Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751904AbaG2SWz (ORCPT ); Tue, 29 Jul 2014 14:22:55 -0400 Received: from mail-la0-f49.google.com ([209.85.215.49]:50668 "EHLO mail-la0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750986AbaG2SWx (ORCPT ); Tue, 29 Jul 2014 14:22:53 -0400 MIME-Version: 1.0 In-Reply-To: <20140729181615.GA4950@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> From: Andy Lutomirski Date: Tue, 29 Jul 2014 11:22:31 -0700 Message-ID: Subject: Re: [PATCH v3 6/8] x86: Split syscall_trace_enter into two phases To: Oleg Nesterov 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" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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: >> > >> > I don't think so (unless I am confused again), note that user_exit() uses >> > jump label. But this doesn't matter. I meant that we should avoid TIF_NOHZ >> > if possible because I think it should die somehow (currently I do not know >> > how ;). And because it is ugly to check the same condition twice: >> > >> > if (work & TIF_NOHZ) { >> > // user_exit() >> > if (context_tracking_is_enabled()) >> > context_tracking_user_exit(); >> > } >> > >> > 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. So adding the condition ought to be safe, barring dumb bugs in my code. --Andy > Oleg. > -- Andy Lutomirski AMA Capital Management, LLC -- 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/