linux-kernel - Re: [PATCH] time: Provide full featured jiffies_to

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Date:	Fri, 16 May 2014 15:02:10 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Shy Shuky <rousya@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Ingo Molnar" <mingo@...hat.com>,
	Mauro Chehab <m.chehab@...sung.com>,
	"xiexiuqi@...wei.com" <xiexiuqi@...wei.com>
Subject: Re: [PATCH] time: Provide full featured jiffies_to_nsecs() function

On Fri, 16 May 2014 17:17:46 +0000
"Luck, Tony" <tony.luck@...el.com> wrote:

> Is this function safe to call in every context (including NMI & machine check)?
> [it uses read_seqcount_begin/read_seqcount_retry ... which I *think* is
> safe ... but this stuff is tricky, so I'd like some reassurance].

No, read_seqcount_begin() is not safe in NMI context. If it interrupts
a write, it goes into an infinite spin (see __read_seqcount_begin()).

> 
> Mauro, Steven: Did we just do math on jiffies because we wanted less overhead
> in a tracepoint?

As I meantioned. read_seqcount_begin() is not safe for tracing, it
had to be reimplemented.

-- Steve

> 
> Bigger question (mostly for Mauro) ... what was the motivation for the "uptime"
> tracer to begin with?  The rasdaemon code that is using it converts the times
> from traces into absolute times (by adding an offset it computes by comparing
> uptime and gettimeofday() when it starts).  But this would seem to be fraught
> with problems:
> 1) Do we get this right for events that happen in daylight saving time shift windows?
> 2) Is there a "drift" problem for systems that stay up for months and rely on ntp
> to keep wall clock time in line with reality?
> 
> -Tony

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