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=-5.4 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 4E4D4C4727E for ; Fri, 25 Sep 2020 08:15:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DC2E5207C4 for ; Fri, 25 Sep 2020 08:15:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="IRQsSJ1v" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727683AbgIYIPP (ORCPT ); Fri, 25 Sep 2020 04:15:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:57798 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727063AbgIYIPN (ORCPT ); Fri, 25 Sep 2020 04:15:13 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1601021712; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=fc8eN87fP8DSToCEtWnmQvSMtw+BUUdgti9YeDiVi0o=; b=IRQsSJ1v9wnKImxTH2bbKUGinMva9jN8JTB8F8qXgf/fecRq+xrOVyA6BJw++v1eaVxt/R b4BQOz6cArjhUhQgJVPjign/mM9VSFv3aIawwPxqXnR8xnAq+Y9b9tT/EIQul+sF5nGSuA VFfL+l7XG/ssA3mtO7OQvzZPtRtkhaU= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 138DDADDF; Fri, 25 Sep 2020 08:15:12 +0000 (UTC) Date: Fri, 25 Sep 2020 10:15:11 +0200 From: Petr Mladek To: Rasmus Villemoes Cc: Sergey Senozhatsky , John Ogness , Sergey Senozhatsky , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers Message-ID: <20200925081511.GK29288@alley> References: <20200922153816.5883-1-john.ogness@linutronix.de> <20200922153816.5883-4-john.ogness@linutronix.de> <20200923151129.GC6442@alley> <20200924054058.GE577@jagdpanzerIV.localdomain> <20200924084555.GJ6442@alley> <20200924085343.GB541@jagdpanzerIV.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 2020-09-24 11:49:06, Rasmus Villemoes wrote: > On 24/09/2020 10.53, Sergey Senozhatsky wrote: > > On (20/09/24 10:45), Petr Mladek wrote: > >> On Thu 2020-09-24 14:40:58, Sergey Senozhatsky wrote: > >>> On (20/09/23 17:11), Petr Mladek wrote: > >>>> > >>>> AFAIK, there is one catch. We need to use va_copy() around > >>>> the 1st call because va_format can be proceed only once. > >>>> > >>> > >>> Current printk() should be good enough for reporting, say, "Kernel > >>> stack overflow" errors. Is extra pressure that va_copy() adds something > >>> that we need to consider? > >> > >> The thing is that vsprintf() traverses the arguments using va_arg(). > >> It modifies internal values so that the next va_arg() will read > >> the next value. > > > > Yes, I understand the purpose of va_copy(). I'm asking if we are > > always on the safe side doing va_copy for every printk (+ potential > > recursive va_copy-s). > > va_copy doesn't really add any extra stack use worth talking about. When > ABI says all arguments are passed on stack, va_list is just a pointer > into the arguments, and va_copy merely copies that pointer. When some > arguments are passed in register, the function prologue sets up a > "register save area" where those registers are stashed, and va_list then > contains two pointers: one to the reg save area, one to the place in the > stack where the rest of the arguments are, plus a bit of control > information on how many of the register arguments have been consumed so > far (and that control info is the only reason one must "reset" a > va_list, or rather use a copy that was made before consumption started). > In either case, an extra va_list doesn't cost more than 24 bytes or so > of stack - even if printk() was called with 100 arguments, those > 90-100ish arguments are only stored once on the stack. Thanks a lot for the detailed information. It is good to know that va_copy() does not cost that much. Best Regards, Petr