From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ Date: Thu, 31 Jul 2014 18:42:46 +0200 Message-ID: <20140731164246.GA15974@redhat.com> References: <7123b2489cc5d1d5abb7bcf1364ca729cab3e6ca.1406604806.git.luto@amacapital.net> <20140729193232.GA8153@redhat.com> <20140730164344.GA27954@localhost.localdomain> <20140731151630.GA7842@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20140731151630.GA7842@localhost.localdomain> 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: Frederic Weisbecker Cc: Andy Lutomirski , "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" List-Id: linux-arch.vger.kernel.org On 07/31, Frederic Weisbecker wrote: > > On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote: > > > > 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. And in my opinion if (work & TIF_XYZ) user_exit(); looks even more confusing. Because, once again, TIF_XYZ is not the reason to call user_exit(). Not to mention this adds a minor performance penalty. > > 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. And if we change this, then the code above becomes racy. The state of TIF_XYZ can be changed right after the check. OK, it is racy anyway ;) but still this adds more confusion. I feel that TIF_XYZ must die. But yes, yes, I know that it is very simple to say this. And no, so far I do not know how we can improve this all. But again, again, I won't insist. Just another "can't resist" email, please ignore. Oleg. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:46884 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751292AbaGaQpt (ORCPT ); Thu, 31 Jul 2014 12:45:49 -0400 Date: Thu, 31 Jul 2014 18:42:46 +0200 From: Oleg Nesterov Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ Message-ID: <20140731164246.GA15974@redhat.com> References: <7123b2489cc5d1d5abb7bcf1364ca729cab3e6ca.1406604806.git.luto@amacapital.net> <20140729193232.GA8153@redhat.com> <20140730164344.GA27954@localhost.localdomain> <20140731151630.GA7842@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140731151630.GA7842@localhost.localdomain> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Frederic Weisbecker Cc: Andy Lutomirski , "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: <20140731164246.h70lDVFLau7_7cjGeyLAkOCwaOq6kINcHsAOWXPfx6I@z> On 07/31, Frederic Weisbecker wrote: > > On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote: > > > > 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. And in my opinion if (work & TIF_XYZ) user_exit(); looks even more confusing. Because, once again, TIF_XYZ is not the reason to call user_exit(). Not to mention this adds a minor performance penalty. > > 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. And if we change this, then the code above becomes racy. The state of TIF_XYZ can be changed right after the check. OK, it is racy anyway ;) but still this adds more confusion. I feel that TIF_XYZ must die. But yes, yes, I know that it is very simple to say this. And no, so far I do not know how we can improve this all. But again, again, I won't insist. Just another "can't resist" email, please ignore. Oleg.