linux-kernel - Re: [PATCH linux-next] tracing/mm: Use raw_smp_processor_id() instead of smp_processor

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Date:	Fri, 15 May 2015 19:02:49 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	"Shreyas B. Prabhu" <shreyas@...ux.vnet.ibm.com>,
	akpm@...ux-foundation.org, rostedt@...dmis.org
CC:	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	mingo@...hat.com
Subject: Re: [PATCH linux-next] tracing/mm: Use raw_smp_processor_id() instead
 of smp_processor_id()

On 05/15/2015 06:57 PM, Shreyas B. Prabhu wrote:
> trace_mm_page_pcpu_drain, trace_kmem_cache_free, trace_mm_page_free
> 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.
> 
> But these checks use smp_processor_id() and since these trace calls can
> happen from preemptable sections, this throws a warning when running
> with CONFIG_DEBUG_PREEMPT set.
> 
> Now consider task gets migrated after calling smp_processor_id()
>  - From an online cpu to another online cpu - No impact
>  - From an online cpu to an offline cpu - Should never happen
>  - From an offline cpu to an online cpu - Once a cpu has been
>    offlined it returns to cpu_idle_loop, discovers its offline and calls
>    arch_cpu_idle_dead. All this happens with preemption disabled. So
>    this scenario too should never happen.
> 
> Thus running with PREEMPT set has no impact on the condition. So use
> raw_smp_processor_id() so that the warnings are suppressed.
> 
> Signed-off-by: Shreyas B. Prabhu <shreyas@...ux.vnet.ibm.com>
> ---
>  include/trace/events/kmem.h | 34 +++++++++++++++++++++++++++++++---
>  1 file changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> index 6cd975f..f7554fd 100644
> --- a/include/trace/events/kmem.h
> +++ b/include/trace/events/kmem.h
> @@ -146,7 +146,16 @@ DEFINE_EVENT_CONDITION(kmem_free, kmem_cache_free,
> 
>  	TP_ARGS(call_site, ptr),
> 
> -	TP_CONDITION(cpu_online(smp_processor_id()))
> +	/*
> +	 * 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.
> +	 * While this trace can be called from a preemptable section,
> +	 * it has no impact on the condition since tasks can migrate
> +	 * only from online cpus to other online cpus. Thus its safe
> +	 * to use raw_smp_processor_id.
> +	 */
> +	TP_CONDITION(cpu_online(raw_smp_processor_id()))
>  );
> 
>  TRACE_EVENT_CONDITION(mm_page_free,
> @@ -155,7 +164,17 @@ TRACE_EVENT_CONDITION(mm_page_free,
> 
>  	TP_ARGS(page, order),
> 
> -	TP_CONDITION(cpu_online(smp_processor_id())),
> +
> +	/*
> +	 * 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.
> +	 * While this trace can be called from a preemptable section,
> +	 * it has no impact on the condition since tasks can migrate
> +	 * only from online cpus to other online cpus. Thus its safe
> +	 * to use raw_smp_processor_id.
> +	 */
> +	TP_CONDITION(cpu_online(raw_smp_processor_id())),
> 
>  	TP_STRUCT__entry(
>  		__field(	unsigned long,	pfn		)
> @@ -263,7 +282,16 @@ TRACE_EVENT_CONDITION(mm_page_pcpu_drain,
> 
>  	TP_ARGS(page, order, migratetype),
> 
> -	TP_CONDITION(cpu_online(smp_processor_id())),
> +	/*
> +	 * 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.
> +	 * While this trace can be called from a preemptable section,
> +	 * it has no impact on the condition since tasks can migrate
> +	 * only from online cpus to other online cpus. Thus its safe
> +	 * to use raw_smp_processor_id.
> +	 */
> +	TP_CONDITION(cpu_online(raw_smp_processor_id())),
> 
>  	TP_STRUCT__entry(
>  		__field(	unsigned long,	pfn		)

Reviewed-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> 

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