linux-kernel - Re: [PATCH 2/3] x86: mm: Change tlb_flushall

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Date:	Thu, 12 Dec 2013 15:11:47 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Alex Shi <alex.shi@...aro.org>
Cc:	Mel Gorman <mgorman@...e.de>, H Peter Anvin <hpa@...or.com>,
	Linux-X86 <x86@...nel.org>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 2/3] x86: mm: Change tlb_flushall_shift for IvyBridge


* Alex Shi <alex.shi@...aro.org> wrote:

> On 12/12/2013 09:13 PM, Ingo Molnar wrote:
> > 
> > * Mel Gorman <mgorman@...e.de> wrote:
> > 
> >> There was a large performance regression that was bisected to commit 611ae8e3
> >> (x86/tlb: enable tlb flush range support for x86). This patch simply changes
> >> the default balance point between a local and global flush for IvyBridge.
> >>
> >> Signed-off-by: Mel Gorman <mgorman@...e.de>
> >> ---
> >>  arch/x86/kernel/cpu/intel.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> >> index dc1ec0d..2d93753 100644
> >> --- a/arch/x86/kernel/cpu/intel.c
> >> +++ b/arch/x86/kernel/cpu/intel.c
> >> @@ -627,7 +627,7 @@ static void intel_tlb_flushall_shift_set(struct cpuinfo_x86 *c)
> >>  		tlb_flushall_shift = 5;
> >>  		break;
> >>  	case 0x63a: /* Ivybridge */
> >> -		tlb_flushall_shift = 1;
> >> +		tlb_flushall_shift = 2;
> >>  		break;
> > 
> > I'd not be surprised if other CPU models showed similar weaknesses 
> > under ebizzy as well.
> > 
> > I don't particularly like the tuning aspect of the whole feature: the 
> > tunings are model specific and they seem to come out of thin air, 
> > without explicit measurements visible.
> > 
> > In particular the first commit that added this optimization:
> > 
> >  commit c4211f42d3e66875298a5e26a75109878c80f15b
> >  Date:   Thu Jun 28 09:02:19 2012 +0800
> > 
> >     x86/tlb: add tlb_flushall_shift for specific CPU
> > 
> > already had these magic tunings, with no explanation about what kind 
> > of measurement was done to back up those tunings.
> > 
> > I don't think this is acceptable and until this is cleared up I think 
> > we might be better off turning off this feature altogether, or making 
> > a constant, very low tuning point.
> > 
> > The original code came via:
> > 
> >   611ae8e3f520 x86/tlb: enable tlb flush range support for x86
> > 
> > which references a couple of benchmarks, in particular a 
> > micro-benchmark:
> > 
> >   My micro benchmark 'mummap' http://lkml.org/lkml/2012/5/17/59
> >   show that the random memory access on other CPU has 0~50% speed up
> >   on a 2P * 4cores * HT NHM EP while do 'munmap'.
> > 
> > if the tunings were done with the micro-benchmark then I think they 
> > are bogus, because AFAICS it does not measure the adversarial case of 
> > the optimization.

You have not replied to this concern of mine: if my concern is valid 
then that invalidates much of the current tunings.

> > So I'd say at minimum we need to remove the per model tunings, and 
> > need to use very conservative defaults, to make sure we don't slow 
> > down reasonable workloads.
> 
> I also hate to depends on mysterious hardware differentiation. But 
> there do have some changes in tlb/cache part on different Intel 
> CPU.(Guess HPA know this more). And the different shift value get 
> from testing not from air. :)

As far as I could see from the changelogs and the code itself the 
various tunings came from nowhere.

So I don't see my concerns addressed. My inclination would be to start 
with something like Mel's known-good tuning value below, we know that 
ebizzy does not regress with that setting. Any more aggressive tuning 
needs to be backed up with ebizzy-alike adversarial workload 
performance numbers.

Thanks,

	Ingo

(Patch totally untested.)

=============>
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index dc1ec0d..c98385d 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -614,23 +614,8 @@ static void intel_tlb_flushall_shift_set(struct cpuinfo_x86 *c)
 	case 0x61d: /* six-core 45 nm xeon "Dunnington" */
 		tlb_flushall_shift = -1;
 		break;
-	case 0x61a: /* 45 nm nehalem, "Bloomfield" */
-	case 0x61e: /* 45 nm nehalem, "Lynnfield" */
-	case 0x625: /* 32 nm nehalem, "Clarkdale" */
-	case 0x62c: /* 32 nm nehalem, "Gulftown" */
-	case 0x62e: /* 45 nm nehalem-ex, "Beckton" */
-	case 0x62f: /* 32 nm Xeon E7 */
-		tlb_flushall_shift = 6;
-		break;
-	case 0x62a: /* SandyBridge */
-	case 0x62d: /* SandyBridge, "Romely-EP" */
-		tlb_flushall_shift = 5;
-		break;
-	case 0x63a: /* Ivybridge */
-		tlb_flushall_shift = 1;
-		break;
 	default:
-		tlb_flushall_shift = 6;
+		tlb_flushall_shift = 2;
 	}
 }
 
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