Re: [RFC PATCH 1/2] time: Introduce one suspend clocksource to compensate the suspend time

[Baolin Wang]

Hi Thomas,

On 16 July 2018 at 16:28, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Thu, 12 Jul 2018, Baolin Wang wrote:
>> On some hardware with multiple clocksources, we have course grained
>> clocksources that support the CLOCK_SOURCE_SUSPEND_NONSTOP flag, but
>> which are less ideal for timekeeping then other clocksources which
>> halt in suspend.
>>
>> Currently, the timekeeping core only supports timing suspend using
>> CLOCK_SOURCE_SUSPEND_NONSTOP clocksources if that clocksource is the
>> current clocksource for timekeeping.
>>
>> As a result, some architectures try to implement read_persisitent_clock64()
>> using those non-stop clocksources, but isn't really ideal. Thus this
>> patch provides logic to allow a registered SUSPEND_NONSTOP clocksource,
>
> Please see Documentation/process/submitting-patches.rst and search for
> 'This patch...'

OK, I will try to improve the commit message.

>
>> +/**
>> + * clocksource_suspend_select - Select the best clocksource for suspend timing
>> + * @fallback:        if select a fallback clocksource
>> + */
>> +static void clocksource_suspend_select(bool fallback)
>> +{
>> +     struct clocksource *cs, *old_suspend;
>> +
>> +     old_suspend = suspend_clocksource;
>> +     if (fallback)
>> +             suspend_clocksource = NULL;
>> +
>> +     list_for_each_entry(cs, &clocksource_list, list) {
>> +             /* Skip current if we were requested for a fallback. */
>> +             if (fallback && cs == old_suspend)
>> +                     continue;
>> +
>> +             __clocksource_suspend_select(cs);
>> +     }
>> +
>> +     /* If we failed to find a fallback restore the old one. */
>> +     if (!suspend_clocksource)
>> +             suspend_clocksource = old_suspend;
>
> That's for the case where something tries to remove a clocksource, right?

Yes.

> The logic here looks odd as the calling code for the fallback case has to
> check whether the clocksource which is about to be removed is the suspend
> clocksource. Why not just returning -EBUSY/0 for the fallback case?
>
> The other question is whether this should be enforced. We might as well
> decide to just let the clocksource go and have no suspend clocksource.

OK.

>
>> +/**
>> + * clocksource_start_suspend_timing - Start measuring the suspend timing
>> + * @cs:                      current clocksource from timekeeping
>> + * @start_cycles:    current cycles from timekeeping
>> + *
>> + * This function will save the start cycle values of suspend timer to calculate
>> + * the suspend time when resuming system.
>> + *
>> + * This function is called late in the suspend process from timekeeping_suspend(),
>> + * that means processes are freezed, non-boot cpus and interrupts are disabled
>> + * now. It is therefore possible to start the suspend timer without taking the
>> + * clocksource mutex.
>> + */
>> +void clocksource_start_suspend_timing(struct clocksource *cs, u64 start_cycles)
>> +{
>> +     if (!suspend_clocksource)
>> +             return;
>> +
>> +     /*
>> +      * If current clocksource is the suspend timer, we should use the
>> +      * tkr_mono.cycle_last value as suspend_start to avoid same reading
>> +      * from suspend timer.
>> +      */
>> +     if (clocksource_is_suspend(cs)) {
>> +             suspend_start = start_cycles;
>> +             return;
>> +     }
>> +
>> +     if (suspend_clocksource->enable &&
>> +         WARN_ON_ONCE(suspend_clocksource->enable(suspend_clocksource))) {
>> +             pr_warn_once("Failed to enable the non-suspend-able clocksource.\n");
>> +             return;
>
> This is horrible to read and the WARN is really not helpful because
> the bracktrace is already known.

Sure, will remove the WARN_ON_ONCE().

>
>> @@ -779,6 +910,16 @@ int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
>>  {
>>       unsigned long flags;
>>
>> +     /*
>> +      * The nonstop clocksource can be selected as the suspend clocksource to
>> +      * calculate the suspend time, so it should not supply suspend/resume
>> +      * interfaces to suspend the nonstop clocksource when system suspends.
>> +      */
>> +     if ((cs->flags & CLOCK_SOURCE_SUSPEND_NONSTOP) &&
>> +         (cs->suspend || cs->resume))
>> +             pr_warn("Nonstop clocksource %s should not supply suspend/resume interfaces\n",
>> +                     cs->name);
>
> Lacks braces.
>
> See https://lkml.kernel.org/r/alpine.DEB.2.20.1701171956290.3645@nanos

OK. I will add braces in next version. Thanks for your comments.

>
> Othar that the few nits this looks good. Nice work!
>
> Thanks,
>
>         tglx



-- 
Baolin Wang
Best Regards