Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752937AbbHUNi6 (ORCPT ); Fri, 21 Aug 2015 09:38:58 -0400 Received: from mail-wi0-f178.google.com ([209.85.212.178]:36797 "EHLO mail-wi0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751402AbbHUNi4 (ORCPT ); Fri, 21 Aug 2015 09:38:56 -0400 Date: Fri, 21 Aug 2015 15:38:53 +0200 From: Frederic Weisbecker To: Ingo Molnar Cc: Andy Lutomirski , x86@kernel.org, Sasha Levin , Brian Gerst , Denys Vlasenko , linux-kernel@vger.kernel.org, Oleg Nesterov , Borislav Petkov , Thomas Gleixner Subject: Re: [PATCH] x86/traps: Weaken context tracking entry assertions Message-ID: <20150821133853.GC3362@lerouge> References: <20150821062328.GA32366@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150821062328.GA32366@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1559 Lines: 35 On Fri, Aug 21, 2015 at 08:23:28AM +0200, Ingo Molnar wrote: > > * Andy Lutomirski wrote: > > > We were asserting that we were all the way in CONTEXT_KERNEL when exception > > handlers were called. While having this be true is, I think, a nice goal (or > > maybe a variant in which we assert that we're in CONTEXT_KERNEL or some new IRQ > > context), we're not quite there. > > > > In particular, if an IRQ interrupts the SYSCALL prologue and the IRQ handler in > > turn causes an exception, the exception entry will be called in RCU IRQ mode but > > with CONTEXT_USER. > > Hm, so what harm would there be in making IRQ handlers enter CONTEXT_KERNEL? > Would nohz-full break? That would imply to double the calls to vtime and RCU that are already in irq generic handlers. Now we can have a CONTEXT_IRQ flag if you guys really want to track irqs, something that takes care of not calling the RCU and time accounting twice. Now exceptions can still happen on irq entry before we run the context tracking call though. I think there will always be this kind of fragility due to the drift between soft context tracking and real context. > > I'd rather have a bit more tracking overhead here than lose such useful sanity > checks. The RCU check should testify enough about sanity here. -- 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/