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Date: Mon, 17 Nov 2008 23:50:18 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Venki Pallipadi <venkatesh.pallipadi@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Arjan van de Ven <arjan@infradead.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>, Mike Galbraith <efault@gmx.de>
Subject: Re: [git pull] scheduler updates
Message-ID: <20081117225018.GA25619@elte.hu>
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* Venki Pallipadi <venkatesh.pallipadi@intel.com> wrote:

> Patch being discussed on this thread (commit 0d12cdd) has a 
> regression on one of the test systems here.
> 
> With the patch, I see
> 
> checking TSC synchronization [CPU#0 -> CPU#1]:
> Measured 28 cycles TSC warp between CPUs, turning off TSC clock.
> Marking TSC unstable due to check_tsc_sync_source failed
> 
> Whereas, without the patch syncs pass fine on all CPUs
> 
> checking TSC synchronization [CPU#0 -> CPU#1]: passed.
> 
> Due to this, TSC is marke unstable, when it is not actually unstable.
> This is because syncs in check_tsc_wrap() goes away due to this commit.
> 
> As per the discussion on this thread, correct way to fix this is to add
> explicit syncs as below?

ah. Yes.

Could you please check whether:

> +	rdtsc_barrier();
>  	start = get_cycles();
> +	rdtsc_barrier();
>  	/*
>  	 * The measurement runs for 20 msecs:
>  	 */
> @@ -61,7 +63,9 @@ static __cpuinit void check_tsc_warp(voi
>  		 */
>  		__raw_spin_lock(&sync_lock);
>  		prev = last_tsc;
> +		rdtsc_barrier();
>  		now = get_cycles();
> +		rdtsc_barrier();

adding the barrier just _after_ the get_cycles() call (but not before 
it) does the trick too? That should be enough in this case.

	Ingo