_sum;
@@ -489,6 +490,7 @@
user = nice = system = idle = iowait =
irq = softirq = steal = cputime64_zero;
guest = cputime64_zero;
+ bh_disabled = cputime64_zero;
getboottime(&boottime);
jif = boottime.tv_sec;

@@ -504,6 +506,8 @@
softirq = cputime64_add(softirq, kstat_cpu(i).cpustat.softirq);
steal = cputime64_add(steal, kstat_cpu(i).cpustat.steal);
guest = cputime64_add(guest, kstat_cpu(i).cpustat.guest);
+ bh_disabled = cputime64_add(bh_disabled,
+ kstat_cpu(i).cpustat.bh_disabled);
for (j = 0; j < NR_IRQS; j++) {
unsigned int temp = kstat_cpu(i).irqs[j];
sum += temp;
@@ -511,7 +515,7 @@
}
}

- seq_printf(p, "cpu %llu %llu %llu %llu %llu %llu %llu %llu %llu\n",
+ seq_printf(p, "cpu %llu %llu %llu %llu %llu %llu %llu %llu %llu %llu\n",
(unsigned long long)cputime64_to_clock_t(user),
(unsigned long long)cputime64_to_clock_t(nice),
(unsigned long long)cputime64_to_clock_t(system),
@@ -520,7 +524,8 @@
(unsigned long long)cputime64_to_clock_t(irq),
(unsigned long long)cputime64_to_clock_t(softirq),
(unsigned long long)cputime64_to_clock_t(steal),
- (unsigned long long)cputime64_to_clock_t(guest));
+ (unsigned long long)cputime64_to_clock_t(guest),
+ (unsigned long long)cputime64_to_clock_t(bh_disabled));
for_each_online_cpu(i) {

/* Copy values here to work around gcc-2.95.3, gcc-2.96 */
@@ -533,8 +538,9 @@
softirq = kstat_cpu(i).cpustat.softirq;
steal = kstat_cpu(i).cpustat.steal;
guest = kstat_cpu(i).cpustat.guest;
+ bh_disabled = kstat_cpu(i).cpustat.bh_disabled;
seq_printf(p,
- "cpu%d %llu %llu %llu %llu %llu %llu %llu %llu %llu\n",
+ "cpu%d %llu %llu %llu %llu %llu %llu %llu %llu %llu %llu\n",
i,
(unsigned long long)cputime64_to_clock_t(user),
(unsigned long long)cputime64_to_clock_t(nice),
@@ -544,7 +550,8 @@
(unsigned long long)cputime64_to_clock_t(irq),
(unsigned long long)cputime64_to_clock_t(softirq),
(unsigned long long)cputime64_to_clock_t(steal),
- (unsigned long long)cputime64_to_clock_t(guest));
+ (unsigned long long)cputime64_to_clock_t(guest),
+ (unsigned long long)cputime64_to_clock_t(bh_disabled));
}
seq_printf(p, "intr %llu", (unsigned long long)sum);

diff -u -r --exclude='*.o' --exclude='*.cmd' linux-2.6.26.5/include/linux/kernel_stat.h
linux-2.6.26.5-bh/include/linux/kernel_stat.h
--- linux-2.6.26.5/include/linux/kernel_stat.h 2008-09-08 13:40:20.000000000 -0400
+++ linux-2.6.26.5-bh/include/linux/kernel_stat.h 2008-09-11 19:32:00.000000000 -0400
@@ -24,6 +24,7 @@
cputime64_t iowait;
cputime64_t steal;
cputime64_t guest;
+ cputime64_t bh_disabled;
};

struct kernel_stat {
@@ -32,6 +33,7 @@
};

DECLARE_PER_CPU(struct kernel_stat, kstat);
+DECLARE_PER_CPU(int, in_softirq);

#define kstat_cpu(cpu) per_cpu(kstat, cpu)
/* Must have preemption disabled for this to be meaningful. */
diff -u -r --exclude='*.o' --exclude='*.cmd' linux-2.6.26.5/kernel/sched.c
linux-2.6.26.5-bh/kernel/sched.c
--- linux-2.6.26.5/kernel/sched.c 2008-09-08 13:40:20.000000000 -0400
+++ linux-2.6.26.5-bh/kernel/sched.c 2008-09-11 20:56:31.000000000 -0400
@@ -3950,8 +3950,12 @@
tmp = cputime_to_cputime64(cputime);
if (hardirq_count() - hardirq_offset)
cpustat->irq = cputime64_add(cpustat->irq, tmp);
- else if (softirq_count())
- cpustat->softirq = cputime64_add(cpustat->softirq, tmp);
+ else if (softirq_count()) {
+ if (__get_cpu_var(in_softirq))
+ cpustat->softirq = cputime64_add(cpustat->softirq, tmp);
+ else
+ cpustat->bh_disabled = cputime64_add(cpustat->bh_disabled, tmp);
+ }
else if (p != rq->idle)
cpustat->system = cputime64_add(cpustat->system, tmp);
else if (atomic_read(&rq->nr_iowait) > 0)
diff -u -r --exclude='*.o' --exclude='*.cmd' linux-2.6.26.5/kernel/softirq.c
linux-2.6.26.5-bh/kernel/softirq.c
--- linux-2.6.26.5/kernel/softirq.c 2008-09-08 13:40:20.000000000 -0400
+++ linux-2.6.26.5-bh/kernel/softirq.c 2008-09-11 19:34:33.000000000 -0400
@@ -49,6 +49,7 @@
static struct softirq_action softirq_vec[32] __cacheline_aligned_in_smp;

static DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
+DEFINE_PER_CPU(int, in_softirq);

/*
* we cannot loop indefinitely here to avoid userspace starvation,
@@ -214,6 +215,8 @@
int max_restart = MAX_SOFTIRQ_RESTART;
int cpu;

+ __get_cpu_var(in_softirq) = 1;
+
pending = local_softirq_pending();
account_system_vtime(current);

@@ -251,6 +254,8 @@

account_system_vtime(current);
_local_bh_enable();
+
+ __get_cpu_var(in_softirq) = 0;
}

#ifndef __ARCH_HAS_DO_SOFTIRQ

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