From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Thu, 31 Jul 2014 17:16:50 +0200 (CEST) Received: from mail-wg0-f47.google.com ([74.125.82.47]:37163 "EHLO mail-wg0-f47.google.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S6860175AbaGaPQoF9J2b (ORCPT ); Thu, 31 Jul 2014 17:16:44 +0200 Received: by mail-wg0-f47.google.com with SMTP id b13so2863441wgh.6 for ; Thu, 31 Jul 2014 08:16:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; bh=Vs8Ua6Xz+iNK1k/h9c4bYIRkChbZ1Ae9e5pBzFN5Rtk=; b=IIFvJwdcozN194s8x/3fesmyFFinQnn1lt6RALZ8Cck+Qcl7oFON+F6ZRc7tOpq/87 AX3EZ0EXi2R4z+P95sSDRzPI4lYHrEtmzoFpSRMlZ6Het+pML+wKyYfsC0bxWQbmiZn0 ZJ1zglSNp89EyeeE8qYGUYwMXHjViVfclzCCI7c0zPNVjnUXVoOvoPuP7rrhhbBMhS6n scXdCdf02l9ErG72f9A1mkPmS3yoJni/8zITGJ40MJjPwpwOsj4QCt/35CNnBUA3O+Po GKnp9tJKEABHVN4vH6a3ZQipuPJ4XN1mBteErUunrg7BI7MUsNWj50+TwR/QBRHfH2pa azug== X-Received: by 10.180.38.51 with SMTP id d19mr16973514wik.10.1406819796408; Thu, 31 Jul 2014 08:16:36 -0700 (PDT) Received: from localhost (8.20.196.77.rev.sfr.net. [77.196.20.8]) by mx.google.com with ESMTPSA id ed14sm67205074wic.10.2014.07.31.08.16.34 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 31 Jul 2014 08:16:35 -0700 (PDT) Date: Thu, 31 Jul 2014 17:16:32 +0200 From: Frederic Weisbecker To: Andy Lutomirski Cc: Oleg Nesterov , "linux-kernel@vger.kernel.org" , Kees Cook , Will Drewry , X86 ML , "linux-arm-kernel@lists.infradead.org" , Linux MIPS Mailing List , linux-arch , LSM List , Alexei Starovoitov , "H. Peter Anvin" Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ Message-ID: <20140731151630.GA7842@localhost.localdomain> References: <7123b2489cc5d1d5abb7bcf1364ca729cab3e6ca.1406604806.git.luto@amacapital.net> <20140729193232.GA8153@redhat.com> <20140730164344.GA27954@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 41841 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: fweisbec@gmail.com Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote: > On Wed, Jul 30, 2014 at 9:43 AM, Frederic Weisbecker wrote: > > On Tue, Jul 29, 2014 at 09:32:32PM +0200, Oleg Nesterov wrote: > >> On 07/28, Andy Lutomirski wrote: > >> > > >> > @@ -1449,7 +1449,12 @@ long syscall_trace_enter(struct pt_regs *regs) > >> > { > >> > long ret = 0; > >> > > >> > - user_exit(); > >> > + /* > >> > + * If TIF_NOHZ is set, we are required to call user_exit() before > >> > + * doing anything that could touch RCU. > >> > + */ > >> > + if (test_thread_flag(TIF_NOHZ)) > >> > + user_exit(); > >> > >> Personally I still think this change just adds more confusion, but I leave > >> this to you and Frederic. > >> > >> It is not that "If TIF_NOHZ is set, we are required to call user_exit()", we > >> need to call user_exit() just because we enter the kernel. TIF_NOHZ is just > >> the implementation detail which triggers this slow path. > >> > >> At least it should be correct, unless I am confused even more than I think. > > > > Agreed, Perhaps the confusion is on the syscall_trace_enter() name which suggests > > this is only about tracing? syscall_slowpath_enter() could be an alternative. > > But that's still tracing in a general sense so... > > At the end of the day, the syscall slowpath code calls a bunch of > functions depending on what TIF_XYZ flags are set. As long as it's > structured like "if (TIF_A) do_a(); if (TIF_B) do_b();" or something > like that, it's comprehensible. But once random functions with no > explicit flag checks or comments start showing up, it gets confusing. Yeah that's a point. I don't mind much the TIF_NOHZ test if you like. > > If it's indeed all-or-nothing, I could remove the check and add a > comment. But please keep in mind that, currently, the slow path is > *slow*, and my patches only improve the entry case. So enabling > context tracking on every task will hurt. That's what we do anyway. I haven't found a safe way to enabled context tracking without tracking all CPUs. > > --Andy From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wg0-f47.google.com ([74.125.82.47]:37163 "EHLO mail-wg0-f47.google.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S6860175AbaGaPQoF9J2b (ORCPT ); Thu, 31 Jul 2014 17:16:44 +0200 Received: by mail-wg0-f47.google.com with SMTP id b13so2863441wgh.6 for ; Thu, 31 Jul 2014 08:16:36 -0700 (PDT) Date: Thu, 31 Jul 2014 17:16:32 +0200 From: Frederic Weisbecker Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ Message-ID: <20140731151630.GA7842@localhost.localdomain> References: <7123b2489cc5d1d5abb7bcf1364ca729cab3e6ca.1406604806.git.luto@amacapital.net> <20140729193232.GA8153@redhat.com> <20140730164344.GA27954@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Return-Path: Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-subscribe: List-owner: List-post: List-archive: To: Andy Lutomirski Cc: Oleg Nesterov , "linux-kernel@vger.kernel.org" , Kees Cook , Will Drewry , X86 ML , "linux-arm-kernel@lists.infradead.org" , Linux MIPS Mailing List , linux-arch , LSM List , Alexei Starovoitov , "H. Peter Anvin" Message-ID: <20140731151632.Kmmtldnq-HexsOyFB-UBxgBK2GqqKNQdYTHjuhhSdhY@z> On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote: > On Wed, Jul 30, 2014 at 9:43 AM, Frederic Weisbecker wrote: > > On Tue, Jul 29, 2014 at 09:32:32PM +0200, Oleg Nesterov wrote: > >> On 07/28, Andy Lutomirski wrote: > >> > > >> > @@ -1449,7 +1449,12 @@ long syscall_trace_enter(struct pt_regs *regs) > >> > { > >> > long ret = 0; > >> > > >> > - user_exit(); > >> > + /* > >> > + * If TIF_NOHZ is set, we are required to call user_exit() before > >> > + * doing anything that could touch RCU. > >> > + */ > >> > + if (test_thread_flag(TIF_NOHZ)) > >> > + user_exit(); > >> > >> Personally I still think this change just adds more confusion, but I leave > >> this to you and Frederic. > >> > >> It is not that "If TIF_NOHZ is set, we are required to call user_exit()", we > >> need to call user_exit() just because we enter the kernel. TIF_NOHZ is just > >> the implementation detail which triggers this slow path. > >> > >> At least it should be correct, unless I am confused even more than I think. > > > > Agreed, Perhaps the confusion is on the syscall_trace_enter() name which suggests > > this is only about tracing? syscall_slowpath_enter() could be an alternative. > > But that's still tracing in a general sense so... > > At the end of the day, the syscall slowpath code calls a bunch of > functions depending on what TIF_XYZ flags are set. As long as it's > structured like "if (TIF_A) do_a(); if (TIF_B) do_b();" or something > like that, it's comprehensible. But once random functions with no > explicit flag checks or comments start showing up, it gets confusing. Yeah that's a point. I don't mind much the TIF_NOHZ test if you like. > > If it's indeed all-or-nothing, I could remove the check and add a > comment. But please keep in mind that, currently, the slow path is > *slow*, and my patches only improve the entry case. So enabling > context tracking on every task will hurt. That's what we do anyway. I haven't found a safe way to enabled context tracking without tracking all CPUs. > > --Andy