From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id A557A189BB2; Tue, 15 Oct 2024 11:09:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728990585; cv=none; b=P8KY7OXuAImGCrnHL2np8qRhkiTeRWiXlJPNEzBHozR2a8T52je5jfhXESJ08d5pAER3kJOQZVl+KBhcMTuJyczPOznob016EuOxD5ZAVgOhc/rmxiDoUgDdfVn8p6tkLvlp62KDLDUynOrtCzhpFVeT066l6o8tj7jMxYwCnlo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728990585; c=relaxed/simple; bh=tXPO7KmjdcWYool96chGvBJ0Wdccziar2tj15Bs1opo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IpepYUxK0M0FlRluwu+JY4FZiinohuatOJEGWGcuSW1EnTbF2U4+n0FbY03QCRGUIt2Y+IFzGta/digB94v6jDzuBuVav2NhNYyGsFWZfiCP1T7WYkbCMDCXy5UmkDJa9K7UXpiSJZGDWHml0+mPE1ClbPHA/so04L6pbo2PxE8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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]; >