Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751964AbaG2R4O (ORCPT ); Tue, 29 Jul 2014 13:56:14 -0400 Received: from mail-la0-f50.google.com ([209.85.215.50]:45943 "EHLO mail-la0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751810AbaG2R4J (ORCPT ); Tue, 29 Jul 2014 13:56:09 -0400 MIME-Version: 1.0 In-Reply-To: <20140729173136.GA2808@redhat.com> References: <3f649f5658a163645e3ce15156176c325283762e.1405992946.git.luto@amacapital.net> <20140728173723.GA20993@redhat.com> <20140729165416.GA967@redhat.com> <20140729173136.GA2808@redhat.com> From: Andy Lutomirski Date: Tue, 29 Jul 2014 10:55:46 -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 10:31 AM, Oleg Nesterov wrote: > On 07/29, Andy Lutomirski wrote: >> >> On Tue, Jul 29, 2014 at 9:54 AM, Oleg Nesterov wrote: >> > >> > Yes, just to trigger the slow path, I guess. >> > >> >> I'll update the code to call user_exit iff TIF_NOHZ is >> >> set. >> > >> > Or perhaps it would be better to not add another user of this (strange) flag >> > and just call user_exit() unconditionally(). But, yes, you need to use >> > from "work = flags & (_TIF_WORK_SYSCALL_ENTRY & ~TIF_NOHZ)" then.\ >> >> user_exit looks slow enough to me that a branch to try to avoid it may >> be worthwhile. I bet that explicitly checking the flag is >> actually both faster and clearer. > > 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. 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? > >> That's what I did for v4. > > I am going to read it today. Not that I think I can help or find something > wrong. > > 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/