From mboxrd@z Thu Jan 1 00:00:00 1970 From: f.fainelli@gmail.com (Florian Fainelli) Date: Wed, 31 Jan 2018 10:53:53 -0800 Subject: [PATCH v3 0/6] 32bit ARM branch predictor hardening In-Reply-To: References: <20180125152139.32431-1-marc.zyngier@arm.com> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 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? -- Florian