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Subject[RFC PATCH 1/4] sched: Track and export per task [hard|soft]irq time
Date
Currently, kernel does not have accounting mechanism for softirq and hardirq
times at the task level. There is irq time info in kstat_cpu which is
accumulated at the cpu level.

Without the task level information, the non irq run time of task(s) would
have to be guessed based on their exec time and CPU on which they were
running recently and assuming that the CPU irq time reported are spread
across all the tasks running there. And this guess can be widely off the mark.

Sample case, considering just the softirq:

If there are varied workloads running on a CPU, say a CPU bound task (loop)
and a network IO bound task (nc) along with the network softirq load,
there is no way for the administrator/user to know the non-irq runtime of each
of these tasks. Only information available is the total runtime for each of the
tasks and kstat_cpu softirq time for the CPU.

In this example, considering a 10 second sample, both loop and nc would have
total run time of ~5s. And kstat_cpu softirq on this cpu increase was
355 (~3.5s).

So, all the information the user gets is that both the tasks are running for
roughly the same amount of time and softirq is around 35%. As a result user
may conclude that irq overhead for both tasks are equal (1.75s) and the
non-irq runtime of both the tasks are around ~3.25s. Yes. There is another
factor of system and user time reported for these tasks that I am ignoring
as that is tough to correlate with irq time, in cases where the tasks have
significant non-irq system time.

This change adds tracking of softirq time on each task and task group.
This information is exported in /proc/<pid>/stat.

So, the user can get info like below, looking at exec_time and si_time in
appropriate /proc/<pid>/stat.
(Taken for a 10s interval)
task exec_time softirqtime (in USER_HZ)
(loop) (nc)
505 0 500 359
502 1 501 363
503 0 502 354
504 0 499 359
503 3 500 360

with this, user can get the non-irq run time as 5s and ~1.45s for
loop and nc, respectively.

Signed-off-by: Venkatesh Pallipadi <venki@google.com>
---
Documentation/filesystems/proc.txt | 5 ++++-
fs/proc/array.c | 17 +++++++++++++++--
include/linux/sched.h | 4 ++++
kernel/exit.c | 2 ++
kernel/sched.c | 9 ++++++---
5 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 9fb6cbe..295f1e3 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -255,7 +255,7 @@ Table 1-3: Contents of the statm files (as of 2.6.8-rc3)
..............................................................................


-Table 1-4: Contents of the stat files (as of 2.6.30-rc7)
+Table 1-4: Contents of the stat files (current kernel version)
..............................................................................
Field Content
pid process id
@@ -303,6 +303,9 @@ Table 1-4: Contents of the stat files (as of 2.6.30-rc7)
blkio_ticks time spent waiting for block IO
gtime guest time of the task in jiffies
cgtime guest time of the task children in jiffies
+ exec_time execution time as accounted by scheduler
+ si_time softirq time that was accounted to this task (or taskgroup)
+ hi_time hardirq time that was accounted to this task (or taskgroup)
..............................................................................

The /proc/PID/maps file containing the currently mapped memory regions and
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 885ab55..401a1c0 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -380,6 +380,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
unsigned long rsslim = 0;
char tcomm[sizeof(task->comm)];
unsigned long flags;
+ cputime64_t exec_time = 0, si_time = 0, hi_time = 0;

state = *get_task_state(task);
vsize = eip = esp = 0;
@@ -427,6 +428,9 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
min_flt += t->min_flt;
maj_flt += t->maj_flt;
gtime = cputime_add(gtime, t->gtime);
+ si_time = cputime64_add(si_time, t->si_time);
+ hi_time = cputime64_add(hi_time, t->hi_time);
+ exec_time += t->se.sum_exec_runtime;
t = next_thread(t);
} while (t != task);

@@ -434,6 +438,9 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
maj_flt += sig->maj_flt;
thread_group_times(task, &utime, &stime);
gtime = cputime_add(gtime, sig->gtime);
+ si_time = cputime64_add(si_time, sig->si_time);
+ hi_time = cputime64_add(hi_time, sig->hi_time);
+ exec_time += sig->sum_sched_runtime;
}

sid = task_session_nr_ns(task, ns);
@@ -448,6 +455,9 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
if (!whole) {
min_flt = task->min_flt;
maj_flt = task->maj_flt;
+ si_time = task->si_time;
+ hi_time = task->hi_time;
+ exec_time = task->se.sum_exec_runtime;
task_times(task, &utime, &stime);
gtime = task->gtime;
}
@@ -467,7 +477,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,

seq_printf(m, "%d (%s) %c %d %d %d %d %d %u %lu \
%lu %lu %lu %lu %lu %ld %ld %ld %ld %d 0 %llu %lu %ld %lu %lu %lu %lu %lu \
-%lu %lu %lu %lu %lu %lu %lu %lu %d %d %u %u %llu %lu %ld\n",
+%lu %lu %lu %lu %lu %lu %lu %lu %d %d %u %u %llu %lu %ld %llu %llu %llu\n",
pid_nr_ns(pid, ns),
tcomm,
state,
@@ -514,7 +524,10 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
task->policy,
(unsigned long long)delayacct_blkio_ticks(task),
cputime_to_clock_t(gtime),
- cputime_to_clock_t(cgtime));
+ cputime_to_clock_t(cgtime),
+ nsec_to_clock_t(exec_time),
+ (unsigned long long)cputime64_to_clock_t(si_time),
+ (unsigned long long)cputime64_to_clock_t(hi_time));
if (mm)
mmput(mm);
return 0;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b55e988..7bd82ab 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -593,6 +593,8 @@ struct signal_struct {
*/
cputime_t utime, stime, cutime, cstime;
cputime_t gtime;
+ cputime64_t si_time;
+ cputime64_t hi_time;
cputime_t cgtime;
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
cputime_t prev_utime, prev_stime;
@@ -1284,6 +1286,8 @@ struct task_struct {

cputime_t utime, stime, utimescaled, stimescaled;
cputime_t gtime;
+ cputime64_t si_time;
+ cputime64_t hi_time;
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
cputime_t prev_utime, prev_stime;
#endif
diff --git a/kernel/exit.c b/kernel/exit.c
index eabca5a..de988b3 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -115,6 +115,8 @@ static void __exit_signal(struct task_struct *tsk)
sig->utime = cputime_add(sig->utime, tsk->utime);
sig->stime = cputime_add(sig->stime, tsk->stime);
sig->gtime = cputime_add(sig->gtime, tsk->gtime);
+ sig->si_time = cputime64_add(sig->si_time, tsk->si_time);
+ sig->hi_time = cputime64_add(sig->hi_time, tsk->hi_time);
sig->min_flt += tsk->min_flt;
sig->maj_flt += tsk->maj_flt;
sig->nvcsw += tsk->nvcsw;
diff --git a/kernel/sched.c b/kernel/sched.c
index d9c0368..b410d5f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3253,12 +3253,15 @@ void account_system_time(struct task_struct *p, int hardirq_offset,

/* Add system time to cpustat. */
tmp = cputime_to_cputime64(cputime);
- if (hardirq_count() - hardirq_offset)
+ if (hardirq_count() - hardirq_offset) {
cpustat->irq = cputime64_add(cpustat->irq, tmp);
- else if (softirq_count())
+ p->hi_time = cputime64_add(p->hi_time, tmp);
+ } else if (softirq_count()) {
cpustat->softirq = cputime64_add(cpustat->softirq, tmp);
- else
+ p->si_time = cputime64_add(p->si_time, tmp);
+ } else {
cpustat->system = cputime64_add(cpustat->system, tmp);
+ }

cpuacct_update_stats(p, CPUACCT_STAT_SYSTEM, cputime);

--
1.7.0.1


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