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Tue, 14 Sep 2021 15:17:33 +0000 Date: Tue, 14 Sep 2021 16:17:24 +0100 From: Joey Gouly To: Mark Rutland Cc: Zhen Lei , Catalin Marinas , Will Deacon , linux-arm-kernel , linux-kernel@vger.kernel.org, nd@arm.com Subject: Re: [PATCH] arm64: entry: Improve the performance of system calls Message-ID: <20210914151724.GA34977@e124191.cambridge.arm.com> References: <20210903121950.2284-1-thunder.leizhen@huawei.com> <20210914095436.GA26544@C02TD0UTHF1T.local> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210914095436.GA26544@C02TD0UTHF1T.local> User-Agent: Mutt/1.9.4 (2018-02-28) X-ClientProxiedBy: SN7PR04CA0076.namprd04.prod.outlook.com (2603:10b6:806:121::21) To DB8PR08MB5433.eurprd08.prod.outlook.com (2603:10a6:10:118::13) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from e124191.cambridge.arm.com (217.140.106.55) by SN7PR04CA0076.namprd04.prod.outlook.com (2603:10b6:806:121::21) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4500.16 via Frontend Transport; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Sep 2021 15:17:49.5048 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 5b24935f-7034-430f-674f-08d97792d078 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: VE1EUR03FT033.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB6PR0801MB1781 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Tue, Sep 14, 2021 at 10:55:16AM +0100, Mark Rutland wrote: > Hi, > > At a high-level, I'm not too keen on special-casing things unless > necessary. > > I wonder if we could get similar results without special-casing by using > a static const array of handlers indexed by the EC, since (with GCC > 11.1.0 from the kernel.org crosstool page) that can result in code like: > > 0000000000001010 : > 1010: d503245f bti c > 1014: d503233f paciasp > 1018: a9bf7bfd stp x29, x30, [sp, #-16]! > 101c: 910003fd mov x29, sp > 1020: d5385201 mrs x1, esr_el1 > 1024: 90000002 adrp x2, 0 > 1028: 531a7c23 lsr w3, w1, #26 > 102c: 91000042 add x2, x2, #:lo12: > 1030: f8637842 ldr x2, [x2, x3, lsl #3] > 1034: d63f0040 blr x2 > 1038: a8c17bfd ldp x29, x30, [sp], #16 > 103c: d50323bf autiasp > 1040: d65f03c0 ret > > ... which might do better by virtue of reducing a chain of potential > mispredicts down to a single potential mispredict, and dynamic branch > prediction hopefully does a good job of predicting the common case at > runtime. That said, the resulting tables will be pretty big... I tested Mark's branch which implements this (found at https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/entry/switch-table) I also took lmbench from https://github.com/intel/lmbench.git and built `lat_syscall` with: gcc lat_syscall.c lib_*.c -l m -o lat_syscall -static These are the results I got from benchmarking on my MacBook Air M1, with the following command: ./lat_syscall null &> /dev/null ; uname -a ; for i in 0 1 2 3 4 ; do ./lat_syscall null ; done The kernel was based on arm64_defconfig that was then stripped of as much as possible. GCC 11.1.0 from kernel.org crosstool page. Clang build fom git b041b613e6fff713fc9ad6dbc73024286fb2fc93. gcc: master: 0.14300 switch-table: 0.14350 likely: 0.13962 clang: master: 0.14354 switch-table: 0.14642 likely: 0.14256 The generated code looks similar to what Leizhen has posted, so I didn't post it again. So it seems the table approach actually performs worse in my testing, and Leizhen's approach is slightly better than master (d0ee23f9d78be5531c4b055ea424ed0b489dfe9b). Thanks, Joey