On Wednesday 13 May 2015 09:51 PM, Steven Rostedt wrote: > On Wed, 13 May 2015 21:37:43 +0530 > "Shreyas B. Prabhu" <[email protected]> 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 <[email protected]> >> --- >> 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? > 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. Thanks, Shreyas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
