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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id B75A8C4332F for ; Thu, 21 Apr 2022 15:15:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=qcDoYl/+Dgjvx+EgWgB217mSca25iZKe0+nY66mspmQ=; b=wvdDdhFtfYMtyo qldg3BuH1CxqET7QtPM824TbIfW3DIqnQrzOtummZpKO9ecR7UKMcwN5LB+54UtxCtnbqwrcIfarP i+swtumWpWNzwK7h4NETgS8Kh0K8HC7Q46EbVP7J37qwI84He9xuXEHEN6WSuwv5o+/LTmwcYnshw ZO1SVEEGepJbhsZqovtZeMbglpxtWSIy8m598gyP03zTXktV5BXeffB3dKaYdyIXEsWh01JaqTnMN OTXEXLXr+2LA6/Rt+lNiFYAUdD1+aplYL6J7HKS7EB1Zv2qrqwrp1dWZzsIrvHz9yRbQOEEOQ9Nh8 ViC1lLJrum+DoVbkRBHA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1nhYVi-00DxrU-Ea; Thu, 21 Apr 2022 15:14:22 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1nhYVe-00DxqX-Uv for linux-arm-kernel@lists.infradead.org; Thu, 21 Apr 2022 15:14:20 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 505791515; Thu, 21 Apr 2022 08:14:17 -0700 (PDT) Received: from lakrids (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D9F963F73B; Thu, 21 Apr 2022 08:14:15 -0700 (PDT) Date: Thu, 21 Apr 2022 16:14:13 +0100 From: Mark Rutland To: Steven Rostedt Cc: Wang ShaoBo , cj.chengjian@huawei.com, huawei.libin@huawei.com, xiexiuqi@huawei.com, liwei391@huawei.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, zengshun.wu@outlook.com Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines Message-ID: References: <20220316100132.244849-1-bobo.shaobowang@huawei.com> <20220316100132.244849-4-bobo.shaobowang@huawei.com> <20220421100639.03c0d123@gandalf.local.home> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20220421100639.03c0d123@gandalf.local.home> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220421_081419_077631_7084EDA5 X-CRM114-Status: GOOD ( 27.61 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Apr 21, 2022 at 10:06:39AM -0400, Steven Rostedt wrote: > On Thu, 21 Apr 2022 14:10:04 +0100 > Mark Rutland wrote: > > > On Wed, Mar 16, 2022 at 06:01:31PM +0800, Wang ShaoBo wrote: > > > From: Cheng Jian > > > > > > When tracing multiple functions customly, a list function is called > > > in ftrace_(regs)_caller, which makes all the other traced functions > > > recheck the hash of the ftrace_ops when tracing happend, apparently > > > it is inefficient. > > > > ... and when does that actually matter? Who does this and why? > > I don't think it was explained properly. What dynamically allocated > trampolines give you is this. Thanks for the, explanation, btw! > Let's say you have 10 ftrace_ops registered (with bpf and kprobes this can > be quite common). But each of these ftrace_ops traces a function (or > functions) that are not being traced by the other ftrace_ops. That is, each > ftrace_ops has its own unique function(s) that they are tracing. One could > be tracing schedule, the other could be tracing ksoftirqd_should_run > (whatever). Ok, so that's when messing around with bpf or kprobes, and not generally when using plain old ftrace functionality under /sys/kernel/tracing/ (unless that's concurrent with one of the former, as per your other reply) ? > Without this change, because the arch does not support dynamically > allocated trampolines, it means that all these ftrace_ops will be > registered to the same trampoline. That means, for every function that is > traced, it will loop through all 10 of theses ftrace_ops and check their > hashes to see if their callback should be called or not. Sure; I can see how that can be quite expensive. What I'm trying to figure out is who this matters to and when, since the implementation is going to come with a bunch of subtle/fractal complexities, and likely a substantial overhead too when enabling or disabling tracing of a patch-site. I'd like to understand the trade-offs better. > With dynamically allocated trampolines, each ftrace_ops will have their own > trampoline, and that trampoline will be called directly if the function > is only being traced by the one ftrace_ops. This is much more efficient. > > If a function is traced by more than one ftrace_ops, then it falls back to > the loop. I see -- so the dynamic trampoline is just to get the ops? Or is that doing additional things? There might be a middle-ground here where we patch the ftrace_ops pointer into a literal pool at the patch-site, which would allow us to handle this atomically, and would avoid the issues with out-of-range trampolines. Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel