linux-kernel - Re: [PATCH 5/9] perf/x86: implement cross-HT corruption bug workaround

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Date:	Thu, 5 Jun 2014 16:04:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...ux.intel.com,
	jolsa@...hat.com, zheng.z.yan@...el.com,
	maria.n.dimakopoulou@...il.com
Subject: Re: [PATCH 5/9] perf/x86: implement cross-HT corruption bug
 workaround

On Wed, Jun 04, 2014 at 11:34:14PM +0200, Stephane Eranian wrote:
> +
> +	/*
> +	 * Modify static constraint with current dynamic
> +	 * state of thread
> +	 *
> +	 * EXCLUSIVE: sibling counter measuring exclusive event
> +	 * SHARED   : sibling counter measuring non-exclusive event
> +	 * UNUSED   : sibling counter unused
> +	 */
> +	for_each_set_bit(i, cx->idxmsk, X86_PMC_IDX_MAX) {
> +		/*
> +		 * exclusive event in sibling counter
> +		 * our corresponding counter cannot be used
> +		 * regardless of our event
> +		 */
> +		if (xl->state[i] == INTEL_EXCL_EXCLUSIVE)
> +			__clear_bit(i, cx->idxmsk);
> +		/*
> +		 * if measuring an exclusive event, sibling
> +		 * measuring non-exclusive, then counter cannot
> +		 * be used
> +		 */
> +		if (is_excl && xl->state[i] == INTEL_EXCL_SHARED)
> +			__clear_bit(i, cx->idxmsk);
> +	}
> +
> +	/*
> +	 * recompute actual bit weight for scheduling algorithm
> +	 */
> +	cx->weight = hweight64(cx->idxmsk64);

So I think we talked about this a bit; what happens if CPU0 (taking your
4 core HSW-client) is first to program its counters and takes all 4 in
exclusive mode?

Then there's none left for CPU4.

Did I miss where we avoid that problem, or is that an actual issue?

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