linux-kernel - Re: [PATCH v2] time: Fix sleeptime injection for non-stop clocksource & persistent clock

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Date:   Mon, 25 Jun 2018 20:08:13 +0530
From:   Mukesh Ojha <mojha@...eaurora.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     john.stultz@...aro.org, linux-kernel@...r.kernel.org,
        neeraju@...eaurora.org, gkohli@...eaurora.org,
        cpandya@...eaurora.org
Subject: Re: [PATCH v2] time: Fix sleeptime injection for non-stop clocksource
 & persistent clock

Hi Thomas,

Thanks you very much for your time and reply.


On 6/23/2018 2:57 AM, Thomas Gleixner wrote:
> On Wed, 30 May 2018, Mukesh Ojha wrote:
>> Currently, for both non-stop clocksource and persistent clock
>> there is a corner case, when a driver failed to go suspend mode.
>> rtc_resume() injects the sleeptime as timekeeping_rtc_skipresume()
>> returned 'false'(sleeptime_injected=false) due to which we can
>> see mismatch in timestamps between system clock and other timers.
>>
>> Fix this by updating sleeptime_injected=true for both non-stop
>> clocksource and persistent clock.
>>
>> Success case:
>> ------------
>>                                           {sleeptime_injected=true}
>> rtc_suspend() => timekeeping_suspend() => timekeeping_resume() =>
>>    rtc_resume()
>>
>> Failure case:
>> ------------
>>               {failure in sleep path} {sleeptime_injected=false}
>> rtc_suspend()          =>            rtc_resume()
> I can see the problem.
>
>> Signed-off-by: Mukesh Ojha <mojha@...eaurora.org>
>> ---
>> Changes in v2:
>>   * Updated the commit text.
>>   * Removed extra variable and used the earlier static
>>     variable 'sleeptime_injected'.
>>
>>   kernel/time/timekeeping.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
>> index 49cbcee..2754c1b 100644
>> --- a/kernel/time/timekeeping.c
>> +++ b/kernel/time/timekeeping.c
>> @@ -1610,6 +1610,17 @@ static void __timekeeping_inject_sleeptime(struct timekeeper *tk,
>>    */
>>   bool timekeeping_rtc_skipresume(void)
>>   {
>> +	struct timekeeper *tk = &tk_core.timekeeper;
>> +	/*
>> +	 * This is to ensure that we don't end up injecting
>> +	 * the sleeptime via rtc_resume() for non-stop clocksource
>> +	 * when we fail to sleep.
>> +	 */
>> +	if (!sleeptime_injected)
>> +		sleeptime_injected = ((tk->tkr_mono.clock->flags &
>> +			CLOCK_SOURCE_SUSPEND_NONSTOP) ||
>> +			(persistent_clock_exists)) ? true : false;
> But this is really a horrible hack. The right thing to do is to keep track
> whether timekeeping_suspend() has been reached in the first place. There is
> a very simple way to do that. Uncompiled and completely untested patch
> below, but you get the idea.

Yeah, missed completely the fact that the issue can also come where only 
clocksource is RTC.
> Thanks,
>
> 	tglx
>
> 8<-------------------
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 4786df904c22..32ae9aea61c3 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1510,8 +1510,20 @@ void __weak read_boot_clock64(struct timespec64 *ts)
>   	ts->tv_nsec = 0;
>   }
>   
> -/* Flag for if timekeeping_resume() has injected sleeptime */
> -static bool sleeptime_injected;
> +/*
> + * Flag reflecting whether timekeeping_resume() has injected sleeptime.
> + *
> + * The flag starts of true and is only cleared when a suspend reaches
> + * timekeeping_suspend(), timekeeping_resume() sets it when the timekeeper
> + * clocksource is not stopping across suspend and has been used to update
> + * sleep time. If the timekeeper clocksource has stopped then the flag
> + * stays false and is used by the RTC resume code to decide whether sleep
> + * time must be injected and if so the flag gets set then.
> + *
> + * If a suspend fails before reaching timekeeping_resume() then the flag
> + * stays true and prevents erroneous sleeptime injection.
> + */
> +static bool sleeptime_injected = true;

This will prevent first sleep failure.

>   
>   /* Flag for if there is a persistent clock on this platform */
>   static bool persistent_clock_exists;
> @@ -1646,6 +1658,8 @@ void timekeeping_inject_sleeptime64(struct timespec64 *delta)
>   	raw_spin_lock_irqsave(&timekeeper_lock, flags);
>   	write_seqcount_begin(&tk_core.seq);
>   
> +	sleeptime_injected = true;

This will prevent further extra sleeptime injection if sleep fails 
(valid for RTC only).
Looks good!

> +
>   	timekeeping_forward_now(tk);
>   
>   	__timekeeping_inject_sleeptime(tk, delta);
> @@ -1671,7 +1685,6 @@ void timekeeping_resume(void)
>   	struct timespec64 ts_new, ts_delta;
>   	u64 cycle_now;
>   
> -	sleeptime_injected = false;
>   	read_persistent_clock64(&ts_new);
>   
>   	clockevents_resume();
> @@ -1743,6 +1756,8 @@ int timekeeping_suspend(void)
>   	if (timekeeping_suspend_time.tv_sec || timekeeping_suspend_time.tv_nsec)
>   		persistent_clock_exists = true;
>   
> +	sleeptime_injected = false;

I did not get the exact valid point of moving it from 
`timekeeping_suspend` to `timekeeping_resume`.
Although it will not have any side effect.

> +
>   	raw_spin_lock_irqsave(&timekeeper_lock, flags);
>   	write_seqcount_begin(&tk_core.seq);
>   	timekeeping_forward_now(tk);
>

Thanks for the change;  will check and update.

Cheers,
Mukesh