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

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Date:	Mon, 24 Aug 2009 10:30:53 +0800
From:	Luming Yu <luming.yu@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Sat, Aug 22, 2009 at 12:37 AM, Peter Zijlstra<peterz@...radead.org> wrote:
> 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?

How comes? Good machine will pass the check without any problem.

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

iirc, in old kernel, there was a function trying to re-sync TSC. But
it has been removed..
Should I re-introduce something that has been deleted. It is odd to do such
kind of thing to me if I don't know why it was deleted.

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

yes.

>
>> 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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