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Date: Tue, 7 Feb 2023 09:03:55 +0100
From: Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
To: Yu Liao <liaoyu15@...wei.com>, fweisbec@...il.com,
tglx@...utronix.de, mingo@...nel.org
Cc: liwei391@...wei.com, adobriyan@...il.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us()
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@...wei.com>
>> ---
>> 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@...vin-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@...vin-IdeaPad-3-15ITL6:~/linux/kernel/linux_torvalds/tools/testing/selftests/proc$ uname -rms
> Linux 6.2.0-rc7-mglru-kmemlk x86_64
> marvin@...vin-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@....unizg.hr>
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
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