Re: 回复: [PATCH] mm: optimize memblock_add_range() for improved performance

[Mike Rapoport <rppt@kernel.org>]

Hi Stephen,

Please don't top post on Linux kernel mailing list.

On Fri, Feb 07, 2025 at 04:03:31PM +0000, Stephen Eta Zhou wrote:
> Hi Mike,
> 
> Given the concerns raised, I would greatly appreciate your guidance on
> whether you think it's worthwhile for me to continue with this approach.
> Should I proceed with further refinements and testing, or would you
> recommend a different direction for optimization? Your input will be
> invaluable in ensuring this patch meets both performance and stability
> goals.

I doubt any optimization for memblock_add_range() will really improve boot
time and even produce a measurable speed up.

With all the fragility and potential of breakage, I don't think it's
worthwhile to attempt optimizations there.
 
> Best regards,  
> Stephen
> ________________________________________
> 发件人: Mike Rapoport <rppt@kernel.org>
> 发送时间: 2025年2月7日 22:58
> 收件人: Stephen Eta Zhou <stephen.eta.zhou@outlook.com>
> 抄送: akpm@linux-foundation.org <akpm@linux-foundation.org>; linux-mm@kvack.org <linux-mm@kvack.org>; linux-kernel@vger.kernel.org <linux-kernel@vger.kernel.org>
> 主题: Re: [PATCH] mm: optimize memblock_add_range() for improved performance
>  
> Hi Stephen,
> 
> On Wed, Feb 05, 2025 at 05:55:50AM +0000, Stephen Eta Zhou wrote:
> > Hi Mike Rapoport、Andrew Morton
> 
> > I have recently been researching the mm subsystem of the Linux kernel,
> > and I came across the memblock_add_range function, which piqued my
> > interest. I found the implementation approach quite interesting, so I
> > analyzed it and identified some areas for optimization. Starting with
> > this part of the code:
> >
> > if (type->cnt * 2 + 1 <= type->max)
> >       insert = true;
> > The idea here is good, but it has a certain flaw. The condition is rather
> > restrictive, and it cannot be executed initially. Moreover, it is only
> > valid when the remaining space is (2/1) + 1. If there is enough memory,
> > but it does not satisfy (2/1) + 1, the insertion operation still needs to
> > be performed twice.
> 
> The code in memblock_add_range() is very fragile, and many attempts to
> remove the second pass that looked correct at the first glance failed for
> some corner case.
> 
> Unfortunately, it's impossible to capture all possible memory
> configurations and reservations in the memblock test suite, so even it it
> passes, there is a chance the kernel will fail to boot on an actual HW.
>  
> > - Before the patch:
> >   - Average: 1.22%
> >   - Max: 1.63%, Min: 0.93%
> >
> > - After the patch:
> >   - Average: 0.69%
> >   - Max: 0.94%, Min: 0.50%
> >
> 
> These numbers do not represent what's actually interesting: the boot time
> speedup.
> 
> --
> Sincerely yours,
> Mike.

-- 
Sincerely yours,
Mike.