by Thomas Gleixner

[permalink] [raw]

Subject: Re: [PATCH v5 02/11] timekeeping: Add function to convert realtime to base clock

On Tue, Mar 19 2024 at 18:35, [email protected] wrote:
> +bool ktime_real_to_base_clock(ktime_t treal, enum clocksource_ids base_id, u64 *cycles)
> +{
> + struct timekeeper *tk = &tk_core.timekeeper;
> + unsigned int seq;
> + u64 delta;
> +
> + do {
> + seq = read_seqcount_begin(&tk_core.seq);
> + delta = (u64)treal - tk->tkr_mono.base_real;
> + if (delta > tk->tkr_mono.clock->max_idle_ns)
> + return false;

I don't think this cutoff is valid. There is no guarantee that this is
linear unless:

Treal[last timekeeper update] <= treal < Treal[next timekeeper update]

Look at the dance in get_device_system_crosststamp() and
adjust_historical_crosststamp() to see why.

> + *cycles = tk->tkr_mono.cycle_last + convert_ns_to_cs(delta);
> + if (!convert_cs_to_base(cycles, base_id))
> + return false;
> + } while (read_seqcount_retry(&tk_core.seq, seq));
> +
> + return true;
> +}
> +EXPORT_SYMBOL_GPL(ktime_real_to_base_clock);

Looking at the usage site:

> +static bool pps_generate_next_pulse(struct pps_tio *tio, ktime_t expires)
> +{
> + u64 art;
> +
> + if (!ktime_real_to_base_clock(expires, CSID_X86_ART, &art)) {
> + pps_tio_disable(tio);

I'm pretty sure this can happen when there is sufficient delay between
the check for (now - expires < SAFE_TIME_NS) and the delta computation
in ktime_real_to_base_clock() if there is a timerkeeper update
interleaving which brings tkr_mono.base_real in front of expires.

Is that intentional and correct?

If so, then it's inconsistent with the behaviour of the hrtimer
callback:

> + return false;
> + }
> +
> + pps_compv_write(tio, art - ART_HW_DELAY_CYCLES);
> + return true;
> +}
> +
> +static enum hrtimer_restart hrtimer_callback(struct hrtimer *timer)
> +{
> + struct pps_tio *tio = container_of(timer, struct pps_tio, timer);
> + ktime_t expires, now;
> +
> + guard(spinlock)(&tio->lock);
> +
> + expires = hrtimer_get_expires(timer);
> + now = ktime_get_real();
> +
> + if (now - expires < SAFE_TIME_NS) {
> + if (!pps_generate_next_pulse(tio, expires + SAFE_TIME_NS))
> + return HRTIMER_NORESTART;
> + }

This safe guard does not care about time being set. I'm not familiar
with the PPS logic, but is it expected that the pulse pattern will be
like this:

---|-----|-----|-----|----------------->
P P ^ P
|
clock_settime(CLOCK_REALTIME, now - 2 seconds)

Obviously the pulse gap will be as big as the time is set
backwards, which might be way more than 2 seconds.

---|-----|-----|-----|----------------->
P P ^ P P
|
clock_settime(CLOCK_REALTIME, now + 2 seconds)

I don't see anything in this code which cares about CLOCK_REALTIME being
set via clock_settime() or adjtimex().

Aside of that I have a question about how the TIO hardware treats this
case:

ktime_real_to_base_clock(expires, &art);

-> GAP which makes @art get into the past

pps_compv_write(tio, art - ART_HW_DELAY_CYCLES);

Will the hardware ignore that already expired value or just emit a pulse
immediately? In the latter case the pulse will be at a random point in
time, which does not sound correct.

Thanks,

tglx

2024-04-01 13:08:18

by D, Lakshmi Sowjanya

[permalink] [raw]

Subject: RE: [PATCH v5 02/11] timekeeping: Add function to convert realtime to base clock

> -----Original Message-----
> From: Thomas Gleixner <[email protected]>
> Sent: Thursday, March 21, 2024 8:23 PM
> To: D, Lakshmi Sowjanya <[email protected]>;
> [email protected]; [email protected]; [email protected]; linux-
> [email protected]
> Cc: [email protected]; [email protected]; [email protected]; intel-
> [email protected]; [email protected]; Dong, Eddie
> <[email protected]>; Hall, Christopher S <[email protected]>;
> Brandeburg, Jesse <[email protected]>; [email protected];
> [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> Nguyen, Anthony L <[email protected]>;
> [email protected]; N, Pandith <[email protected]>;
> Sangannavar, Mallikarjunappa <[email protected]>;
> Mohan, Subramanian <[email protected]>; Goudar, Basavaraj
> <[email protected]>; T R, Thejesh Reddy
> <[email protected]>; D, Lakshmi Sowjanya
> <[email protected]>
> Subject: Re: [PATCH v5 02/11] timekeeping: Add function to convert realtime to
> base clock
>
> On Tue, Mar 19 2024 at 18:35, [email protected] wrote:
> > +bool ktime_real_to_base_clock(ktime_t treal, enum clocksource_ids
> > +base_id, u64 *cycles) {
> > + struct timekeeper *tk = &tk_core.timekeeper;
> > + unsigned int seq;
> > + u64 delta;
> > +
> > + do {
> > + seq = read_seqcount_begin(&tk_core.seq);
> > + delta = (u64)treal - tk->tkr_mono.base_real;
> > + if (delta > tk->tkr_mono.clock->max_idle_ns)
> > + return false;
>
> I don't think this cutoff is valid. There is no guarantee that this is linear unless:
>
> Treal[last timekeeper update] <= treal < Treal[next timekeeper update]
>
> Look at the dance in get_device_system_crosststamp() and
> adjust_historical_crosststamp() to see why.
>
> > + *cycles = tk->tkr_mono.cycle_last + convert_ns_to_cs(delta);
> > + if (!convert_cs_to_base(cycles, base_id))
> > + return false;
> > + } while (read_seqcount_retry(&tk_core.seq, seq));
> > +
> > + return true;
> > +}
> > +EXPORT_SYMBOL_GPL(ktime_real_to_base_clock);
>
> Looking at the usage site:
>
> > +static bool pps_generate_next_pulse(struct pps_tio *tio, ktime_t
> > +expires) {
> > + u64 art;
> > +
> > + if (!ktime_real_to_base_clock(expires, CSID_X86_ART, &art)) {
> > + pps_tio_disable(tio);
>
> I'm pretty sure this can happen when there is sufficient delay between the check
> for (now - expires < SAFE_TIME_NS) and the delta computation in
> ktime_real_to_base_clock() if there is a timerkeeper update interleaving which
> brings tkr_mono.base_real in front of expires.
>
> Is that intentional and correct?
>
> If so, then it's inconsistent with the behaviour of the hrtimer
> callback:
>
> > + return false;
> > + }
> > +
> > + pps_compv_write(tio, art - ART_HW_DELAY_CYCLES);
> > + return true;
> > +}
> > +
> > +static enum hrtimer_restart hrtimer_callback(struct hrtimer *timer) {
> > + struct pps_tio *tio = container_of(timer, struct pps_tio, timer);
> > + ktime_t expires, now;
> > +
> > + guard(spinlock)(&tio->lock);
> > +
> > + expires = hrtimer_get_expires(timer);
> > + now = ktime_get_real();
> > +
> > + if (now - expires < SAFE_TIME_NS) {
> > + if (!pps_generate_next_pulse(tio, expires + SAFE_TIME_NS))
> > + return HRTIMER_NORESTART;
> > + }
>
> This safe guard does not care about time being set. I'm not familiar with the PPS
> logic, but is it expected that the pulse pattern will be like this:
>
>
>
> ---|-----|-----|-----|----------------->
> P P ^ P
> |
> clock_settime(CLOCK_REALTIME, now - 2 seconds)
>
> Obviously the pulse gap will be as big as the time is set
> backwards, which might be way more than 2 seconds.
>
>
> ---|-----|-----|-----|----------------->
> P P ^ P P
> |
> clock_settime(CLOCK_REALTIME, now + 2 seconds)
>
> I don't see anything in this code which cares about CLOCK_REALTIME being set
> via clock_settime() or adjtimex().
>
> Aside of that I have a question about how the TIO hardware treats this
> case:
>
> ktime_real_to_base_clock(expires, &art);
>
> -> GAP which makes @art get into the past
>
> pps_compv_write(tio, art - ART_HW_DELAY_CYCLES);
>
> Will the hardware ignore that already expired value or just emit a pulse
> immediately? In the latter case the pulse will be at a random point in time, which
> does not sound correct.

Thanks for the review Thomas,

If an expired time is programmed to hardware, it ignores and no pulse is generated.
There is an event counter feature in hardware, this increments on every pulse sent. In next version of driver, we will be monitoring this counter to detect any missed pulse and disable the hardware.

Regards,
Sowjanya

>
> Thanks,
>
> tglx