From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753449Ab1LLXdC (ORCPT ); Mon, 12 Dec 2011 18:33:02 -0500 Received: from e8.ny.us.ibm.com ([32.97.182.138]:57161 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752402Ab1LLXc7 (ORCPT ); Mon, 12 Dec 2011 18:32:59 -0500 Message-ID: <1323732772.4078.113.camel@work-vm> Subject: Re: [PATCH 2/2] Add a thread cpu time implementation to vDSO From: john stultz To: Arun Sharma Cc: linux-kernel@vger.kernel.org, Kumar Sundararajan , Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Andy Lutomirski Date: Mon, 12 Dec 2011 15:32:52 -0800 In-Reply-To: <4EE68C31.1030207@fb.com> References: <1323718578-1157-1-git-send-email-asharma@fb.com> <1323718578-1157-3-git-send-email-asharma@fb.com> <1323731361.4078.102.camel@work-vm> <4EE68C31.1030207@fb.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.1- Content-Transfer-Encoding: 7bit Mime-Version: 1.0 x-cbid: 11121223-9360-0000-0000-00000174C17F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2011-12-12 at 15:20 -0800, Arun Sharma wrote: > On 12/12/11 3:09 PM, john stultz wrote: > > On Mon, 2011-12-12 at 11:36 -0800, Arun Sharma wrote: > >> From: Kumar Sundararajan > >> > >> This primarily speeds up clock_gettime(CLOCK_THREAD_CPUTIME_ID, ..) > >> via a new vsyscall. We also add a direct vsyscall that returns > >> time in ns (RFC: the direct vsyscall doesn't have a corresponding > >> regular syscall, although clock_gettime() is pretty close). > > > > I'm still not super psyched about providing a vdso-only API. > > > > If a nanosecond interface like thread_cpu_time() is actually a big win > > over clock_gettime(CLOCK_THREAD_CPUTIME,...) it seems it should have its > > own syscall as well, no? > > The win is relatively small when we're dealing with syscalls. But with > vsyscalls, it starts showing up in micro benchmarks. > > Happy to post patches for regular syscalls (assuming I can get them > allocated :). > > > > > Possibly something like clock_gettime_ns(), which would return the same > > values as clock_gettime() but in nanoseconds rather then a timespec? > > > > If we're doing non-POSIXy things there, how about allocating one syscall > per clock instead of multiplexing them through a single syscall? > > This would be a nice to have (clock_gettime_ns() should get us most of > the perf benefit). Well, it makes it a little easier to extend if we get a new clockid, rather then having to add a whole new syscall. Keeps parity between the timespec and ns interfaces. Is it just that you're concerned about the clockid switch costs being too high? thanks -john