From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 365D4C4360F for ; Mon, 18 Feb 2019 14:07:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1115A21901 for ; Mon, 18 Feb 2019 14:07:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389339AbfBROHk (ORCPT ); Mon, 18 Feb 2019 09:07:40 -0500 Received: from foss.arm.com ([217.140.101.70]:59262 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389283AbfBROHi (ORCPT ); Mon, 18 Feb 2019 09:07:38 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 084E7A78; Mon, 18 Feb 2019 06:07:34 -0800 (PST) Received: from [10.1.196.50] (e108454-lin.cambridge.arm.com [10.1.196.50]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7A6C33F720; Mon, 18 Feb 2019 06:07:27 -0800 (PST) Subject: Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state To: Julia Cartwright Cc: "linux-arm-kernel@lists.infradead.org" , "bigeasy@linutronix.de" , "Dave.Martin@arm.com" , "linux-rt-users@vger.kernel.org" , "catalin.marinas@arm.com" , "will.deacon@arm.com" , "ard.biesheuvel@linaro.org" , "linux-kernel@vger.kernel.org" References: <20190208165513.8435-1-julien.grall@arm.com> <20190212171350.GB1002@jcartwri.amer.corp.natinst.com> From: Julien Grall Message-ID: <7671d868-9d33-dfe1-851a-b72271f12d5f@arm.com> Date: Mon, 18 Feb 2019 14:07:26 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190212171350.GB1002@jcartwri.amer.corp.natinst.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/02/2019 17:13, Julia Cartwright wrote: > Hello Julien- Hello Julien, > > On Fri, Feb 08, 2019 at 04:55:13PM +0000, Julien Grall wrote: >> When the kernel is compiled with CONFIG_KERNEL_MODE_NEON, some part of >> the kernel may be able to use FPSIMD/SVE. This is for instance the case >> for crypto code. >> >> Any use of FPSIMD/SVE in the kernel are clearly marked by using the >> function kernel_neon_{begin, end}. Furthermore, this can only be used >> when may_use_simd() returns true. >> >> The current implementation of may_use_simd() allows softirq to use >> FPSIMD/SVE unless it is currently in used (i.e kernel_neon_busy is true). >> When in used, softirqs usually fallback to a software method. >> >> At the moment, as a softirq may use FPSIMD/SVE, softirqs are disabled >> when touching the FPSIMD/SVE context. This has the drawback to disable >> all softirqs even if they are not using FPSIMD/SVE. >> >> As a softirq should not rely on been able to use simd at a given time, >> there are limited reason to keep softirq disabled when touching the >> FPSIMD/SVE context. Instead, we can only disable preemption and tell >> the NEON unit is currently in use. >> >> This patch introduces two new helpers kernel_neon_{disable, enable} to >> mark the area using FPSIMD/SVE context and use them in replacement of >> local_bh_{disable, enable}. The functions kernel_neon_{begin, end} are >> also re-implemented to use the new helpers. >> >> Signed-off-by: Julien Grall >> >> --- >> >> I have been exploring this solution as an alternative approach to the RT >> patch "arm64: fpsimd: use preemp_disable in addition to local_bh_disable()". >> >> So far, the patch has only been lightly tested. >> >> For RT-linux, it might be possible to use migrate_{enable, disable}. I >> am quite new with RT and have some trouble to understand the semantics >> of migrate_{enable, disable}. So far, I am still unsure if it is possible >> to run another userspace task on the same CPU while getting preempted >> when the migration is disabled. > > Yes, a thread in a migrate_disable()-protected critical section can be > preempted, and another thread on the same CPU may enter the critical > section. > > If it's necessary to prevent this from happening, then you need to also > make use of a per-CPU mutex. On RT, this would do the right thing > w.r.t. priority inheritance. Thank you for the explanation, I now understand better the concept of migrate_disable. Best regards, -- Julien Grall