linux-kernel - Re: [PATCH] x86: add back the alignment of the destination to 8 bytes in copy_user

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Message-ID: <Z9axe9Ac5biyJjCC@gmail.com>
Date: Sun, 16 Mar 2025 12:09:47 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Herton Krzesinski <hkrzesin@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, olichtne@...hat.com,
	atomasov@...hat.com, aokuliar@...hat.com
Subject: Re: [PATCH] x86: add back the alignment of the destination to 8
 bytes in copy_user_generic()


* Ingo Molnar <mingo@...nel.org> wrote:

> > It does look good in my testing here, I built same kernel I was 
> > using for testing the original patch (based on 6.14-rc6), this is 
> > one of the results I got in one of the runs testing on the same 
> > machine:
> > 
> >              CPU      RATE          SYS          TIME     sender-receiver
> > Server bind   19: 20.8Gbits/sec 14.832313000 20.863476111 75.4%-89.2%
> > Server bind   21: 18.0Gbits/sec 18.705221000 23.996913032 80.8%-89.7%
> > Server bind   23: 20.1Gbits/sec 15.331761000 21.536657212 75.0%-89.7%
> > Server bind none: 24.1Gbits/sec 14.164226000 18.043132731 82.3%-87.1%
> > 
> > There are still some variations between runs, which is expected as 
> > was the same when I tested my patch or in the not aligned case, but 
> > it's consistently better/higher than the no align case. Looks 
> > really it's sufficient to align for the higher than or equal 64 
> > bytes copy case.
> 
> Mind sending a v2 patch with a changelog and these benchmark numbers 
> added in, and perhaps a Co-developed-by tag with Linus or so?

BTW., if you have a test system available, it would be nice to test a 
server CPU in the Intel spectrum as well. (For completeness mostly, I'd 
not expect there to be as much alignment sensitivity.)

The CPU you tested, AMD Epyc 7742 was launched ~6 years ago so it's 
still within the window of microarchitectures we care about. An Intel 
test would be nice from a similar timeframe as well. Older is probably 
better in this case, but not too old. :-)

( Note that the Intel test is not required to apply the fix IMO - we 
  did change alignment patterns ~2 years ago in a5624566431d which 
  regressed. )

Thanks,

	Ingo