On 07/23/2015 08:58 AM, Andrea Arcangeli wrote: > You wrote the patch that uses the tlb_single_page_flush_ceiling, so if > the above discussion would be relevant with regard to flush_tlb_page, > are you implying that the above optimization in the kernel, should > also be removed?
When I put that in, my goal was to bring consistency to how we handled things without regressing anything. I was never able to measure any nice macro-level benefits to a particular flush behavior. We can also now just easily disable the ranged flushes if we want to, or leave them in place for small flushes only. > When these flush_tlb_range optimizations were introduced, it was > measured with benchmark that they helped IIRC. If it's not true > anymore with latest CPU I don't know but there should be at least a > subset of those CPUs where this helps. So I doubt it should be removed > for all CPUs out there. I tried to reproduce the results and had a difficult time doing so. > The tlb_single_page_flush_ceiling optimization has nothing to do with > 2MB pages. But if that is still valid (or if it has ever been valid > for older CPUs), why is flush_tlb_page not a valid optimization at > least for those older CPUS? Why is it worth doing single invalidates > on 4k pages and not on 2MB pages? I haven't seen any solid evidence that we should do it for one and not the other. > It surely was helpful to do invlpg invalidated on 4k pages, up to 33 > in a row, with x86 CPUs as you wrote the code quoted above to do > that, and it is still in the current kernel. So why are 2MB pages > different? They were originally different because the work that introduced 'invlpg' didn't see a benefit from using 'invlpg' on 2M pages. I didn't reevaluate it when I hacked on the code and just left it as it was. It would be great if someone would go and collect some recent data on using 'invlpg' on 2M pages! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
