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SubjectRe: [PATCH] PM / suspend: Count suspend-to-idle loop as sleep time
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On 09/14/2018 11:46 AM, Rafael J. Wysocki wrote:
> On Friday, September 14, 2018 10:28:44 AM CEST Mika Penttilä wrote:
>> Hi!
>>
>>
>> On 09/14/2018 09:59 AM, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>>
>>> There is a difference in behavior between suspend-to-idle and
>>> suspend-to-RAM in the timekeeping handling that leads to functional
>>> issues. Namely, every iteration of the loop in s2idle_loop()
>>> increases the monotinic clock somewhat, even if timekeeping_suspend()
>>> and timekeeping_resume() are invoked from s2idle_enter(), and if
>>> many of them are carried out in a row, the monotonic clock can grow
>>> significantly while the system is regarded as suspended, which
>>> doesn't happen during suspend-to-RAM and so it is unexpected and
>>> leads to confusion and misbehavior in user space (similar to what
>>> ensued when we tried to combine the boottime and monotonic clocks).
>>>
>>> To avoid that, count all iterations of the loop in s2idle_loop()
>>> as "sleep time" and adjust the clock for that on exit from
>>> suspend-to-idle.
>>>
>>> [That also covers systems on which timekeeping is not suspended
>>> by by s2idle_enter().]
>>>
>>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>> ---
>>>
>>> This is a replacement for https://patchwork.kernel.org/patch/10599209/
>>>
>>> I decided to count the entire loop in s2idle_loop() as "sleep time" as the
>>> patch is then simpler and it also covers systems where timekeeping is not
>>> suspended in the final step of suspend-to-idle.
>>>
>>> I dropped the "Fixes:" tag, because the monotonic clock delta problem
>>> has been present on the latter since the very introduction of "freeze"
>>> (as suspend-to-idle was referred to previously) and so this doesn't fix
>>> any particular later commits.
>>>
>>> ---
>>> kernel/power/suspend.c | 18 ++++++++++++++++++
>>> 1 file changed, 18 insertions(+)
>>>
>>> Index: linux-pm/kernel/power/suspend.c
>>> ===================================================================
>>> --- linux-pm.orig/kernel/power/suspend.c
>>> +++ linux-pm/kernel/power/suspend.c
>>> @@ -109,8 +109,12 @@ static void s2idle_enter(void)
>>>
>>> static void s2idle_loop(void)
>>> {
>>> + ktime_t start, delta;
>>> +
>>> pm_pr_dbg("suspend-to-idle\n");
>>>
>>> + start = ktime_get();
>>> +
>>> for (;;) {
>>> int error;
>>>
>>> @@ -150,6 +154,20 @@ static void s2idle_loop(void)
>>> pm_wakeup_clear(false);
>>> }
>>>
>>> + /*
>>> + * If the monotonic clock difference between the start of the loop and
>>> + * this point is too large, user space may get confused about whether or
>>> + * not the system has been suspended and tasks may get killed by
>>> + * watchdogs etc., so count the loop as "sleep time" to compensate for
>>> + * that.
>>> + */
>>> + delta = ktime_sub(ktime_get(), start);
>>> + if (ktime_to_ns(delta) > 0) {
>>> + struct timespec64 timespec64_delta = ktime_to_timespec64(delta);
>>> +
>>> + timekeeping_inject_sleeptime64(&timespec64_delta);
>>> + }
>>
>> But doesn't injecting sleep time here make monotonic clock too large by the amount of sleeptime?
>> tick_freeze() / tick_unfreeze() already injects the sleeptime (otherwise delta would be 0).
>
> No, it doesn't.
>
> The delta here is the extra time taken by the loop which hasn't been counted
> as sleep time yet.

I said incorrectly monotonic clock, but timekeeping_inject_sleeptime64() forwards the wall time, by the amount of delta.
Why wouldn't some other cpu update xtime when one cpu is in the loop? And if all cpus enter s2idle, tick_unfreeze()
injects sleeptime. My point is that this extra injection makes wall time wrong, no?

>
> Thanks,
> Rafael
>

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