Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one ofsiblings of SMT

[Jan Beulich]

>>> On 23.05.12 at 16:15, Alex Shi <alex.shi@xxxxxxxxx> wrote:
> +	/* doing flush on both siblings of SMT is just wasting time */
> +	cpumask_copy(&flush_mask, cpumask);
> +	if (likely(smp_num_siblings > 1)) {
> +		rand = jiffies;
> +		/* See "Numerical Recipes in C", second edition, p. 284 */
> +		rand = rand * 1664525L + 1013904223L;
> +		rand &= 0x1;
> +
> +		for_each_cpu(cpu, &flush_mask) {
> +			sblmask = cpu_sibling_mask(cpu);
> +			if (cpumask_subset(sblmask, &flush_mask)) {
> +				if (rand == 0)
> +					cpu_clear(cpu, flush_mask);
> +				else
> +					cpu_clear(cpumask_next(cpu, sblmask),
> +								flush_mask);
> +			}
> +		}
> +	}
> +

There is no comment or anything else indicating that this is
suitable for dual-thread CPUs only - when there are more than
2 threads per core, the intended effect won't be achieved. I'd
recommend making the logic generic from the beginning, but if
that doesn't seem feasible to you, at least a comment stating
the limitation should be added imo.

Jan

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