linux-kernel - Re: [PATCH] tracing: Add comments explaining cpu online filter for trace events

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Date:	Wed, 13 May 2015 12:21:47 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Shreyas B. Prabhu" <shreyas@...ux.vnet.ibm.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	preeti@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] tracing: Add comments explaining cpu online filter for
 trace events

On Wed, 13 May 2015 21:37:43 +0530
"Shreyas B. Prabhu" <shreyas@...ux.vnet.ibm.com> wrote:

> trace_mm_page_pcpu_drain, trace_kmem_cache_free, trace_mm_page_free
> and trace_tlb_flush can be potentially called from an offlined cpu.
> Since trace points use RCU and RCU should not be used from offlined
> cpus, we have checks to filter out such calls. Add comments to explain
> this.
> 
> Signed-off-by: Shreyas B. Prabhu <shreyas@...ux.vnet.ibm.com>
> ---
> This applies on top of patches posted here:
> https://lkml.org/lkml/2015/5/8/527
> 
>  include/trace/events/kmem.h | 15 +++++++++++++++
>  include/trace/events/tlb.h  |  5 +++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> index 6cd975f..9883f2f 100644
> --- a/include/trace/events/kmem.h
> +++ b/include/trace/events/kmem.h
> @@ -146,6 +146,11 @@ DEFINE_EVENT_CONDITION(kmem_free, kmem_cache_free,
>  
>  	TP_ARGS(call_site, ptr),
>  
> +	/*
> +	 * This trace can be potentially called from an offlined cpu.
> +	 * Since trace points use RCU and RCU should not be used from
> +	 * offline cpus, filter such calls out.
> +	 */
>  	TP_CONDITION(cpu_online(smp_processor_id()))
>  );
>  

Thanks for the comments, but can't these still be called with
preemption enabled. What happens when CONFIG_DEBUG_PREEMPT is set and
you enable these tracepoints. Wont it trigger a warning about
smp_processor_id() being used in preemptible code?

-- Steve
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