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 ED51FCFC289 for ; Tue, 15 Oct 2024 11:15:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=lM+5oTniwkHcvtmkO8BSwiRrPYY1OP6u7tcToruu1Vo=; b=1Sz3SvkmVWGGil1ELZ7QaYvn0+ OrmiBjmPVUPrxgwmrNkpm66wLJ4KIr6iMH10308o/suHkK/YRzGn8S7uSssDLqk0Pd/PLw4DS1MYa MKD1jiQwocOcIQC4owW8JW6pH6UNCvfvsRDRCnPKHBcWOQnYU0WFwzt21FAqYZzoNivYOvOrqNzJ4 QRWW386pvSHGp5cgd8NKstwAIOtikK7m099to5G2NnHzhiQE6XiWqmWiXzYdQCunJatTtDeg87eY8 lSl3C1yOr0HU29oWpXCGcnwFYdAyZckg2U1VpCfuO1N48TxtadIFLjddp83ZPQFQ1Gt6cPjg3PBdL Lc6gjHPw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1t0fW2-00000007z1t-080s; Tue, 15 Oct 2024 11:15:02 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1t0fQt-00000007xtH-1mh6 for linux-arm-kernel@lists.infradead.org; Tue, 15 Oct 2024 11:09:44 +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 60B211007; Tue, 15 Oct 2024 04:10:12 -0700 (PDT) Received: from [10.57.86.207] (unknown [10.57.86.207]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id DE1ED3F51B; Tue, 15 Oct 2024 04:09:39 -0700 (PDT) Message-ID: Date: Tue, 15 Oct 2024 12:09:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 18/57] trace: Remove PAGE_SIZE compile-time constant assumption Content-Language: en-GB To: Steven Rostedt Cc: Andrew Morton , Anshuman Khandual , Ard Biesheuvel , Catalin Marinas , David Hildenbrand , Greg Marsden , Ivan Ivanov , Kalesh Singh , Marc Zyngier , Mark Rutland , Masami Hiramatsu , Matthias Brugger , Miroslav Benes , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org References: <20241014105514.3206191-1-ryan.roberts@arm.com> <20241014105912.3207374-1-ryan.roberts@arm.com> <20241014105912.3207374-18-ryan.roberts@arm.com> <20241014124656.3ffb0f65@gandalf.local.home> From: Ryan Roberts In-Reply-To: <20241014124656.3ffb0f65@gandalf.local.home> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241015_040943_613186_FE670BFF X-CRM114-Status: GOOD ( 24.43 ) 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: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 14/10/2024 17:46, Steven Rostedt wrote: > On Mon, 14 Oct 2024 11:58:25 +0100 > Ryan Roberts wrote: > >> To prepare for supporting boot-time page size selection, refactor code >> to remove assumptions about PAGE_SIZE being compile-time constant. Code >> intended to be equivalent when compile-time page size is active. >> >> Convert BUILD_BUG_ON() BUG_ON() since the argument depends on PAGE_SIZE >> and its not trivial to test against a page size limit. >> >> Redefine FTRACE_KSTACK_ENTRIES so that "struct ftrace_stacks" is always >> sized at 32K for 64-bit and 16K for 32-bit. It was previously defined in >> terms of PAGE_SIZE (and worked out at the quoted sizes for a 4K page >> size). But for 64K pages, the size expanded to 512K. Given the ftrace >> stacks should be invariant to page size, this seemed like a waste. As a >> side effect, it removes the PAGE_SIZE compile-time constant assumption >> from this code. >> >> Signed-off-by: Ryan Roberts >> --- >> >> ***NOTE*** >> Any confused maintainers may want to read the cover note here for context: >> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/ >> >> kernel/trace/fgraph.c | 2 +- >> kernel/trace/trace.c | 2 +- >> 2 files changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c >> index d7d4fb403f6f0..47aa5c8d8090e 100644 >> --- a/kernel/trace/fgraph.c >> +++ b/kernel/trace/fgraph.c >> @@ -534,7 +534,7 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func, >> if (!current->ret_stack) >> return -EBUSY; >> >> - BUILD_BUG_ON(SHADOW_STACK_SIZE % sizeof(long)); >> + BUG_ON(SHADOW_STACK_SIZE % sizeof(long)); > > Absolutely not! > > BUG_ON() is in no way a substitution of any BUILD_BUG_ON(). BUILD_BUG_ON() > is a non intrusive way to see if something isn't lined up correctly, and > can fix it before you execute any code. BUG_ON() is the most intrusive way > to say something is wrong and you crash the system. Yep, totally agree. I'm afraid this was me being lazy, and there are a couple of other instances where I have done this in other patches that I'll need to fix. Most of the time, I've been able to keep BUILD_BUG_ON() and simply compare against a page size limit. Looking at this again, perhaps the better solution is to define SHADOW_STACK_SIZE as PAGE_SIZE_MIN? Then it remains a compile-time constant. Is there any need for SHADOW_STACK_SIZE to increase with page size? > > Not to mention, when function graph tracing is enabled, this gets triggered > for *every* function call! So I do not want any runtime test done. Every > nanosecond counts in this code path. > > If anything, this needs to be moved to initialization and checked once, if > it fails, gives a WARN_ON() and disables function graph tracing. I'm hoping my suggestion above to decouple SHADOW_STACK_SIZE from PAGE_SIZE is acceptable and simpler? If not, happy to do as you suggest here. Thanks, Ryan > > -- Steve > > >> >> /* Set val to "reserved" with the delta to the new fgraph frame */ >> val = (FGRAPH_TYPE_RESERVED << FGRAPH_TYPE_SHIFT) | FGRAPH_FRAME_OFFSET; >> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c >> index c3b2c7dfadef1..0f2ec3d30579f 100644 >> --- a/kernel/trace/trace.c >> +++ b/kernel/trace/trace.c >> @@ -2887,7 +2887,7 @@ trace_function(struct trace_array *tr, unsigned long ip, unsigned long >> /* Allow 4 levels of nesting: normal, softirq, irq, NMI */ >> #define FTRACE_KSTACK_NESTING 4 >> >> -#define FTRACE_KSTACK_ENTRIES (PAGE_SIZE / FTRACE_KSTACK_NESTING) >> +#define FTRACE_KSTACK_ENTRIES (SZ_4K / FTRACE_KSTACK_NESTING) >> >> struct ftrace_stack { >> unsigned long calls[FTRACE_KSTACK_ENTRIES]; >