Re: [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us()

From: Mirsad Goran Todorovac
Date: Tue Feb 07 2023 - 03:04:17 EST

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In reply to: Mirsad Goran Todorovac: "Re: [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us()"
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On 7.2.2023. 6:25, Mirsad Goran Todorovac wrote:

Hi,Yu Liao,

On 28. 01. 2023. 03:00, Yu Liao wrote:

selftest/proc/proc-uptime-001 complains:
Euler:/mnt # while true; do ./proc-uptime-001; done
proc-uptime-001: proc-uptime-001.c:41: main: Assertion `i1 >= i0' failed.
proc-uptime-001: proc-uptime-001.c:41: main: Assertion `i1 >= i0' failed.

/proc/uptime should be monotonically increasing. This occurs because
the data races between get_cpu_idle_time_us and
tick_nohz_stop_idle/tick_nohz_start_idle, for example:

CPU0 CPU1
get_cpu_idle_time_us

tick_nohz_idle_exit
now = ktime_get();
tick_nohz_stop_idle
update_ts_time_stats
delta = ktime_sub(now, ts->idle_entrytime);
ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta)
ts->idle_entrytime = now

now = ktime_get();
if (ts->idle_active && !nr_iowait_cpu(cpu)) {
ktime_t delta = ktime_sub(now, ts->idle_entrytime);
idle = ktime_add(ts->idle_sleeptime, delta);
//idle is slightly greater than the actual value
} else {
idle = ts->idle_sleeptime;
}
ts->idle_active = 0

After this, idle = idle_sleeptime(actual idle value) + now(CPU0) - now(CPU1).
If get_cpu_idle_time_us() is called immediately after ts->idle_active = 0,
only ts->idle_sleeptime is returned, which is smaller than the previously
read one, resulting in a non-monotonically increasing idle time. In
addition, there are other data race scenarios not listed here.

This patch introduce a lock to prevent data races.

Fixes: a130e8fbc7de ("fs/proc/uptime.c: Fix idle time reporting in /proc/uptime")
Signed-off-by: Yu Liao <liaoyu15@xxxxxxxxxx>
---
kernel/time/tick-sched.c | 15 ++++++++++++++-
kernel/time/tick-sched.h | 1 +
2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b0e3c9205946..ad7d47098a58 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -36,7 +36,9 @@
/*
* Per-CPU nohz control structure
*/
-static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
+static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched) = {
+ .idle_time_lock = __SPIN_LOCK_UNLOCKED(tick_cpu_sched.idle_time_lock),
+};
struct tick_sched *tick_get_tick_sched(int cpu)
{
@@ -661,16 +663,24 @@ update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, u64 *last_upda
static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&ts->idle_time_lock, flags);
update_ts_time_stats(smp_processor_id(), ts, now, NULL);
ts->idle_active = 0;
+ spin_unlock_irqrestore(&ts->idle_time_lock, flags);
sched_clock_idle_wakeup_event();
}
static void tick_nohz_start_idle(struct tick_sched *ts)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&ts->idle_time_lock, flags);
ts->idle_entrytime = ktime_get();
ts->idle_active = 1;
+ spin_unlock_irqrestore(&ts->idle_time_lock, flags);
sched_clock_idle_sleep_event();
}
@@ -691,12 +701,14 @@ static void tick_nohz_start_idle(struct tick_sched *ts)
u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+ unsigned long flags;
ktime_t now, idle;
if (!tick_nohz_active)
return -1;
now = ktime_get();
+ spin_lock_irqsave(&ts->idle_time_lock, flags);
if (last_update_time) {
update_ts_time_stats(cpu, ts, now, last_update_time);
idle = ts->idle_sleeptime;
@@ -709,6 +721,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
idle = ts->idle_sleeptime;
}
}
+ spin_unlock_irqrestore(&ts->idle_time_lock, flags);
return ktime_to_us(idle);
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index 504649513399..a64d4781e7af 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -81,6 +81,7 @@ struct tick_sched {
atomic_t tick_dep_mask;
unsigned long last_tick_jiffies;
unsigned int stalled_jiffies;
+ spinlock_t idle_time_lock;
};
extern struct tick_sched *tick_get_tick_sched(int cpu);

Thank you for the patch RFC.

I can confirm that v6.2-rc7 still exhibits the issue:
marvin@marvin-IdeaPad-3-15ITL6:~/linux/kernel/linux_torvalds/tools/testing/selftests/proc$ while true; do ./proc-uptime-001; done
proc-uptime-001: proc-uptime-001.c:39: main: Assertion `i1 >= i0' failed.
Aborted (core dumped)
proc-uptime-001: proc-uptime-001.c:39: main: Assertion `i1 >= i0' failed.
Aborted (core dumped)
proc-uptime-001: proc-uptime-001.c:39: main: Assertion `i1 >= i0' failed.
Aborted (core dumped)
^C
marvin@marvin-IdeaPad-3-15ITL6:~/linux/kernel/linux_torvalds/tools/testing/selftests/proc$ uname -rms
Linux 6.2.0-rc7-mglru-kmemlk x86_64
marvin@marvin-IdeaPad-3-15ITL6:~/linux/kernel/linux_torvalds/tools/testing/selftests/proc$

Your patch, however, apparently fixes the problem.

Rebooting with the patched kernel eliminates the Assertion above, on the same hardware platform.

All the programs in the tools/testing/seltest/proc now appear to pass the test:

make[2]: Entering directory '/home/marvin/linux/kernel/linux_torvalds/tools/testing/selftests/proc'
TAP version 13
1..21
# selftests: proc: fd-001-lookup
ok 1 selftests: proc: fd-001-lookup
# selftests: proc: fd-002-posix-eq
ok 2 selftests: proc: fd-002-posix-eq
# selftests: proc: fd-003-kthread
ok 3 selftests: proc: fd-003-kthread
# selftests: proc: proc-loadavg-001
ok 4 selftests: proc: proc-loadavg-001
# selftests: proc: proc-empty-vm
ok 5 selftests: proc: proc-empty-vm
# selftests: proc: proc-pid-vm
ok 6 selftests: proc: proc-pid-vm
# selftests: proc: proc-self-map-files-001
ok 7 selftests: proc: proc-self-map-files-001
# selftests: proc: proc-self-map-files-002
ok 8 selftests: proc: proc-self-map-files-002
# selftests: proc: proc-self-syscall
ok 9 selftests: proc: proc-self-syscall
# selftests: proc: proc-self-wchan
ok 10 selftests: proc: proc-self-wchan
# selftests: proc: proc-subset-pid
ok 11 selftests: proc: proc-subset-pid
# selftests: proc: proc-tid0
ok 12 selftests: proc: proc-tid0
# selftests: proc: proc-uptime-001
ok 13 selftests: proc: proc-uptime-001
# selftests: proc: proc-uptime-002
ok 14 selftests: proc: proc-uptime-002
# selftests: proc: read
ok 15 selftests: proc: read
# selftests: proc: self
ok 16 selftests: proc: self
# selftests: proc: setns-dcache
ok 17 selftests: proc: setns-dcache
# selftests: proc: setns-sysvipc
ok 18 selftests: proc: setns-sysvipc
# selftests: proc: thread-self
ok 19 selftests: proc: thread-self
# selftests: proc: proc-multiple-procfs
ok 20 selftests: proc: proc-multiple-procfs
# selftests: proc: proc-fsconfig-hidepid
ok 21 selftests: proc: proc-fsconfig-hidepid
make[2]: Leaving directory '/home/marvin/linux/kernel/linux_torvalds/tools/testing/selftests/proc'

What was applied was:

# git diff
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b0e3c9205946..ad7d47098a58 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -36,7 +36,9 @@
/*
* Per-CPU nohz control structure
*/
-static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
+static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched) = {
+ .idle_time_lock = __SPIN_LOCK_UNLOCKED(tick_cpu_sched.idle_time_lock),
+};
struct tick_sched *tick_get_tick_sched(int cpu)
{
@@ -661,16 +663,24 @@ update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, u64 *last_upda
static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&ts->idle_time_lock, flags);
update_ts_time_stats(smp_processor_id(), ts, now, NULL);
ts->idle_active = 0;
+ spin_unlock_irqrestore(&ts->idle_time_lock, flags);
sched_clock_idle_wakeup_event();
}
static void tick_nohz_start_idle(struct tick_sched *ts)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&ts->idle_time_lock, flags);
ts->idle_entrytime = ktime_get();
ts->idle_active = 1;
+ spin_unlock_irqrestore(&ts->idle_time_lock, flags);
sched_clock_idle_sleep_event();
}
@@ -691,12 +701,14 @@ static void tick_nohz_start_idle(struct tick_sched *ts)
u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+ unsigned long flags;
ktime_t now, idle;
if (!tick_nohz_active)
return -1;
now = ktime_get();
+ spin_lock_irqsave(&ts->idle_time_lock, flags);
if (last_update_time) {
update_ts_time_stats(cpu, ts, now, last_update_time);
idle = ts->idle_sleeptime;
@@ -709,6 +721,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
idle = ts->idle_sleeptime;
}
}
+ spin_unlock_irqrestore(&ts->idle_time_lock, flags);
return ktime_to_us(idle);
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index 504649513399..a64d4781e7af 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -81,6 +81,7 @@ struct tick_sched {
atomic_t tick_dep_mask;
unsigned long last_tick_jiffies;
unsigned int stalled_jiffies;
+ spinlock_t idle_time_lock;
};
extern struct tick_sched *tick_get_tick_sched(int cpu);

Feel free to add: Tested-by: Mirsad Goran Todorovac <mirsad.todorovac@xxxxxxxxxxxx>

P.S.

The fact that it works for me didn't take into account the kernel test robot result
I overlooked, which makes my test irrelevant.

Thanks,

--
Mirsad Todorovac
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb
Republic of Croatia, the European Union
--
Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu

Next message: Krzysztof Kozlowski: "Re: [PATCH v4 1/4] dt-bindings: net: Add WCN6855 Bluetooth"
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In reply to: Mirsad Goran Todorovac: "Re: [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us()"
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