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=-4.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 416FBC433E0 for ; Tue, 21 Jul 2020 13:27:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1D45D21702 for ; Tue, 21 Jul 2020 13:27:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595338028; bh=VMw8YE5x3FLHqpQGkH/mADRhXIT9v2v+BirY/MVEvvA=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=dA6AZ62mRrI1+ZkvhN0HFF5rXtclRnd5+uzF10qhlCli0B3B4X66l3pbiyE1yVjsW NteOz+7bokXWzE7aPXnX8nyMzNbqsjbSlRiu1LDEWZcS/PbEjOUaKYDlcqO0lnFGmL Fv+rpEQwu8avbItXb+uLjYAaJmy0tvEdjuMUyvUI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726312AbgGUN1I (ORCPT ); Tue, 21 Jul 2020 09:27:08 -0400 Received: from mail.kernel.org ([198.145.29.99]:60368 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726014AbgGUN1H (ORCPT ); Tue, 21 Jul 2020 09:27:07 -0400 Received: from devnote2 (NE2965lan1.rev.em-net.ne.jp [210.141.244.193]) (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 9DB0B206E9; Tue, 21 Jul 2020 13:27:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595338026; bh=VMw8YE5x3FLHqpQGkH/mADRhXIT9v2v+BirY/MVEvvA=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=BjmXkpeMj7JySjDEpkGsYN3u59UQW0/R5lAcuVxbLMI4lu8y3U3un1Zbis1T2oWpR SDAVBuGUc9o5iTFEmzgnGXEtvuSA20k1ntorTYvIX7W9ctgzCgmlDQB2umeE1yFWS9 7dgDPUjCRPor0Brr/UVjFZG3Uvo6WPP2W2SEuaLY= Date: Tue, 21 Jul 2020 22:27:01 +0900 From: Masami Hiramatsu To: Guo Ren Cc: Palmer Dabbelt , Paul Walmsley , Oleg Nesterov , linux-riscv , Linux Kernel Mailing List , Anup Patel , linux-csky@vger.kernel.org, Greentime Hu , Zong Li , Patrick =?UTF-8?B?U3TDpGhsaW4=?= , =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , Guo Ren , Pekka Enberg Subject: Re: [PATCH v3 6/7] riscv: Add KPROBES_ON_FTRACE supported Message-Id: <20200721222701.3074315f6a9f6c42c5963f40@kernel.org> In-Reply-To: References: <1594683562-68149-1-git-send-email-guoren@kernel.org> <1594683562-68149-7-git-send-email-guoren@kernel.org> <20200714203757.512ce7fb5fa61a88b1dbb2f3@kernel.org> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-csky-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-csky@vger.kernel.org On Wed, 15 Jul 2020 00:24:54 +0800 Guo Ren wrote: > Thx Masami, > > On Tue, Jul 14, 2020 at 7:38 PM Masami Hiramatsu wrote: > > > > On Mon, 13 Jul 2020 23:39:21 +0000 > > guoren@kernel.org wrote: > > > > > From: Guo Ren > > > > > > This patch adds support for kprobes on ftrace call sites to avoids > > > much of the overhead with regular kprobes. Try it with simple > > > steps: > > > > > > 1. Get _do_fork ftrace call site. > > > Dump of assembler code for function _do_fork: > > > 0xffffffe00020af64 <+0>: addi sp,sp,-128 > > > 0xffffffe00020af66 <+2>: sd s0,112(sp) > > > 0xffffffe00020af68 <+4>: sd ra,120(sp) > > > 0xffffffe00020af6a <+6>: addi s0,sp,128 > > > 0xffffffe00020af6c <+8>: sd s1,104(sp) > > > 0xffffffe00020af6e <+10>: sd s2,96(sp) > > > 0xffffffe00020af70 <+12>: sd s3,88(sp) > > > 0xffffffe00020af72 <+14>: sd s4,80(sp) > > > 0xffffffe00020af74 <+16>: sd s5,72(sp) > > > 0xffffffe00020af76 <+18>: sd s6,64(sp) > > > 0xffffffe00020af78 <+20>: sd s7,56(sp) > > > 0xffffffe00020af7a <+22>: mv s4,a0 > > > 0xffffffe00020af7c <+24>: mv a0,ra > > > 0xffffffe00020af7e <+26>: nop <<<<<<<< here! > > > 0xffffffe00020af82 <+30>: nop > > > 0xffffffe00020af86 <+34>: ld s3,0(s4) > > > > > > 2. Set _do_fork+26 as the kprobe. > > > echo 'p:myprobe _do_fork+26 dfd=%a0 filename=%a1 flags=%a2 mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events > > > echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable > > > cat /sys/kernel/debug/tracing/trace > > > tracer: nop > > > > > > entries-in-buffer/entries-written: 3/3 #P:1 > > > > > > _-----=> irqs-off > > > / _----=> need-resched > > > | / _---=> hardirq/softirq > > > || / _--=> preempt-depth > > > ||| / delay > > > TASK-PID CPU# |||| TIMESTAMP FUNCTION > > > | | | |||| | | > > > sh-87 [000] .... 551.557031: myprobe: (_do_fork+0x1a/0x2e6) dfd=0xffffffe00020af7e filename=0xffffffe00020b34e flags=0xffffffe00101e7c0 mode=0x20af86ffffffe0 > > > > > > cat /sys/kernel/debug/kprobes/list > > > ffffffe00020af7e k _do_fork+0x1a [FTRACE] > > > ^^^^^^ > > > > Hmm, this seems fentry is not supported on RISC-V yet. But anyway, > > it will be useful for users (if they can find the offset). > > Seems only x86 & ⬆️90 use fentry，can you elaborate more about fentry's > benefit and how the user could set kprobe on ftrace call site without > disassemble? On x86, the fentry replaces the mcount with just one call instruction, without saving any arguments. This means all probes which are puts on the address of target symbol, are automatically using ftrace. IOW, all probes on _do_fork+0 will use ftrace. We don't need any disassembling. I think if RISC-V already support "-fpatchable-function-entry=2" option on GCC, you can easily enable it as same as arm64. See https://lkml.org/lkml/2019/6/18/648 Thank you, -- Masami Hiramatsu