Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031196AbbDWWzv (ORCPT ); Thu, 23 Apr 2015 18:55:51 -0400 Received: from mail-lb0-f181.google.com ([209.85.217.181]:35090 "EHLO mail-lb0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1031132AbbDWWzs (ORCPT ); Thu, 23 Apr 2015 18:55:48 -0400 MIME-Version: 1.0 In-Reply-To: <553977AA.9040802@zytor.com> References: <1429792491-5978-1-git-send-email-dvlasenk@redhat.com> <20150423211015.GS28327@pd.tnic> <55396617.4020303@zytor.com> <553972D5.2070104@zytor.com> <553977AA.9040802@zytor.com> From: Andy Lutomirski Date: Thu, 23 Apr 2015 15:55:26 -0700 Message-ID: Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary To: "H. Peter Anvin" Cc: Borislav Petkov , Denys Vlasenko , Linus Torvalds , Brian Gerst , Denys Vlasenko , Ingo Molnar , Steven Rostedt , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , "the arch/x86 maintainers" , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1286 Lines: 34 On Thu, Apr 23, 2015 at 3:52 PM, H. Peter Anvin wrote: > On 04/23/2015 03:38 PM, Andy Lutomirski wrote: >>> >>> Because there are way more sysrets than context switches, and Linux is >>> particularly sensitive to system call latency, by design. >> > > Just to clarify: why would Linux be more sensitive to system call by > design? It enables much simpler APIs and avoids hacks like sending down > a syscall task list (which was genuinely proposed at one point.) If > kernel entry/exit is too expensive, then the APIs get more complex > because they *have* to do everything in the smallest number of system calls. > It's a matter of the ratio, right? One cycle of syscall overhead saved is worth some number of context switch cycles added, and the ratio probably varies by workload. If we do syscall, two context switches, and sysret, then we wouldn't have been better off fixing it on sysret. But maybe most workloads still prefer the fixup on context switch. --Andy -- 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/