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Date: Thu, 23 Feb 2017 10:17:22 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Thomas Gleixner <tglx@linutronix.de>, Olof Johansson <olof@lixom.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [GIT pull] x86/timers for 4.10
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On Thu, Feb 23, 2017 at 09:20:06AM +0100, Mike Galbraith wrote:
> On Thu, 2017-02-09 at 16:07 +0100, Thomas Gleixner wrote:

> > Can you share your cheatery ?
> 
> I can do better than that... sorta ;-)
> 
> x86/tsc: Fix unreliable tsc adjust
> 
> On a 4 socket BIOS challenged box (4x18), the magic number '3' does
> not work reliably, resulting in TSC being disabled more often than not.
> 
> Replace defective magic number '3' with functional magic number '5',
> derived via scientific method number sockets in afflicted box, plus
> one for good luck, and reboot box a lot to validate (poke 'n' hope).
> 
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> ---
>  arch/x86/kernel/tsc_sync.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/arch/x86/kernel/tsc_sync.c
> +++ b/arch/x86/kernel/tsc_sync.c
> @@ -294,7 +294,7 @@ void check_tsc_sync_source(int cpu)
>  	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
>  		atomic_set(&test_runs, 1);
>  	else
> -		atomic_set(&test_runs, 3);
> +		atomic_set(&test_runs, 5);
>  retry:
>  	/*
>  	 * Wait for the target to start or to skip the test:

Confirmed on another box too; the default of 3 wasn't sufficient to
achieve sync. When upping it to 16, sync was achieved in around 4 runs
or so.