linux-kernel - Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

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Message-ID: <c62985530811131206i5fd982c3r36f637412bb5ab95@mail.gmail.com>
Date:	Thu, 13 Nov 2008 21:06:24 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

2008/11/13 Ingo Molnar <mingo@...e.hu>:
> you are right - it needs a bit more logic.
>
> I think the simplest would be something like this:
>
>  atomic64_t global_clock = INIT_ATOMIC64(0);
>
>  u64 global_time()
>  {
>        u64 now, delta, now_global, prev_global;
>
>        do {
>                prev_global = atomic64_read(&global_clock);
>                now = cpu_clock(raw_smp_processor_id());
>
>                if ((s64)(now - prev_global) < 0) {
>                        now = prev_global;
>                        break;
>                }
>        } while (atomic64_cmpxchg(&global_clock,
>                                   prev_global, now) != prev_global);
>
>        return now;
>  }
>
> This is the simplest way of implementing monotonic time: we only allow
> global_clock to go forwards. If all cpu_clock()s are perfectly in
> sync, we've got no problem: then "now - prev_global" will never be
> negative and we can return the local clock as the latest global time.
>
> But if one of the CPU clocks is "behind", the function returns the
> latest global time up until the local clock catches up. Time wont be
> allowed to jump around by going back. If the clock is behind for a
> long time, then we get a lot of timestamps with the same value -


Ok! I understand now this approach.
So, if global ordering flag is set, we return this kind of protected value,
and on the opposite, we return the normal cpu_clock() local value.

> Would that work? [ Would you be interested in sending patches? :-) ]

Yes :-)

> that
> will be very visible in the trace and we'll then work in improving the
> cpu_clock() implementation.
>
> So i think we could start with this simplest approach, and see how
> often we get the same timestamp for a long time (indication of the
> clocks being not perfectly in sync).


Ok, good idea.
Thanks for the explanations!
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