linux-kernel - Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of siblings of SMT

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Date:	Wed, 23 May 2012 16:05:22 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Alex Shi" <alex.shi@...el.com>
Cc:	<borislav.petkov@....com>, <arnd@...db.de>,
	<a.p.zijlstra@...llo.nl>, <akinobu.mita@...il.com>,
	<eric.dumazet@...il.com>, <fweisbec@...il.com>,
	<rostedt@...dmis.org>, <hughd@...gle.com>, <jeremy@...p.org>,
	<len.brown@...el.com>, <tony.luck@...el.com>,
	<yongjie.ren@...el.com>, <kamezawa.hiroyu@...fujitsu.com>,
	<seto.hidetoshi@...fujitsu.com>, <penberg@...nel.org>,
	<yinghai@...nel.org>, <tglx@...utronix.de>,
	<akpm@...ux-foundation.org>, <ak@...ux.intel.com>, <luto@....edu>,
	<avi@...hat.com>, <dhowells@...hat.com>, <mingo@...hat.com>,
	<riel@...hat.com>, <cpw@....com>, <steiner@....com>,
	<linux-kernel@...r.kernel.org>, <viro@...iv.linux.org.uk>,
	<hpa@...or.com>
Subject: Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of
 siblings of SMT

>>> On 23.05.12 at 16:15, Alex Shi <alex.shi@...el.com> 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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