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

["Jan Beulich" <JBeulich@suse.com>]

>>> On 24.05.12 at 10:32, Alex Shi <alex.shi@intel.com> wrote:
> On 05/24/2012 01:09 AM, Peter Zijlstra wrote:
> 
>> On Wed, 2012-05-23 at 16:05 +0100, Jan Beulich wrote:
>>>>>> On 23.05.12 at 16:15, Alex Shi <alex.shi@intel.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.
>> 
>> Why would that be? Won't higher thread count still share the same
>> resources just more so?
>> 
>>>  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.
> 
> 
> Sure. but just want to know how many commercial x86 CPU uses >2 SMTs?
> Write a short, quick function to do random selection in SMT is quite
> complicate considering cpumask maybe just contain random number SMT
> siblings in a core.

Which is why I wrote that a second best solution would be to
merely document the restriction in the source.

However, picking one out of more than 2 siblings shouldn't be
_that_ difficult.

>> My objection to the whole lot is that its looks mightily expensive on
>> large machines, cpumask operations aren't cheap when you've got 4k cpus
>> etc..
>> 
>> Also, you very much cannot put cpumask_t on stack.
> 
> 
> Sure, and do you has related data for this?
> 
> I just measured the cost of this function on my Romely EP(32 LCPUs) with
> cpumask_t and NR_CPUS = 32/256/512/4096, the cost are similar with
> 256/512/4096 and that increased about 20% time cost from 32.
> 
> I also tried to use cpumask_var_t and alloc it in heap(use
> CPUMASK_OFFSTACK), actually, it cost same time with cpumask_t in stack.
> But, the allocation bring another big cost. So, I use cpumask_t in stack.
> The performance gain data in commit log is getting with NR_CPUS = 256.

Perhaps using a per-CPU cpumask would be the better choice here
(I can't see how preemption could validly be enabled when this
code is utilized).

Jan