linux-kernel - Re: [PATCH] mm: eliminate function call overhead during copy_page

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Date:   Mon, 6 Feb 2023 09:17:59 +0000
From:   Hao Lee <haolee.swjtu@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: eliminate function call overhead during
 copy_page_range()

On Sun, Feb 05, 2023 at 09:53:53PM +0000, Matthew Wilcox wrote:
> On Sun, Feb 05, 2023 at 03:06:02PM +0000, Hao Lee wrote:
> > vm_normal_page() is called so many times that its overhead is very high.
> > After changing this call site to an inline function, copy_page_range()
> > runs 3~5 times faster than before.
> 
> So you're saying that your compiler is making bad decisions?  What
> architecture, what compiler, what version?  Do you have
> CONFIG_ARCH_HAS_PTE_SPECIAL set?
> 
> Is there something about inlining it that makes the compiler able to
> optimise away code, or is it really the function call overhead?  Can
> you share any perf results?

I am so embarrassed; I forgot to disable function_graph when timing the
non-inlined function so my test was interfered. And the actual
performance improvement is only ~3%.
Please ignore this patch. Sorry...

>