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

From: Shreyas B Prabhu
Date: Thu May 14 2015 - 10:17:19 EST

On Wednesday 13 May 2015 09:51 PM, Steven Rostedt wrote:
> On Wed, 13 May 2015 21:37:43 +0530
> "Shreyas B. Prabhu" <shreyas@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
>> ---
>> This applies on top of patches posted here:
>> 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?
Yes. It does trigger "using smp_processor_id() in preemptible code"
warnings. But as you mentioned in the previous comments, we should be
safe even if the trace call happens from a preemptible section. Let me
play out the scenarios here again-

The task gets migrated after the smp_processor_id()
1. From an online cpu to another online cpu - No impact
2. From an online cpu to an offline cpu - Should never happen
3. From an offline cpu to an online cpu - IIUC, 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.

So I don't see any downside to changing smp_processor_id() to
raw_smp_processor_id() which will suppress the warnings. If you agree
I'll send a patch doing this.

Another alternative which is perhaps worth considering is to change
__DO_TRACE itself to check for offline cpu, without a trace event
specifying the check. This will prevent any currently uncaught and any
future tracepoints from using RCU on offline cpus. But I guess it's
little extreme considering only a low fraction of tracepoints have
potential of being called from offline cpus.


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