Re: [PATCH 3/3] ring-buffer: make cpu buffer entries counteratomic

From: Steven Rostedt
Date: Fri May 01 2009 - 10:23:54 EST



On Fri, 1 May 2009, Ingo Molnar wrote:

>
> * Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > From: Steven Rostedt <srostedt@xxxxxxxxxx>
> >
> > The entries counter in cpu buffer is not atomic. Although it only
> > gets updated by a single CPU, interrupts may come in and update
> > the counter too. This would cause missing entries to be added.
>
> > - unsigned long entries;
> > + atomic_t entries;
>
> Hm, that's not really good as atomics can be rather expensive and
> this is the fastpath.

Actually, it could be local_t. I used that in a lot of the other places.
The race is with on CPU not other CPUs, and on archs like x86 there
is not cost of the "LOCK".

>
> This is the upteenth time or so that the fact that we do not disable
> irqs while generating trace entries bites us in one way or another.
> IRQs can come in and confuse function trace output, etc. etc.

Note, this race is on a simple counter used for stats. It never was
exposed to user land except in the latency output, and that tracer
disables interrupts anyway.

>
> Please lets do what i suggested a long time ago: disable irqs _once_
> in any trace point and run atomically from that point on, and enable
> them once, at the end.
>
> The cost is very small and it turns into a win immediately by
> elimination of a _single_ atomic instruction. (even on Nehalem they
> cost 20 cycles. More on older CPUs.) We can drop the preempt-count
> disable/enable as well and a lot of racy code as well. Please.

If we punt and simply disable interrupts in the ring buffer, I would then
have to disable all tracing of NMIs. Yes it will make the code simpler,
but the new code would also have:

ring_buffer_lock_reserve() {

if (in_nmi())
return NULL;

If that is acceptible, then fine. I'll make the change.

I will also throw away the lockless ring buffer since it would no long er
be needed.

-- Steve

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