Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter constraint

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 22, 2015 at 01:21:46PM +0200, Peter Zijlstra wrote:
> @@ -821,8 +828,24 @@ int x86_schedule_events(struct cpu_hw_ev
>  
>  	/* slow path */
>  	if (i != n) {
> +		int gpmax = x86_pmu.num_counters / 2;
> +
> +		/*
> +		 * Do not allow scheduling of more than half the available
> +		 * generic counters.
> +		 *
> +		 * This helps avoid counter starvation of sibling thread by
> +		 * ensuring at most half the counters cannot be in exclusive
> +		 * mode. There is no designated counters for the limits. Any
> +		 * N/2 counters can be used. This helps with events with
> +		 * specific counter constraints.
> +		 */
> +		if (is_ht_workaround_enabled() && !cpuc->is_fake &&
> +		    READ_ONCE(cpuc->excl_cntrs->exclusive_present))
> +			gpmax /= 2;
> +
>  		unsched = perf_assign_events(cpuc->event_constraint, n, wmin,
> -					     wmax, assign);
> +					     wmax, gpmax, assign);
>  	}
>  

Hmm, I divide by 2 twice.. no wonder it doesn't quite work as expected.