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From: Sagi Maimon <maimon.sagi@gmail.com>
Date: Thu, 14 Mar 2024 10:05:04 +0200
Message-ID: <CAMuE1bEGcPPQUHdZ-sodCZyMragvSRtKfENOZ4wphQggr1fJWg@mail.gmail.com>
Subject: Re: [PATCH v6] posix-timers: add clock_compare system call
To: Arnd Bergmann <arnd@arndb.de>
Cc: Richard Cochran <richardcochran@gmail.com>, Andy Lutomirski <luto@kernel.org>, datglx@linutronix.de, 
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, Geert Uytterhoeven <geert@linux-m68k.org>, 
	Peter Zijlstra <peterz@infradead.org>, Johannes Weiner <hannes@cmpxchg.org>, 
	Sohil Mehta <sohil.mehta@intel.com>, Rick Edgecombe <rick.p.edgecombe@intel.com>, 
	Nhat Pham <nphamcs@gmail.com>, Palmer Dabbelt <palmer@sifive.com>, Kees Cook <keescook@chromium.org>, 
	Alexey Gladkov <legion@kernel.org>, Mark Rutland <mark.rutland@arm.com>, 
	Miklos Szeredi <mszeredi@redhat.com>, Casey Schaufler <casey@schaufler-ca.com>, reibax@gmail.com, 
	"David S . Miller" <davem@davemloft.net>, Christian Brauner <brauner@kernel.org>, linux-kernel@vger.kernel.org, 
	linux-api@vger.kernel.org, Linux-Arch <linux-arch@vger.kernel.org>, 
	Netdev <netdev@vger.kernel.org>
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On Tue, Mar 12, 2024 at 3:47=E2=80=AFPM Arnd Bergmann <arnd@arndb.de> wrote=
:
>
> On Tue, Mar 12, 2024, at 13:15, Sagi Maimon wrote:
> > On Tue, Mar 12, 2024 at 1:19=E2=80=AFPM Arnd Bergmann <arnd@arndb.de> w=
rote:
> >> On Tue, Mar 12, 2024, at 10:50, Sagi Maimon wrote:
> >> > +     kc_a =3D clockid_to_kclock(clock_a);
> >> > +     if (!kc_a) {
> >> > +             error =3D -EINVAL;
> >> > +             return error;
> >> > +     }
> >> > +
> >> > +     kc_b =3D clockid_to_kclock(clock_b);
> >> > +     if (!kc_b) {
> >> > +             error =3D -EINVAL;
> >> > +             return error;
> >> > +     }
> >>
> >> I'm not sure if we really need to have it generic enough to
> >> support any combination of clocks here. It complicates the
> >> implementation a bit but it also generalizes the user space
> >> side of it.
> >>
> >> Can you think of cases where you want to compare against
> >> something other than CLOCK_MONOTONIC_RAW or CLOCK_REALTIME,
> >> or are these going to be the ones that you expect to
> >> be used anyway?
> >>
> > sure, one example is syncing two different PHCs (which was originally
> > why we needed this syscall)
> > I hope that I have understand your note and that answers your question.
>
> Right, that is clearly a sensible use case.
>
> I'm still trying to understand the implementation for the case
> where you have two different PHCs and both implement
> clock_get_crosstimespec(). Rather than averaging between
> two snapshots here, I would expect this to result in
> something like
>
>       ktime_a1 +=3D xtstamp_b.sys_monoraw - xtstamp_a1.sys_monoraw;
>
> in order get two device timestamps ktime_a1 and ktime_b
> that reflect the snapshots as if they were taken
> simulatenously. Am I missing some finer detail here,
> or is this something you should do?
>
Since the raw monotonic clock and the PHC are not synthesized, that
won't be accurate at all and depends on the frequency delta between
them.

> >> > +     if (crosstime_support_a) {
> >> > +             ktime_a1 =3D xtstamp_a1.device;
> >> > +             ktime_a2 =3D xtstamp_a2.device;
> >> > +     } else {
> >> > +             ktime_a1 =3D timespec64_to_ktime(ts_a1);
> >> > +             ktime_a2 =3D timespec64_to_ktime(ts_a2);
> >> > +     }
> >> > +
> >> > +     ktime_a =3D ktime_add(ktime_a1, ktime_a2);
> >> > +
> >> > +     ts_offs =3D ktime_divns(ktime_a, 2);
> >> > +
> >> > +     ts_a1 =3D ns_to_timespec64(ts_offs);
> >>
> >> Converting nanoseconds to timespec64 is rather expensive,
> >> so I wonder if this could be changed to something cheaper,
> >> either by returning nanoseconds in the end and consistently
> >> working on those, or by doing the calculation on the
> >> timespec64 itself.
> >>
> > I prefer returning timespec64, so this system call aligns with other
> > system calls like clock_gettime for example.
> > As far as doing the calculation on timespec64 itself, that looks more
> > expansive to me, but I might be wrong.
>
> In the general case, dividing a 64-bit variable by some other
> variable is really expensive and will take hundreds of cycles.
> This one is a bit cheaper because the division is done using
> a constant divider of NS_PER_SEC, which can get optimized fairly
> well on many systems by turning it into an equivalent 128-bit
> multiplication plus shift.
>
> For the case where you start out with a timespec64, I would
> expect it to be cheaper to calculate the nanosecond difference
> between ts_a1 and ts_a2 to add half of that to the timespec
> than to average two large 64-bit values and convert that back
> to a timespec afterwards. This should be fairly easy to try
> out if you can test a 32-bit kernel. We could decide that
> there is no need to care about anything bug 64-bit kernels
> here, in which case your current version should be just as
> good for both the crosstime_support_a and !crosstime_support_a
> cases.
>
sounds good, it will be done on the next patch.
>      Arnd