Messages in this thread | | | Date | Thu, 5 Feb 2015 06:37:07 -0800 | From | "Paul E. McKenney" <> | Subject | Re: linux-next: Tree for Feb 4 |
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On Wed, Feb 04, 2015 at 11:14:55PM -0800, Dave Hansen wrote: > On 02/04/2015 05:53 PM, Sedat Dilek wrote: > > The architecture-specific switch_mm() function can be called by offline > > CPUs, but includes event tracing, which cannot be legally carried out > > on offline CPUs. This results in a lockdep-RCU splat. This commit fixes > > this splat by omitting the tracing when the CPU is offline. > ... > >>> >> > load_cr3(next->pgd); > >>> >> > - trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL); > >>> >> > + if (cpu_online(smp_processor_id())) > >>> >> > + trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL); > > Is this, perhaps, something that we should be doing in the generic trace > code so that all of the trace users don't have to worry about it? Also, > this patch will add overhead to the code when tracing is off. It would > be best if we could manage to make the cpu_online() check only in the > cases where the tracepoint is on.
I considered doing this in the _rcuidle piece of the trace code, but unlike the RCU idle exit/entry in the _rcuidle stuff, the work required to get through the RCU online/offline code is pretty heavyweight. You end up having 16 CPUs contending for an rcu_node lock, for example.
But maybe you are instead suggesting pushing only the cpu_online() check into the trace infrastructure. If so, fair point, and I will take a look at this.
Thanx, Paul
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