linux-kernel - Re: [x86/mm/tlb] 6035152d8e: will-it-scale.per_thread

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Date:   Thu, 17 Mar 2022 17:16:18 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <namit@...are.com>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "lkp@...ts.01.org" <lkp@...ts.01.org>,
        "lkp@...el.com" <lkp@...el.com>,
        "ying.huang@...el.com" <ying.huang@...el.com>,
        "feng.tang@...el.com" <feng.tang@...el.com>,
        "zhengjun.xing@...ux.intel.com" <zhengjun.xing@...ux.intel.com>,
        "fengwei.yin@...el.com" <fengwei.yin@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [x86/mm/tlb] 6035152d8e: will-it-scale.per_thread_ops -13.2%
 regression

On 3/17/22 13:32, Nadav Amit wrote:
> I’m not married to this patch, but before a revert it would be good
> to know why it even matters. I wonder whether you can confirm that
> reverting the patch (without the rest of the series) even helps. If
> it does, I’ll try to run some tests to understand what the heck is
> going on.

I went back and tested on a "Intel(R) Core(TM) i7-8086K CPU @ 4.00GHz"
which is evidently a 6-core "Coffee Lake".  It needs retpolines:

> /sys/devices/system/cpu/vulnerabilities/spectre_v2:Mitigation: Full
generic retpoline, IBPB: conditional, IBRS_FW, STIBP: conditional, RSB
filling

I ran the will-it-scale test:

	./malloc1_threads -s 30 -t 12

and took the 30-second average "ops/sec" at the two commits:

	4c1ba3923e:197876
	6035152d8e:199367 +0.75%

Where bigger is better.  So, a small win, but probably mostly in the
noise.  The number of IPIs definitely went up, probably 3-4% to get that
win.

IPI costs go up the more threads you throw at it.  The retpolines do
too, though because you do *more* of them.  Systems with no retpolines
get hit harder by the IPI costs and have no upsides from removing the
retpoline.

So, we've got a small (<1%, possibly zero) win on the bulk of systems
(which have retpolines).  Newer, retpoline-free systems see a
double-digit regression.  The bigger the system, the bigger the
regression (probably).

I tend to think the bigger regression wins and we should probably revert
the patch, or at least back out its behavior.

Nadav, do you have some different data or a different take?