Re: [PATCH v2] mm: memory: check userfaultfd_wp() in vmf_orig_pte_uffd_wp()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/4/19 0:32, Peter Xu wrote:
> Hi, Kefeng,
> 
> On Thu, Apr 18, 2024 at 08:06:41PM +0800, Kefeng Wang wrote:
>> Add userfaultfd_wp() check in vmf_orig_pte_uffd_wp() to avoid the
>> unnecessary pte_marker_entry_uffd_wp() in most pagefault, difference
>> as shows below from perf data of lat_pagefault, note, the function
>> vmf_orig_pte_uffd_wp() is not inlined in the two kernel versions.
>>
>>    perf report -i perf.data.before | grep vmf
>>       0.17%     0.13%  lat_pagefault  [kernel.kallsyms]      [k] vmf_orig_pte_uffd_wp.part.0.isra.0
>>    perf report -i perf.data.after  | grep vmf
> 
> Any real number to share too besides the perf greps?  I meant, even if perf
> report will not report such function anymore, it doesn't mean it'll be
> faster, and how much it improves?

dd if=/dev/zero of=/tmp/XXX bs=512M count=1
./lat_pagefault -W 5 -N 5 /tmp/XXX

	before		after	
1	0.2623		0.2605	
2	0.2622		0.2598	
3	0.2621		0.2595	
4	0.2622		0.2600	
5	0.2651		0.2598	
6	0.2624		0.2594	
7	0.2624		0.2605	
8	0.2627		0.2608	
average	0.262675	0.2600375	-0.0026375

The lat_pagefault does show some improvement(also I reboot and retest,
the results are same).

> 
> Now we're switching from pte_marker_uffd_wp() check into a vma flag check.
> I think it makes more sense to compare the number rather than the perf
> reports, as the vma flag check instructions will be buried under other
> entries IIUC.
> 
> Thanks,
>