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SubjectRe: [RFC patch] need check TSC wrap unconditionally
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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@intel.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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