linux-kernel - Re: [RFC patch] need check TSC wrap unconditionally

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Date:	Fri, 21 Aug 2009 18:37:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Luming Yu <luming.yu@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC patch] need check TSC wrap unconditionally

On Fri, 2009-08-14 at 12:15 +0800, Luming Yu wrote:
> Hi there,

Hi, thanks for CC'ing the right folks.

> we disabled tsc wrap check on any platform that has NOSTOP_TSC cpu. But this
> will cause some real problem.For example, 1.CPU does has constant and
> non_stop tsc, which means different CPU ticks at same rate in same domain,but
> have been given different initial TSC value.Then at any given time,
> CPUsare unsynchronized.
> 2. if those CPUs are sit in different domain..(multi-chassis cluster system?)
> 
> Please review. If make sense, please apply.

Right, so because your machine is funny, everybody with a good machine
should suffer?

If the TSCs run at identical frequency (one time domain) but at
different offsets, you can fix that by using cyc2ns_offset.

When the machine has unsynchonized frequencies (multiple time domains),
detect that during cpu enumeration and use that to disable the
optimization.

> Signed-off-by: Yu Luming <luming.yu@...el.com>

>  arch/x86/kernel/tsc_sync.c |    7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
> index 027b5b4..312ba84 100644
> --- a/arch/x86/kernel/tsc_sync.c
> +++ b/arch/x86/kernel/tsc_sync.c
> @@ -113,11 +113,6 @@ void __cpuinit check_tsc_sync_source(int cpu)
>         if (unsynchronized_tsc())
>                 return;
> 
> -       if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) {
> -               pr_info("Skipping synchronization checks as TSC is
> reliable.\n");
> -               return;
> -       }
> -
>         pr_info("checking TSC synchronization [CPU#%d -> CPU#%d]:",
>                 smp_processor_id(), cpu);
> 
> @@ -171,7 +166,7 @@ void __cpuinit check_tsc_sync_target(void)
>  {
>         int cpus = 2;
> 
> -       if (unsynchronized_tsc() || boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
> +       if (unsynchronized_tsc())
>                 return;
> 
>         /*
--
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