From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: John Stultz <john.stultz@linaro.org>,
Andy Lutomirski <luto@kernel.org>,
LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Matt Rickard <matt@softrans.com.au>,
Stephen Boyd <sboyd@kernel.org>,
Florian Weimer <fweimer@redhat.com>,
"K. Y. Srinivasan" <kys@microsoft.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
devel@linuxdriverproject.org,
Linux Virtualization <virtualization@lists.linux-foundation.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case
Date: Wed, 19 Sep 2018 00:46:54 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.21.1809190037570.1468@nanos.tec.linutronix.de> (raw)
In-Reply-To: <863331ED-B04A-4B94-91A2-D34002C9CCDC@amacapital.net>
[-- Attachment #1: Type: text/plain, Size: 1734 bytes --]
On Tue, 18 Sep 2018, Andy Lutomirski wrote:
> > On Sep 18, 2018, at 12:52 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> >> On Mon, 17 Sep 2018, John Stultz wrote:
> >>> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto@kernel.org> wrote:
> >>> Also, I'm not entirely convinced that this "last" thing is needed at
> >>> all. John, what's the scenario under which we need it?
> >>
> >> So my memory is probably a bit foggy, but I recall that as we
> >> accelerated gettimeofday, we found that even on systems that claimed
> >> to have synced TSCs, they were actually just slightly out of sync.
> >> Enough that right after cycles_last had been updated, a read on
> >> another cpu could come in just behind cycles_last, resulting in a
> >> negative interval causing lots of havoc.
> >>
> >> So the sanity check is needed to avoid that case.
> >
> > Your memory serves you right. That's indeed observable on CPUs which
> > lack TSC_ADJUST.
> >
> > @Andy: Welcome to the wonderful world of TSC.
> >
>
> Do we do better if we use signed arithmetic for the whole calculation?
> Then a small backwards movement would result in a small backwards result.
> Or we could offset everything so that we’d have to go back several
> hundred ms before we cross zero.
That would be probably the better solution as signed math would be
problematic when the resulting ns value becomes negative. As the delta is
really small, otherwise the TSC sync check would have caught it, the caller
should never be able to observe time going backwards.
I'll have a look into that. It needs some thought vs. the fractional part
of the base time, but it should be not rocket science to get that
correct. Famous last words...
Thanks,
tglx
next prev parent reply other threads:[~2018-09-18 22:47 UTC|newest]
Thread overview: 81+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-14 12:50 [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Thomas Gleixner
2018-09-14 12:50 ` [patch 01/11] clocksource: Provide clocksource_arch_init() Thomas Gleixner
2018-09-14 12:50 ` [patch 02/11] x86/time: Implement clocksource_arch_init() Thomas Gleixner
2018-09-14 15:45 ` Vitaly Kuznetsov
2018-09-15 6:05 ` Thomas Gleixner
2018-09-14 12:50 ` [patch 03/11] x86/vdso: Enforce 64bit clocksource Thomas Gleixner
2018-09-14 12:50 ` [patch 04/11] x86/vdso: Use unsigned int consistently for vsyscall_gtod_data::seq Thomas Gleixner
2018-09-14 12:50 ` [patch 05/11] x86/vdso: Introduce and use vgtod_ts Thomas Gleixner
2018-09-14 12:50 ` [patch 06/11] x86/vdso: Collapse high resolution functions Thomas Gleixner
2018-09-14 12:50 ` [patch 07/11] x86/vdso: Collapse coarse functions Thomas Gleixner
2018-09-14 12:50 ` [patch 08/11] x86/vdso: Replace the clockid switch case Thomas Gleixner
2018-09-14 12:50 ` [patch 09/11] x86/vdso: Simplify the invalid vclock case Thomas Gleixner
2018-09-17 19:25 ` Andy Lutomirski
2018-09-17 20:12 ` John Stultz
2018-09-18 7:52 ` Thomas Gleixner
2018-09-18 8:30 ` Peter Zijlstra
2018-09-18 8:52 ` Thomas Gleixner
2018-09-18 10:06 ` Thomas Gleixner
2018-09-18 10:41 ` Thomas Gleixner
2018-09-18 12:48 ` Peter Zijlstra
2018-09-18 13:23 ` Thomas Gleixner
2018-09-18 13:38 ` Peter Zijlstra
2018-09-18 15:52 ` Thomas Gleixner
2018-09-27 14:41 ` Thomas Gleixner
2018-09-18 14:01 ` Andy Lutomirski
2018-09-18 22:46 ` Thomas Gleixner [this message]
2018-09-18 23:03 ` Andy Lutomirski
2018-09-18 23:16 ` Thomas Gleixner
2018-09-27 14:36 ` Thomas Gleixner
2018-09-27 14:39 ` Andy Lutomirski
2018-09-19 9:08 ` Rasmus Villemoes
2018-09-19 13:29 ` Thomas Gleixner
2018-09-14 12:50 ` [patch 10/11] x86/vdso: Move cycle_last handling into the caller Thomas Gleixner
2018-09-14 15:26 ` Vitaly Kuznetsov
2018-09-14 12:50 ` [patch 11/11] x66/vdso: Add CLOCK_TAI support Thomas Gleixner
2018-09-14 14:04 ` Andy Lutomirski
2018-09-14 14:27 ` Thomas Gleixner
2018-09-14 14:59 ` Andy Lutomirski
2018-09-16 9:39 ` Thomas Gleixner
2018-09-14 12:56 ` [patch 00/11] x86/vdso: Cleanups, simmplifications and " Florian Weimer
2018-09-14 13:05 ` Thomas Gleixner
2018-09-14 13:06 ` Florian Weimer
2018-09-14 13:19 ` Thomas Gleixner
2018-09-14 13:09 ` Peter Zijlstra
2018-09-14 14:22 ` Arnd Bergmann
2018-09-17 13:00 ` Thomas Gleixner
2018-09-24 21:08 ` Arnd Bergmann
2018-10-03 5:15 ` Andy Lutomirski
2018-10-03 9:22 ` Vitaly Kuznetsov
2018-10-03 10:20 ` Andy Lutomirski
2018-10-03 12:01 ` Vitaly Kuznetsov
2018-10-03 14:20 ` Andy Lutomirski
2018-10-03 15:10 ` Thomas Gleixner
2018-10-03 16:18 ` Andy Lutomirski
2018-10-03 19:06 ` Marcelo Tosatti
2018-10-04 7:54 ` Vitaly Kuznetsov
2018-10-04 8:11 ` Peter Zijlstra
2018-10-04 14:00 ` Andy Lutomirski
2018-10-04 19:31 ` Peter Zijlstra
2018-10-04 20:05 ` Andy Lutomirski
2018-10-04 22:15 ` Andy Lutomirski
2018-10-06 20:27 ` Marcelo Tosatti
2018-10-06 22:28 ` Andy Lutomirski
2018-10-08 15:26 ` Marcelo Tosatti
2018-10-08 17:38 ` Andy Lutomirski
2018-10-08 19:36 ` Marcelo Tosatti
2018-10-09 20:09 ` Andy Lutomirski
2018-10-11 22:27 ` Marcelo Tosatti
2018-10-11 23:00 ` Andy Lutomirski
2018-10-15 13:39 ` Marcelo Tosatti
2018-10-06 20:49 ` Marcelo Tosatti
2018-10-04 12:00 ` Paolo Bonzini
2018-10-04 14:04 ` Andy Lutomirski
2018-10-05 21:18 ` Marcelo Tosatti
2018-10-03 19:00 ` Marcelo Tosatti
2018-10-03 19:05 ` [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support\ Marcelo Tosatti
2018-10-03 22:32 ` [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Andy Lutomirski
2018-10-04 16:37 ` Marcelo Tosatti
2018-10-04 17:08 ` Andy Lutomirski
2018-10-04 17:28 ` Vitaly Kuznetsov
2018-10-04 20:32 ` Andy Lutomirski
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.DEB.2.21.1809190037570.1468@nanos.tec.linutronix.de \
--to=tglx@linutronix.de \
--cc=arnd@arndb.de \
--cc=devel@linuxdriverproject.org \
--cc=fweimer@redhat.com \
--cc=jgross@suse.com \
--cc=john.stultz@linaro.org \
--cc=kys@microsoft.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=luto@kernel.org \
--cc=matt@softrans.com.au \
--cc=pbonzini@redhat.com \
--cc=peterz@infradead.org \
--cc=sboyd@kernel.org \
--cc=virtualization@lists.linux-foundation.org \
--cc=vkuznets@redhat.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).