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

[Minchan Kim]

On Mon, Jun 13, 2016 at 05:02:15PM +0800, Huang, Ying wrote:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
> 
> > 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.
> 
> I done some simple counting, and found that about 3.15e9 PTE are set to
> old during the test after the commit.  This may interpret the user_time
> increase as below, because these accessed bit microfault is accounted as
> user time.
> 
>     387.66 .  0%      +5.4%     408.49 .  0%  unixbench.time.user_time
> 
> I also make a one line debug patch as below on top of the commit to set
> the PTE to young unconditionally, which recover the regression.

With this patch, meminfo.Active(file) is almost same unlike previous
experiment?

> 
> modified   mm/filemap.c
> @@ -2193,7 +2193,7 @@ repeat:
>  		if (file->f_ra.mmap_miss > 0)
>  			file->f_ra.mmap_miss--;
>  		addr = address + (page->index - vmf->pgoff) * PAGE_SIZE;
> -		do_set_pte(vma, addr, page, pte, false, false, true);
> +		do_set_pte(vma, addr, page, pte, false, false, false);
>  		unlock_page(page);
>  		atomic64_inc(&old_pte_count);
>  		goto next;
> 
> Best Regards,
> Huang, Ying