Re: [LKP] [lkp] [mm] 5c0a85fad9: unixbench.score -6.3% regression

[Linus Torvalds]

On Sat, Jun 11, 2016 at 5:49 PM, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> From perf profile, the time spent in page_fault and its children
> functions are almost same (7.85% vs 7.81%).  So the time spent in page
> fault and page table operation itself doesn't changed much.  So, you
> mean CPU may be slower to load the page table entry to TLB if accessed
> bit is not set?

So the CPU does take a microfault internally when it needs to set the
accessed/dirty bit. It's not architecturally visible, but you can see
it when you do timing loops.

I've timed it at over a thousand cycles on at least some CPU's, but
that's still peanuts compared to a real page fault. It shouldn't be
*that* noticeable, ie no way it's a 6% regression on its own.

           Linus