On 04/01/2014 09:20 AM, Ingo Molnar wrote:
>
> * Rik van Riel <riel@xxxxxxxxxx> wrote:
>
>>>> int ptep_clear_flush_young(struct vm_area_struct *vma,
>>>> unsigned long address, pte_t *ptep)
>>>> {
>>>> - int young;
>>>> + int young, cpu;
>>>>
>>>> young = ptep_test_and_clear_young(vma, address, ptep);
>>>> - if (young)
>>>> - flush_tlb_page(vma, address);
>>>> + if (young) {
>>>> + for_each_cpu(cpu, vma->vm_mm->cpu_vm_mask_var)
>>>> + tlb_set_force_flush(cpu);
>>>
>>> Hm, just to play the devil's advocate - what happens when we have
>>> a va that is used on a few dozen, a few hundred or a few thousand
>>> CPUs? Will the savings be dwarved by the O(nr_cpus_used) loop
>>> overhead?
>>>
>>> Especially as this is touching cachelines on other CPUs and likely
>>> creating the worst kind of cachemisses. That can really kill
>>> performance.
>>
>> flush_tlb_page does the same O(nr_cpus_used) loop, but it sends an
>> IPI to each CPU every time, instead of dirtying a cache line once
>> per pageout run (or until the next context switch).
>>
>> Does that address your concern?
>
> That depends on the platform - which could implement flush_tlb_page()
> as a broadcast IPI - but yes, it was bad before as well, now it became
> more visible and I noticed it :)
>
> Wouldn't it be more scalable to use a generation count as a timestamp,
> and set that in the mm? mm that last flushed before that timestamp
> need to flush, or so. That gets rid of the mask logic and the loop,
> AFAICS.
More scalable in the page eviction code, sure.
However, that would cause the context switch code to load an
additional cache line, so I am not convinced that is a good
tradeoff...
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