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

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Date:	Thu, 3 Mar 2016 20:35:50 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	"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>
Subject: Re: x86 memory barrier: why does Linux prefer MFENCE to Locked ADD?

On Thu, Mar 03, 2016 at 04:34:53PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 03, 2016 at 04:27:39PM +0100, Ingo Molnar wrote:
> > 
> > * 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.
> 
> Lots of additional chatter here:
> 
>   lkml.kernel.org/r/20160112150032-mutt-send-email-mst@...hat.com
> 
> And some useful bits here:
> 
>   lkml.kernel.org/r/56957D54.5000602@...or.com
> 
> latest version here:
> 
>   lkml.kernel.org/r/1453921746-16178-1-git-send-email-mst@...hat.comZ

It's ready as far as I am concerned.
Basically we are just waiting for ack from hpa.

-- 
MST