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=-10.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 35999C433E6 for ; Tue, 1 Sep 2020 09:25:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 142C32087D for ; Tue, 1 Sep 2020 09:25:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725989AbgIAJZ4 (ORCPT ); Tue, 1 Sep 2020 05:25:56 -0400 Received: from mail.kernel.org ([198.145.29.99]:48160 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726091AbgIAJZ4 (ORCPT ); Tue, 1 Sep 2020 05:25:56 -0400 Received: from gaia (unknown [46.69.195.127]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 8FE6D206A5; Tue, 1 Sep 2020 09:25:54 +0000 (UTC) Date: Tue, 1 Sep 2020 10:25:52 +0100 From: Catalin Marinas To: Shuah Khan Cc: Mark Brown , Will Deacon , linux-arm-kernel@lists.infradead.org, Vincenzo Frascino , linux-kselftest@vger.kernel.org, Shuah Khan Subject: Re: [PATCH v3 0/5] arm64: vdso: getcpu() support Message-ID: <20200901092551.GG5561@gaia> References: <20200819121318.52158-1-broonie@kernel.org> <315d97af-715a-9942-a731-11de2fbbbded@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <315d97af-715a-9942-a731-11de2fbbbded@linuxfoundation.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kselftest-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On Mon, Aug 31, 2020 at 03:47:17PM -0600, Shuah Khan wrote: > On 8/19/20 6:13 AM, Mark Brown wrote: > > Some applications, especially tracing ones, benefit from avoiding the > > syscall overhead for getcpu() so it is common for architectures to have > > vDSO implementations. Add one for arm64, using TPIDRRO_EL0 to pass a > > pointer to per-CPU data rather than just store the immediate value in > > order to allow for future extensibility. > > > > It is questionable if something TPIDRRO_EL0 based is worthwhile at all > > on current kernels, since v4.18 we have had support for restartable > > sequences which can be used to provide a sched_getcpu() implementation > > with generally better performance than the vDSO approach on > > architectures which have that[1]. Work is ongoing to implement this for > > glibc: > > > > https://lore.kernel.org/lkml/20200527185130.5604-3-mathieu.desnoyers@efficios.com/ > > > > but is not yet merged and will need similar work for other userspaces. > > The main advantages for the vDSO implementation are the node parameter > > (though this is a static mapping to CPU number so could be looked up > > separately when processing data if it's needed, it shouldn't need to be > > in the hot path) and ease of implementation for users. > > > > This is currently not compatible with KPTI due to the use of TPIDRRO_EL0 > > by the KPTI trampoline, this could be addressed by reinitializing that > > system register in the return path but I have found it hard to justify > > adding that overhead for all users for something that is essentially a > > profiling optimization which is likely to get superceeded by a more > > modern implementation - if there are other uses for the per-CPU data > > then the balance might change here. > > > > This builds on work done by Kristina Martsenko some time ago but is a > > new implementation. > > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d7822b1e24f2df5df98c76f0e94a5416349ff759 > > > > v3: > > - Rebase on v5.9-rc1. > > - Drop in progress portions of the series. > > v2: > > - Rebase on v5.8-rc3. > > - Add further cleanup patches & a first draft of multi-page support. > > > > Mark Brown (5): > > arm64: vdso: Provide a define when building the vDSO > > arm64: vdso: Add per-CPU data > > arm64: vdso: Initialise the per-CPU vDSO data > > arm64: vdso: Add getcpu() implementation > > selftests: vdso: Support arm64 in getcpu() test > > > > arch/arm64/include/asm/processor.h | 12 +---- > > arch/arm64/include/asm/vdso/datapage.h | 54 +++++++++++++++++++ > > arch/arm64/kernel/process.c | 26 ++++++++- > > arch/arm64/kernel/vdso.c | 33 +++++++++++- > > arch/arm64/kernel/vdso/Makefile | 4 +- > > arch/arm64/kernel/vdso/vdso.lds.S | 1 + > > arch/arm64/kernel/vdso/vgetcpu.c | 48 +++++++++++++++++ > > .../testing/selftests/vDSO/vdso_test_getcpu.c | 10 ++++ > > 8 files changed, 172 insertions(+), 16 deletions(-) > > create mode 100644 arch/arm64/include/asm/vdso/datapage.h > > create mode 100644 arch/arm64/kernel/vdso/vgetcpu.c > > > > Patches look good to me from selftests perspective. My acked by > for these patches to go through arm64. > > Acked-by: Shuah Khan > > If you would like me to take these through kselftest tree, give > me your Acks. I can queue these up for 5.10-rc1 Thanks Shuah for the ack. We are still pondering whether the merge these patches as they have some limitations (the per-CPU data structures may not fit in the sole data vDSO page). -- Catalin