From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1755441AbcHSPGG (ORCPT <rfc822;w@1wt.eu>);
        Fri, 19 Aug 2016 11:06:06 -0400
Received: from merlin.infradead.org ([205.233.59.134]:36420 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1754040AbcHSPGE (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 19 Aug 2016 11:06:04 -0400
Date: Fri, 19 Aug 2016 17:05:51 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Clark Williams <williams@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Jon Masters <jcm@redhat.com>,
        Daniel Wagner <wagi@monom.org>, Carsten Emde <C.Emde@osadl.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        John Stultz <john.stultz@linaro.org>
Subject: Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer
Message-ID: <20160819150551.GE10153@twins.programming.kicks-ass.net>
References: <20160810135356.265780824@goodmis.org>
 <20160819140928.GK10121@twins.programming.kicks-ass.net>
 <20160819102620.763bd7a0@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160819102620.763bd7a0@gandalf.local.home>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Aug 19, 2016 at 10:26:20AM -0400, Steven Rostedt wrote:
> > In any case, for those you could probably use ktime_get_mono_fast_ns().
> 
> Is that safe to call from NMI?

It should be, we were very careful to make it so. Also read the comment
with __ktime_get_fast_ns().

> Looking into the code I see:
> 
> 	now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
> 
> where timekeeping_get_ns() has:
> 
> 	timekeeping_get_delta()
> 
> which does:
> 
> 	read_seqcount_begin()
> 
> which has (eventually):

That's a bug in CONFIG_DEBUG_TIMEKEEPING, if you look at the
!timekeeping_get_delta() it does no such thing.

John, looks like 4ca22c2648f9 ("timekeeping: Add warnings when overflows
or underflows are observed") buggered things.