From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754421AbZIQGcM (ORCPT ); Thu, 17 Sep 2009 02:32:12 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754041AbZIQGcL (ORCPT ); Thu, 17 Sep 2009 02:32:11 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:56804 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1753542AbZIQGcK (ORCPT ); Thu, 17 Sep 2009 02:32:10 -0400 Message-ID: <4AB1D887.90300@cn.fujitsu.com> Date: Thu, 17 Sep 2009 14:34:47 +0800 From: Zhaolei User-Agent: Thunderbird 2.0.0.6 (Windows/20070728) MIME-Version: 1.0 To: john stultz CC: rostedt@goodmis.org, KOSAKI Motohiro , Frederic Weisbecker , Ingo Molnar , LKML Subject: Re: Re: [PATCH 2/3] ftrace: add tracepoint for xtime References: <4A89213C.5090109@cn.fujitsu.com> <20090818215620.A63C.A69D9226@jp.fujitsu.com> <4A939CDF.2000407@cn.fujitsu.com> <4A939D4A.1020706@cn.fujitsu.com> <1f1b08da0909161256m376b1292ia4b2f3b2b4e82c34@mail.gmail.com> <1f1b08da0909161258g3e57a579iac2238c2f0916b07@mail.gmail.com> <1253133136.20020.245.camel@gandalf.stny.rr.com> <1253134155.3335.13.camel@work-vm> In-Reply-To: <1253134155.3335.13.camel@work-vm> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org john stultz wrote: > On Wed, 2009-09-16 at 16:32 -0400, Steven Rostedt wrote: >> On Wed, 2009-09-16 at 12:58 -0700, john stultz wrote: >>> On Wed, Sep 16, 2009 at 12:56 PM, john stultz wrote: >>>> On Tue, Aug 25, 2009 at 1:14 AM, Zhaolei wrote: >>>>> From: Xiao Guangrong >>>>> /* Structure holding internal timekeeping values. */ >>>>> struct timekeeper { >>>>> /* Current clocksource used for timekeeping. */ >>>>> @@ -338,6 +341,8 @@ int do_settimeofday(struct timespec *tv) >>>>> >>>>> update_vsyscall(&xtime, timekeeper.clock); >>>>> >>>>> + trace_gtod_update_xtime(&xtime, &wall_to_monotonic); >>>>> + >>>> So the only thing to watch out on here is that xtime doesn't hold the >>>> current time, but the accumulated time. There is some unaccumulated >>>> time still kept in the clocksource structure. >>>> >>>> You probably want (assuming you only need tick granularity time) to >>>> use current_kernel_time(). >>>> >>>> As an aside, is there a reason you have to have update callbacks and >>>> duplicate the time storage instead of using the existing interfaces? >>>> (ie: Is this due to locking or something else?) >>> Doh. Sorry, you're actually tracing the timekeeping code. Not using >>> this to assist tracing. Got this confused. >>> >>> So yea, I think this should be ok, plus or minus determining if you >>> really want xtime or xtime_cache. >> Well this may be a real concern. It's not about tracing timekeeping >> (although it adds that functionality). His second patch (I didn't Cc you >> on that one) hooks to these tracepoints to update time accordingly. >> >> What is being done is a way to have a "wall time" value being added to >> the ring buffer. But this needs to be very carefully done, because the >> all tracers use this, including the function tracer in NMI code. So the >> clock source can not take locks or do anything fancy. >> >> What the idea is, is to have a semi clock that is read with some kind of >> fast increment, and then at clock ticks, this clock is synced up. > > Hmm.. Yea, if that's the case, then I'm not a big fan of this approach. > > It sounds like what's really needed is a lock-free variant of > current_kernel_time() or something close to the CLOCK_MONOTONIC_COARSE > functionality currently queued. Yes. And it will be better if I can get current walltime fast and with high resolution. I see your patch of __current_kernel_time() in Aug 19, and I think it can be used to get walltime in ftrace. But IMHO, use tracepoint to do it maybe better for its high resolution. Thanks Zhaolei > Doing it without locks might have some downsides, and I guess that's the > point of the callback method (updates happen at prescribed times and > likely under locks that the trace code understands so it avoids races > and deadlocks). > > -john >