Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933115AbaLJTLv (ORCPT ); Wed, 10 Dec 2014 14:11:51 -0500 Received: from mail-la0-f44.google.com ([209.85.215.44]:45208 "EHLO mail-la0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933002AbaLJTLt (ORCPT ); Wed, 10 Dec 2014 14:11:49 -0500 MIME-Version: 1.0 In-Reply-To: <20141210185116.GA12098@devbig257.prn2.facebook.com> References: <238c10795d7be218fea8d16ceb183e1e7450da0b.1418006970.git.shli@fb.com> <20141210185116.GA12098@devbig257.prn2.facebook.com> From: Andy Lutomirski Date: Wed, 10 Dec 2014 11:11:27 -0800 Message-ID: Subject: Re: [PATCH 2/3] X86: add a generic API to let vdso code detect context switch To: Shaohua Li Cc: "linux-kernel@vger.kernel.org" , X86 ML , kernel-team@fb.com, "H. Peter Anvin" , Ingo Molnar 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 Wed, Dec 10, 2014 at 10:51 AM, Shaohua Li wrote: > On Wed, Dec 10, 2014 at 10:38:41AM -0800, Andy Lutomirski wrote: >> On Sun, Dec 7, 2014 at 7:03 PM, Shaohua Li wrote: >> > vdso code can't disable preempt, so it can be preempted at any time. >> > This makes a challenge to implement specific features. This patch adds a >> > generic API to let vdso code detect context switch. >> > >> > With this patch, every cpu maintains a context switch count. The upper >> > bits of the count is the logical cpu id, so the count can't be identical >> > for any cpu. The low bits of the count will be increased for each >> > context switch. For a x86_64 cpu with 4096 cpus, the context switch will >> > be overflowed for 2^(64 - 12) context switch, which is a long time and can be >> > ignored. The change of the count in giving time can be used to detect if >> > context switch occurs. >> >> Why do you need those high bits? I don't understand how you could >> possibly confuse one cpu's count with another's unless you fail to >> make sure that you're reading the same address both times. >> >> That being said, I don't like this patch. I'm not sure I have a much >> better idea, though. More thoughts in the 0/0 email to follow. > the vdso code doesn't disable preemption, so it can be migrated between > cpus at any time, the usage is: > > get_countext_switch_count (in cpu A) > do_something (in cpu B) > get_countext_switch_count (in cpu C) > > The cpu A, B, C could be completely different. We want to make sure > there is no preemption here and we use the context switch count to judge > this. If the high bits is ignored, the context switch count could be > identical even A != C, then our judgement using the switch count is > wrong. Sure, but you could compare the cpu numbers, too. --Andy > > Thanks, > Shaohua -- 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/