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

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Date:	Thu, 13 Nov 2008 18:26:49 +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>:
> "prev_global_time" also acts as a global serializer: it ensures that
> events are timestamped in a monotonic and ordered way.
>
> i.e. something like this (pseudocode, without the cmpxchg):
>
>  u64 prev_global_time;
>
>  DEFINE_PER_CPU(prev_local_time);
>
>  u64 global_time()
>  {
>        u64 now, delta, now_global;
>
>        prev_global = prev_global_time;
>        now = sched_clock();
>        delta = now - per_cpu(prev_local_time, this_cpu);
>        per_cpu(prev_local_time, this_cpu) = now;
>
>        now_global = prev_global + delta;
>        prev_global = now_global;
>
>        return now_global;
>  }
>
> note how we build "global time" out of "local time".
>
> The cmpxchg would be used to put the above one into a loop, and
> instead of updating the global time in a racy way:
>
>        prev_global = now_global;
>
> We'd update it via the cmpxchg:
>
>        atomic64_t prev_global_time;
>
>        ...
>
>        while (atomic64_cmpxchg(&prev_global_time,
>                                 prev_global, now_global) != prev_global) {
>                [...]
>        }
>
> To make sure the global time goes monotonic. (this way we also avoid a
> spinlock - locks are fragile for instrumentation)

Ok, I understand better.
But consider the following:

 u64 global_time()
 {
       u64 now, delta, now_global;
       prev_global = prev_global_time;

       while (atomic64_cmpxchg(&prev_global_time,
                                 prev_global, now_global) != prev_global) {

           now = sched_clock();
           delta = now - per_cpu(prev_local_time, this_cpu);
           per_cpu(prev_local_time, this_cpu) = now;
           now_global = prev_global + delta;
           prev_global = now_global;
       }
       return now_global;
 }

Sarting with prev_global_time = 0
If we have two cpu and the above function is executed 5 times on the first cpu.
We couldl have per_cpu(prev_local_time) = 50 for example. And so
prev_global_time will be equal to 50.

Just after that, almost at the same time, cpu2 calls global_time()

delta will be equal to 50 (sched_clock() - per_cpu(prev_local_time)
which is 0) and prev_global_time will be 50 + 50 = 100.
This is not consistent.
I don't know where but I'm pretty sure I missed something....
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