linux-kernel - Re: x86 memory barrier: why does Linux prefer MFENCE to Locked ADD?

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Date:	Thu, 3 Mar 2016 16:27:39 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Dexuan Cui <decui@...rosoft.com>
Cc:	"linux-x86_64@...r.kernel.org" <linux-x86_64@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: x86 memory barrier: why does Linux prefer MFENCE to Locked ADD?


* Dexuan Cui <decui@...rosoft.com> wrote:

> Hi,
> My understanding about arch/x86/include/asm/barrier.h is: obviously Linux
> more likes {L,S,M}FENCE -- Locked ADD is only used in x86_32 platforms that
> don't support XMM2.
> 
> However, it looks people say Locked Add is much faster than the FENCE
> instructions, even on modern Intel CPUs like Haswell, e.g., please see
> the three sources:
> 
> " 11.5.1 Locked Instructions as Memory Barriers
> Optimization
> Use locked instructions to implement Store/Store and Store/Load barriers.
> "
> http://support.amd.com/TechDocs/47414_15h_sw_opt_guide.pdf
> 
> "lock addl %(rsp), 0 is a better solution for StoreLoad barrier ":
> http://shipilev.net/blog/2014/on-the-fence-with-dependencies/
> 
> "...locked instruction are more efficient barriers...":
> http://www.pvk.ca/Blog/2014/10/19/performance-optimisation-~-writing-an-essay/
> 
> I also found that FreeBSD prefers Locked Add.
> 
> So, I'm curious why Linux prefers MFENCE.
> I guess I may be missing something.
> 
> I tried to google the question, but didn't find an answer.

It's being worked on, see this thread on lkml from a few weeks ago:

   C Jan 13 Michael S. Tsir    | [PATCH v3 0/4] x86: faster mb()+documentation tweaks
   C Jan 13 Michael S. Tsir    | ├─>[PATCH v3 1/4] x86: add cc clobber for addl
   C Jan 13 Michael S. Tsir    | ├─>[PATCH v3 2/4] x86: drop a comment left over from X86_OOSTORE
   C Jan 13 Michael S. Tsir    | ├─>[PATCH v3 3/4] x86: tweak the comment about use of wmb for IO
   C Jan 13 Michael S. Tsir    | ├─>[PATCH v3 4/4] x86: drop mfence in favor of lock+addl

The 4th patch changes MFENCE to a LOCK ADDL locked instruction.

Thanks,

	Ingo