Messages in this thread |  | | | Date | Thu, 24 May 2012 16:32:00 +0800 | | From | Alex Shi <> | | Subject | Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of siblings of SMT |
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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.
> > 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.
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