Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752631AbbKPWu4 (ORCPT ); Mon, 16 Nov 2015 17:50:56 -0500 Received: from mail-wm0-f54.google.com ([74.125.82.54]:36843 "EHLO mail-wm0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750878AbbKPWuw (ORCPT ); Mon, 16 Nov 2015 17:50:52 -0500 Date: Mon, 16 Nov 2015 23:50:49 +0100 From: Frederic Weisbecker To: Andy Lutomirski Cc: Borislav Petkov , Brian Gerst , "linux-kernel@vger.kernel.org" , X86 ML , Peter Zijlstra , Linus Torvalds Subject: Re: [PATCH v3 5/5] x86/entry/64: Bypass enter_from_user_mode on non-context-tracking boots Message-ID: <20151116225048.GA5212@lerouge> References: <73ee804fff48cd8c66b65b724f9f728a11a8c686.1447361906.git.luto@kernel.org> <20151113152612.GA14397@lerouge> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2250 Lines: 55 On Mon, Nov 16, 2015 at 11:10:55AM -0800, Andy Lutomirski wrote: > On Nov 13, 2015 7:26 AM, "Frederic Weisbecker" wrote: > > > > On Thu, Nov 12, 2015 at 12:59:04PM -0800, Andy Lutomirski wrote: > > > On CONFIG_CONTEXT_TRACKING kernels that have context tracking > > > disabled at runtime (which includes most distro kernels), we still > > > have the overhead of a call to enter_from_user_mode in interrupt and > > > exception entries. > > > > > > If jump labels are available, this uses the jump label > > > infrastructure to skip the call. > > > > Looks good. But why are we still calling context tracking code on IRQs at all? > > Same reasons as before: > > 1. This way the IRQ exit path is almost completely shared with all the > other exit paths. I'm all for consolidation in general. Unless it brings bad middle states. If I knew before that I would have to argue endlessly in order to protest against these context tracking changes, I would have NACK'ed the x86 consolidation rework in the state it was while it got merged. > > 2. It combines the checks for which context we were in with what CPL > we entered from. > > Part 2 should be complete across the whole x86 kernel soon once the > 64-bit syscall code gets fixed up. > > We should get rid of the duplication in the irq entry hooks. Want to > help with that? Which one? The duplication against irq_enter() and irq_exit()? I think that irq_exit() should be moved to the IRQ very end and perform the final signal/schedule/preempt_schedule_irq() loop. But it requires a bit of rework on all archs in order to do that. This could be done iteratively though. > Presumably we should do the massive remote polling speedup to the nohz code, Hmm, I don't get what you mean here. > and we should also teach enter_from_user_mode to transition directly to IRQ state as > appropriate. Then irq_enter can be much faster. I don't get what you mean here either. You mean calling irq_enter() from enter_from_user_mode()? -- 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/