Re: [PATCH v3] posix-timers: add multi_clock_gettime system call

[Sagi Maimon <maimon.sagi@gmail.com>]

On Tue, Jan 2, 2024 at 1:30 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Sun, Dec 31, 2023, at 17:00, Sagi Maimon wrote:
> > On Fri, Dec 29, 2023 at 5:27 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> >> > +struct __ptp_multi_clock_get {
> >> > +     unsigned int n_clocks; /* Desired number of clocks. */
> >> > +     unsigned int n_samples; /* Desired number of measurements per clock. */
> >> > +     clockid_t clkid_arr[MULTI_PTP_MAX_CLOCKS]; /* list of clock IDs */
> >> > +     /*
> >> > +      * Array of list of n_clocks clocks time samples n_samples times.
> >> > +      */
> >> > +     struct  __kernel_timespec ts[MULTI_PTP_MAX_SAMPLES][MULTI_PTP_MAX_CLOCKS];
> >> > +};
> >>
> >> The fixed size arrays here seem to be an unnecessary limitation,
> >> both MULTI_PTP_MAX_SAMPLES and MULTI_PTP_MAX_CLOCKS are small
> >> enough that one can come up with scenarios where you would want
> >> a higher number, but at the same time the structure is already
> >> 808 bytes long, which is more than you'd normally want to put
> >> on the kernel stack, and which may take a significant time to
> >> copy to and from userspace.
> >>
> >> Since n_clocks and n_samples are always inputs to the syscall,
> >> you can just pass them as register arguments and use a dynamically
> >> sized array instead.
> >>
> > Both MULTI_PTP_MAX_SAMPLES and MULTI_PTP_MAX_CLOCKS are enough of any
> > usage we can think of,
> > But I think you are right, it is better to use a dynamically sized
> > array for future use, plus to use less stack memory.
> > On patch v4 a dynamically sized array will be used .
> > I leaving both MULTI_PTP_MAX_SAMPLES and MULTI_PTP_MAX_CLOCKS but
> > increasing their values, since there should be some limitation.
>
> I think having an implementation specific limit in the kernel is
> fine, but it would be nice to hardcode that limit in the API.
>
> If both clkidarr[] and ts[] are passed as pointer arguments
> in registers, they can be arbitrarily long in the API and
> still have a documented maximum that we can extend in the
> future without changing the interface.
>
> >> It's not clear to me what you gain from having the n_samples
> >> argument over just calling the syscall repeatedly. Does
> >> this offer a benefit for accuracy or is this just meant to
> >> avoid syscall overhead.
> > It is mainly to avoid syscall overhead which also slightly
> > improve the accuracy.
>
> This is not a big deal as far as I'm concerned, but it
> would be nice to back this up with some numbers if you
> think it's worthwhile, as my impression is that the effect
> is barely measurable: my guess would be that the syscall
> overhead is always much less than the cost for the hardware
> access.
>
> >> On the other hand, this will still give less accuracy than the
> >> getcrosststamp() callback with ioctl(PTP_SYS_OFFSET_PRECISE),
> >> so either the last bit of accuracy isn't all that important,
> >> or you need to refine the interface to actually be an
> >> improvement over the chardev.
> >>
> > I don't understand this comment, please explain.
> > The ioctl(PTP_SYS_OFFSET_PRECISE) is one specific case that can be
> > done by multi_clock_gettime syscall (which cover many more cases)
> > Plus the ioctl(PTP_SYS_OFFSET_PRECISE) works only on drivers that
> > support this feature.
>
> My point here is that on drivers that do support
> PTP_SYS_OFFSET_PRECISE, the extra accuracy should be maintained
> by the new interface, ideally in a way that does not have any
> other downsides.
>
> I think Andy's suggestion of exposing time offsets instead
> of absolute times would actually achieve that: If the
> interface is changed to return the offset against
> CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW or CLOCK_BOOTTIME
> (not sure what is best here), then the new syscall can use
> getcrosststamp() where supported for the best results or
> fall back to gettimex64() or gettime64() otherwise to
> provide a consistent user interface.
>
> Returning an offset would also allow easily calculating an
> average over multiple calls in the kernel, instead of
> returning a two-dimensional array.
>
PTP_SYS_OFFSET_PRECISE returns the systime and PHC time and not offset.
But you are right , in the next patch I will use this IOCTL .

>     Arnd