From patchwork Wed Aug 7 17:28:28 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Steven Rostedt X-Patchwork-Id: 11082531 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 2466B13A4 for ; Wed, 7 Aug 2019 17:29:36 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 1212F28A5E for ; Wed, 7 Aug 2019 17:29:36 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 0350F28A62; Wed, 7 Aug 2019 17:29:36 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 697DE28A5E for ; Wed, 7 Aug 2019 17:29:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:Subject:To:From :Date:Message-Id:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To: List-Owner; bh=HTVYMp25Ufhc2B+S/PBRDlAX1olo+xRUy8hwCnVN3Pk=; b=kO+wYvNbiKgilL 5UNXruwPH1fm/Ut97TW9NO7+DhSQ7sqQ7dNDiCXQemxOSoq7PYZVg0bwZ2HFOW1/idrYTn1dZ9C+M IfK1HUaB41qxRJwURQvs2KHgNpo6lmGXKFh5Fp1ONd6bLUBRjUH4atpOZzgaeAXV1fgwNSnMFDKJg jCK/+HLwV3z9nWOdyo16O9WAIOQEjCL7TGFaNBIKnwHmljOdjt5c0k7k2xzVBz5MkoagiIjbEQ6ph VZ8AvOa5rFooqMIw6fyAqiuNPuvgVlAHfOwPyiQGl7w3EA4Bw+3AW6uz3uM/lmAhisZViijy5olt9 gnQuNWbaILaZVizT3kGA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hvPkj-0007F9-1Q; Wed, 07 Aug 2019 17:29:33 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1hvPkK-0006qX-JW for linux-arm-kernel@lists.infradead.org; Wed, 07 Aug 2019 17:29:11 +0000 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (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 48C7322305; Wed, 7 Aug 2019 17:29:08 +0000 (UTC) Received: from rostedt by gandalf.local.home with local (Exim 4.92) (envelope-from ) id 1hvPkJ-0007zy-De; Wed, 07 Aug 2019 13:29:07 -0400 Message-Id: <20190807172907.310138647@goodmis.org> User-Agent: quilt/0.65 Date: Wed, 07 Aug 2019 13:28:28 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org Subject: [PATCH 2/2 v2] tracing: Document the stack trace algorithm in the comments References: <20190807172826.352574408@goodmis.org> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190807_102908_680995_1E56F499 X-CRM114-Status: GOOD ( 16.83 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Jiping Ma , catalin.marinas@arm.com, will.deacon@arm.com, mingo@redhat.com, Joel Fernandes , linux-arm-kernel@lists.infradead.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Virus-Scanned: ClamAV using ClamSMTP From: "Steven Rostedt (VMware)" As the max stack tracer algorithm is not that easy to understand from the code, add comments that explain the algorithm and mentions how ARCH_RET_ADDR_AFTER_LOCAL_VARS affects it. Link: http://lkml.kernel.org/r/20190806123455.487ac02b@gandalf.local.home Suggested-by: Joel Fernandes Signed-off-by: Steven Rostedt (VMware) Acked-by: Joel Fernandes (Google) --- kernel/trace/trace_stack.c | 98 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 98 insertions(+) diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c index 40e4a88eea8f..f94a2fc567de 100644 --- a/kernel/trace/trace_stack.c +++ b/kernel/trace/trace_stack.c @@ -53,6 +53,104 @@ static void print_max_stack(void) } } +/* + * The stack tracer looks for a maximum stack at each call from a function. It + * registers a callback from ftrace, and in that callback it examines the stack + * size. It determines the stack size from the variable passed in, which is the + * address of a local variable in the stack_trace_call() callback function. + * The stack size is calculated by the address of the local variable to the top + * of the current stack. If that size is smaller than the currently saved max + * stack size, nothing more is done. + * + * If the size of the stack is greater than the maximum recorded size, then the + * following algorithm takes place. + * + * For architectures (like x86) that store the function's return address before + * saving the function's local variables, the stack will look something like + * this: + * + * [ top of stack ] + * 0: sys call entry frame + * 10: return addr to entry code + * 11: start of sys_foo frame + * 20: return addr to sys_foo + * 21: start of kernel_func_bar frame + * 30: return addr to kernel_func_bar + * 31: [ do trace stack here ] + * + * The save_stack_trace() is called returning all the functions it finds in the + * current stack. Which would be (from the bottom of the stack to the top): + * + * return addr to kernel_func_bar + * return addr to sys_foo + * return addr to entry code + * + * Now to figure out how much each of these functions' local variable size is, + * a search of the stack is made to find these values. When a match is made, it + * is added to the stack_dump_trace[] array. The offset into the stack is saved + * in the stack_trace_index[] array. The above example would show: + * + * stack_dump_trace[] | stack_trace_index[] + * ------------------ + ------------------- + * return addr to kernel_func_bar | 30 + * return addr to sys_foo | 20 + * return addr to entry | 10 + * + * The print_max_stack() function above, uses these values to print the size of + * each function's portion of the stack. + * + * for (i = 0; i < nr_entries; i++) { + * size = i == nr_entries - 1 ? stack_trace_index[i] : + * stack_trace_index[i] - stack_trace_index[i+1] + * print "%d %d %d %s\n", i, stack_trace_index[i], size, stack_dump_trace[i]); + * } + * + * The above shows + * + * depth size location + * ----- ---- -------- + * 0 30 10 kernel_func_bar + * 1 20 10 sys_foo + * 2 10 10 entry code + * + * Now for architectures that might save the return address after the functions + * local variables (saving the link register before calling nested functions), + * this will cause the stack to look a little different: + * + * [ top of stack ] + * 0: sys call entry frame + * 10: start of sys_foo_frame + * 19: return addr to entry code << lr saved before calling kernel_func_bar + * 20: start of kernel_func_bar frame + * 29: return addr to sys_foo_frame << lr saved before calling next function + * 30: [ do trace stack here ] + * + * Although the functions returned by save_stack_trace() may be the same, the + * placement in the stack will be different. Using the same algorithm as above + * would yield: + * + * stack_dump_trace[] | stack_trace_index[] + * ------------------ + ------------------- + * return addr to kernel_func_bar | 30 + * return addr to sys_foo | 29 + * return addr to entry | 19 + * + * Where the mapping is off by one: + * + * kernel_func_bar stack frame size is 29 - 19 not 30 - 29! + * + * To fix this, if the architecture sets ARCH_RET_ADDR_AFTER_LOCAL_VARS the + * values in stack_trace_index[] are shifted by one to and the number of + * stack trace entries is decremented by one. + * + * stack_dump_trace[] | stack_trace_index[] + * ------------------ + ------------------- + * return addr to kernel_func_bar | 29 + * return addr to sys_foo | 19 + * + * Although the entry function is not displayed, the first function (sys_foo) + * will still include the stack size of it. + */ static void check_stack(unsigned long ip, unsigned long *stack) { unsigned long this_size, flags; unsigned long *p, *top, *start;