[v5] posix-timers: add multi_clock_gettime system call

  Some user space applications need to read some clocks.
Each read requires moving from user space to kernel space.
The syscall overhead causes unpredictable delay between N clocks reads
Removing this delay causes better synchronization between N clocks.

Introduce a new system call multi_clock_gettime, which can be used to measure
the offset between multiple clocks, from variety of types: PHC, virtual PHC
and various system clocks (CLOCK_REALTIME, CLOCK_MONOTONIC, etc).
The offset includes the total time that the driver needs to read the clock
timestamp.

New system call allows the reading of a list of clocks - up to PTP_MAX_CLOCKS.
Supported clocks IDs: PHC, virtual PHC and various system clocks.
Up to PTP_MAX_SAMPLES times (per clock) in a single system call read.
The system call returns n_clocks timestamps for each measurement:
- clock 0 timestamp
- ...
- clock n timestamp

Signed-off-by: Sagi Maimon <maimon.sagi@gmail.com>
---
 Changes since version 4:
 - fix error  : 'struct __ptp_multi_clock_get' declared inside parameter list 
   will not be visible outside of this definition or declaration

 arch/x86/entry/syscalls/syscall_64.tbl |  1 +
 include/linux/syscalls.h               |  3 +-
 include/uapi/asm-generic/unistd.h      |  4 +-
 include/uapi/linux/multi_clock.h       | 21 +++++++++
 kernel/time/posix-timers.c             | 59 ++++++++++++++++++++++++++
 5 files changed, 86 insertions(+), 2 deletions(-)
 create mode 100644 include/uapi/linux/multi_clock.h
  

On Tue, Jan 2, 2024, at 10:18, Sagi Maimon wrote:
> Some user space applications need to read some clocks.
> Each read requires moving from user space to kernel space.
> The syscall overhead causes unpredictable delay between N clocks reads
> Removing this delay causes better synchronization between N clocks.
>
> Introduce a new system call multi_clock_gettime, which can be used to measure
> the offset between multiple clocks, from variety of types: PHC, virtual PHC
> and various system clocks (CLOCK_REALTIME, CLOCK_MONOTONIC, etc).
> The offset includes the total time that the driver needs to read the clock
> timestamp.
>
> New system call allows the reading of a list of clocks - up to PTP_MAX_CLOCKS.
> Supported clocks IDs: PHC, virtual PHC and various system clocks.
> Up to PTP_MAX_SAMPLES times (per clock) in a single system call read.
> The system call returns n_clocks timestamps for each measurement:
> - clock 0 timestamp
> - ...
> - clock n timestamp
>
> Signed-off-by: Sagi Maimon <maimon.sagi@gmail.com>
> ---
>  Changes since version 4:
>  - fix error  : 'struct __ptp_multi_clock_get' declared inside parameter list 
>    will not be visible outside of this definition or declaration

I usually put all the changes for previous versions in a
list here, it helps reviewers.

The changes you made for previous versions all look good
to me, but I think there is still a few things worth
considering. I'll also follow up on the earlier threads.

> +#define MULTI_PTP_MAX_CLOCKS 32 /* Max number of clocks */
> +#define MULTI_PTP_MAX_SAMPLES 32 /* Max allowed offset measurement samples. */
> +
> +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];
> +};

Since you now access each member individually, I think it
makes more sense here to just pass these as four
register arguments. It helps with argument introspection,
avoids a couple of get_user(), and lets you remove the fixed
array dimensions.

> +SYSCALL_DEFINE1(multi_clock_gettime, struct __ptp_multi_clock_get 
> __user *, ptp_multi_clk_get)
> +{
> +	const struct k_clock *kc;
> +	struct timespec64 *kernel_tp;
> +	struct timespec64 *kernel_tp_base;
> +	unsigned int n_clocks; /* Desired number of clocks. */
> +	unsigned int n_samples; /* Desired number of measurements per clock. 
> */
> +	unsigned int i, j;
> +	clockid_t clkid_arr[MULTI_PTP_MAX_CLOCKS]; /* list of clock IDs */
> +	int error = 0;
> +
> +	if (copy_from_user(&n_clocks, &ptp_multi_clk_get->n_clocks, 
> sizeof(n_clocks)))
> +		return -EFAULT;
> +	if (copy_from_user(&n_samples, &ptp_multi_clk_get->n_samples, 
> sizeof(n_samples)))

If these remain as struct members rather than register arguments,
you should use get_user() instead of copy_from_user().

> +	kernel_tp_base = kmalloc_array(n_clocks * n_samples,
> +				       sizeof(struct timespec64), GFP_KERNEL);
> +	if (!kernel_tp_base)
> +		return -ENOMEM;

To be on the safe side regarding possible data leak, maybe use
kcalloc() instead of kmalloc_array() here.

> +	kernel_tp = kernel_tp_base;
> +	for (j = 0; j < n_samples; j++) {
> +		for (i = 0; i < n_clocks; i++) {
> +			if (put_timespec64(kernel_tp++, (struct __kernel_timespec __user *)
> +					&ptp_multi_clk_get->ts[j][i])) {

I think the typecast here can be removed.

      Arnd
  

On Tue, Jan 2, 2024 at 12:22 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Tue, Jan 2, 2024, at 10:18, Sagi Maimon wrote:
> > Some user space applications need to read some clocks.
> > Each read requires moving from user space to kernel space.
> > The syscall overhead causes unpredictable delay between N clocks reads
> > Removing this delay causes better synchronization between N clocks.
> >
> > Introduce a new system call multi_clock_gettime, which can be used to measure
> > the offset between multiple clocks, from variety of types: PHC, virtual PHC
> > and various system clocks (CLOCK_REALTIME, CLOCK_MONOTONIC, etc).
> > The offset includes the total time that the driver needs to read the clock
> > timestamp.
> >
> > New system call allows the reading of a list of clocks - up to PTP_MAX_CLOCKS.
> > Supported clocks IDs: PHC, virtual PHC and various system clocks.
> > Up to PTP_MAX_SAMPLES times (per clock) in a single system call read.
> > The system call returns n_clocks timestamps for each measurement:
> > - clock 0 timestamp
> > - ...
> > - clock n timestamp
> >
> > Signed-off-by: Sagi Maimon <maimon.sagi@gmail.com>
> > ---
> >  Changes since version 4:
> >  - fix error  : 'struct __ptp_multi_clock_get' declared inside parameter list
> >    will not be visible outside of this definition or declaration
>
> I usually put all the changes for previous versions in a
> list here, it helps reviewers.
>
Will be done on patch V6.
> The changes you made for previous versions all look good
> to me, but I think there is still a few things worth
> considering. I'll also follow up on the earlier threads.
>
> > +#define MULTI_PTP_MAX_CLOCKS 32 /* Max number of clocks */
> > +#define MULTI_PTP_MAX_SAMPLES 32 /* Max allowed offset measurement samples. */
> > +
> > +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];
> > +};
>
> Since you now access each member individually, I think it
> makes more sense here to just pass these as four
> register arguments. It helps with argument introspection,
> avoids a couple of get_user(), and lets you remove the fixed
> array dimensions.
>
I prefer the use of  get_user(), I will use it to remove  the fixed
array dimensions.
which will be done on patch V6.
> > +SYSCALL_DEFINE1(multi_clock_gettime, struct __ptp_multi_clock_get
> > __user *, ptp_multi_clk_get)
> > +{
> > +     const struct k_clock *kc;
> > +     struct timespec64 *kernel_tp;
> > +     struct timespec64 *kernel_tp_base;
> > +     unsigned int n_clocks; /* Desired number of clocks. */
> > +     unsigned int n_samples; /* Desired number of measurements per clock.
> > */
> > +     unsigned int i, j;
> > +     clockid_t clkid_arr[MULTI_PTP_MAX_CLOCKS]; /* list of clock IDs */
> > +     int error = 0;
> > +
> > +     if (copy_from_user(&n_clocks, &ptp_multi_clk_get->n_clocks,
> > sizeof(n_clocks)))
> > +             return -EFAULT;
> > +     if (copy_from_user(&n_samples, &ptp_multi_clk_get->n_samples,
> > sizeof(n_samples)))
>
> If these remain as struct members rather than register arguments,
> you should use get_user() instead of copy_from_user().
>
Will be done on patch V6
> > +     kernel_tp_base = kmalloc_array(n_clocks * n_samples,
> > +                                    sizeof(struct timespec64), GFP_KERNEL);
> > +     if (!kernel_tp_base)
> > +             return -ENOMEM;
>
> To be on the safe side regarding possible data leak, maybe use
> kcalloc() instead of kmalloc_array() here.
>
Will be done on patch V6.
> > +     kernel_tp = kernel_tp_base;
> > +     for (j = 0; j < n_samples; j++) {
> > +             for (i = 0; i < n_clocks; i++) {
> > +                     if (put_timespec64(kernel_tp++, (struct __kernel_timespec __user *)
> > +                                     &ptp_multi_clk_get->ts[j][i])) {
>
> I think the typecast here can be removed.
>
You are right, will be fixed on patch V6.
>       Arnd
Thanks for your Notes.
  

Hi Sagi,

kernel test robot noticed the following build errors:

[auto build test ERROR on tip/x86/asm]
[also build test ERROR on arnd-asm-generic/master tip/timers/core linus/master v6.7-rc8]
[cannot apply to next-20240103]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Sagi-Maimon/posix-timers-add-multi_clock_gettime-system-call/20240102-172105
base:   tip/x86/asm
patch link:    https://lore.kernel.org/r/20240102091855.70418-1-maimon.sagi%40gmail.com
patch subject: [PATCH v5] posix-timers: add multi_clock_gettime system call
config: csky-randconfig-002-20240104 (https://download.01.org/0day-ci/archive/20240104/202401041000.T0GQPIBW-lkp@intel.com/config)
compiler: csky-linux-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240104/202401041000.T0GQPIBW-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202401041000.T0GQPIBW-lkp@intel.com/

All errors (new ones prefixed by >>):

>> csky-linux-ld: arch/csky/kernel/syscall_table.o:(.data..page_aligned+0x724): undefined reference to `sys_multi_clock_gettime'
  

On Tue, Jan 02 2024 at 11:18, Sagi Maimon wrote:
> Some user space applications need to read some clocks.
> Each read requires moving from user space to kernel space.
> The syscall overhead causes unpredictable delay between N clocks reads
> Removing this delay causes better synchronization between N clocks.

As I explained to you before: This is wishful thinking.

There is absolutely no guarantee that the syscall will yield better
results. It might on average, but that's a useless measure.

You also still fail to explain what this is going to solve and how it's
used.

> Some user space applications need to read some clocks.

Is just not an explanation at all.

Thanks,

        tglx