[PATCH v3 0/6] 32bit ARM branch predictor hardening

[marc.zyngier@arm.com (Marc Zyngier)]

On 31/01/18 18:53, Florian Fainelli wrote:
> On 01/31/2018 04:45 AM, Hanjun Guo wrote:
>> On 2018/1/29 22:58, Nishanth Menon wrote:
>>> On Mon, Jan 29, 2018 at 5:36 AM, Hanjun Guo <guohanjun@huawei.com> wrote:
>>> [...]
>>>
>>>> By the way, this patch set just enable branch predictor hardening
>>>> on arm32 unconditionally, but some of machines (such as wireless
>>>> network base station) will not be exposed to user to take advantage
>>>> of variant 2, and those machines will be pretty sensitive for
>>>> performance, so can we introduce Kconfig or boot option to disable
>>>> branch predictor hardening as an option?
>>>
>>> I am curious: Have you seen performance degradation with this series?
>>> If yes, is it possible to share the information?
>>
>> Sorry for the late reply, the performance data for context switch (CFS)
>> is about 6%~12% drop (A9 based machine) for the first around test, but
>> the data is not stable, I need to retest then I will update here.
> 
> What tool did you use to measure this? On a Brahma-B15 platform clocked
> at 1.5Ghz, across kernels 4.1, 4.9 (4.15 in progress as we speak), I
> measured the following, with two memory configurations, one giving 256MB
> of usable memory, another giving 3GB of usable memory, results below are
> only the most extreme 256MB case. This is running 13 groups because the
> ASID space is 256bits so this should force at least two full ASID
> generation rollovers (assuming the logic is correct here).
> 
> for i in $(seq 0 9)
> do
> 	hackbench 13 process 10000
> done
> 
> Average values, in seconds:
> 
> 1) 4.1.45, ACTLR[0] = 0, no spectre variant 2 patches: 114,2666
> 2) 4.1.45, ACTLR[0] = 1, no spectre variant 2 patches: 114,2952
> 3) 4.1.45, ACTLR[0] =1 , spectre variant 2 patches: 115,5853
> 
> => 3) is a 1.15% degradation against 1)
> 
> 4.9.51, ACTLR[0] = 0, no spectre variant 2 patches: 130,7676
> 4.9.51, ACTLR[0] = 1, no spectre variant 2 patches: 130,6848
> 4.9.51, ACTLR[0] =1 , spectre variant 2 patches: 132,4274
> 
> => 3) is a 1.26% degradation against 1)
> 
> The relative differences between 4.1 and 4.9 appear consistent (with 4.9
> being slower for a reason I ignore).
> 
> Marc, are there any performance tests/results that you ran that you
> could share?

None. I usually don't run benchmarks, because they are not
representative of a real workload. I urge people to run their own real
workload, as it is very unlikely to have hackbench's profile...

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...