Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754079AbaG2Sqa (ORCPT ); Tue, 29 Jul 2014 14:46:30 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57467 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751165AbaG2Sq2 (ORCPT ); Tue, 29 Jul 2014 14:46:28 -0400 Date: Tue, 29 Jul 2014 20:44:11 +0200 From: Oleg Nesterov To: Andy Lutomirski Cc: "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 v3 6/8] x86: Split syscall_trace_enter into two phases Message-ID: <20140729184411.GA6035@redhat.com> References: <3f649f5658a163645e3ce15156176c325283762e.1405992946.git.luto@amacapital.net> <20140728173723.GA20993@redhat.com> <20140729165416.GA967@redhat.com> <20140729173136.GA2808@redhat.com> <20140729181615.GA4950@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/29, Andy Lutomirski wrote: > > On Tue, Jul 29, 2014 at 11:16 AM, Oleg Nesterov wrote: > > On 07/29, Andy Lutomirski wrote: > >> > >> On Tue, Jul 29, 2014 at 10:31 AM, Oleg Nesterov wrote: > >> > > >> > TIF_NOHZ is set if and only if context_tracking_is_enabled() is true. > >> > So I think that > >> > > >> > work = current_thread_info()->flags & (_TIF_WORK_SYSCALL_ENTRY & ~TIF_NOHZ); > >> > > >> > user_exit(); > >> > > >> > looks a bit better. But I won't argue. > >> > >> I don't get it. > > > > Don't worry, you are not alone. > > > >> context_tracking_is_enabled is global, and TIF_NOHZ > >> is per-task. Isn't this stuff determined per-task or per-cpu or > >> something? > >> > >> IOW, if one CPU is running something that's very heavily > >> userspace-oriented and another CPU is doing something syscall- or > >> sleep-heavy, then shouldn't only the first CPU end up paying the price > >> of context tracking? > > > > Please see another email I sent to Frederic. > > > I'll add at least this argument in favor of my approach: if context > tracking works at all, then it had better not demand that syscall > entry call user_exit if TIF_NOHZ is *not* set. I disagree. At least I disagree with that you should enforce this in syscall_trace_enter() paths, and in any case this has nothing to do with these changes. But again, I won't insist, so please forget. > So adding the > condition ought to be safe, barring dumb bugs in my code. Yes, I think it is technically correct. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/