Re: [RFC 2/2] khugepaged: use upgrade_read() to optimize collapse_huge_page

Matthew Wilcox
2024-10-17T13:20:17Z

On Thu, Oct 17, 2024 at 02:18:41PM +0800, lizhe.67@bytedance.com wrote:
> On Wed, 16 Oct 2024 12:53:15 +0100, willy@infradead.org wrote:
>
> >On Wed, Oct 16, 2024 at 12:36:00PM +0800, lizhe.67@bytedance.com wrote:
> >> From: Li Zhe <lizhe.67@bytedance.com>
> >>
> >> In function collapse_huge_page(), we drop mmap read lock and get
> >> mmap write lock to prevent most accesses to pagetables. There is
> >> a small time window to allow other tasks to acquire the mmap lock.
> >> With the use of upgrade_read(), we don't need to check vma and pmd
> >> again in most cases.
> >
> >This is clearly a performance optimisation. So you must have some
> >numebrs that justify this, please include them.
>
> Yes, I will add the relevant data to v2 patch.

How about telling us all now so we know whether to continue discussing
this?