Re: [PATCH 1/2] tracing/mm: don't trace kfree on offline CPUs

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Feb 16, 2016 at 08:52:24AM +0300, Denis Kirjanov wrote:
> On 2/15/16, Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Sun, 14 Feb 2016 23:19:42 +0300
> > Denis Kirjanov <kda@linux-powerpc.org> wrote:
> >
> >
> >> >>  	TP_PROTO(unsigned long call_site, const void *ptr),
> >> >>
> >> >> -	TP_ARGS(call_site, ptr)
> >> >> +	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.
> >> >> +	 * 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()))
> >> >
> >> > This is starting to become a common occurrence. Perhaps it is best to
> >> > just hardcode this into the tracepoint code itself?
> >>
> >> Can you take it as a fix for now. I'll post the follow-up patch then
> >> for rcu and offline cpus
> >> issue.
> >
> > Actually, I prefer not to add more checks for cpu_online(), if its
> > going to become the default.
> >
> > Does this work for you instead?
> 
> Hm, somehow it doesn't work:
> 
> [   84.692846] ===============================
> [   84.692847] [ INFO: suspicious RCU usage. ]
> [   84.692849] 4.5.0-rc3-00765-g78b70d4-dirty #36 Tainted: G S
> [   84.692850] -------------------------------
> [   84.692851] include/trace/events/rcu.h:457 suspicious
> rcu_dereference_check() usage!
> [   84.692852]
> other info that might help us debug this:
> 
> [   84.692854]
> RCU used illegally from offline CPU!

The problem is that the CPU really is offline as far as RCU is concerned.
The usual idle-loop workarounds do not apply here.

> rcu_scheduler_active = 1, debug_locks = 1
> [   84.692855] no locks held by swapper/11/0.
> [   84.692856]
> stack backtrace:
> [   84.692858] CPU: 11 PID: 0 Comm: swapper/11 Tainted: G S
>   4.5.0-rc3-00765-g78b70d4-dirty #36
> [   84.692860] Call Trace:
> [   84.692868] [c0000005b76d37f0] [c0000000008c57ac]
> .dump_stack+0xe0/0x14c (unreliable)
> [   84.692872] [c0000005b76d3880] [c00000000010d0c8]
> .lockdep_rcu_suspicious+0x108/0x170
> [   84.692876] [c0000005b76d3910] [c00000000013cab4]
> .__call_rcu.constprop.60+0x244/0x680
> [   84.692880] [c0000005b76d39e0] [c0000000002c38ec] .put_object+0x5c/0x80
> [   84.692884] [c0000005b76d3a60] [c00000000029e748]
> .kmem_cache_free+0x318/0x4c0
> [   84.692888] [c0000005b76d3b00] [c0000000000936b4] .__mmdrop+0x54/0x150

Ah, this is the problem.  We are calling __mmdrop() on a CPU that RCU
believes to be offline.

Can we pull the call to __mmdrop() forward?  (Trying to recall what
x86 did about a very similar problem...)

							Thanx, Paul

> [   84.692891] [c0000005b76d3b90] [c0000000000e46e0] .idle_task_exit+0x120/0x130
> [   84.692895] [c0000005b76d3c20] [c000000000075a14]
> .pseries_mach_cpu_die+0x64/0x310
> [   84.692899] [c0000005b76d3cd0] [c00000000004401c] .cpu_die+0x3c/0x60
> [   84.692902] [c0000005b76d3d40] [c000000000018998]
> .arch_cpu_idle_dead+0x28/0x40
> [   84.692906] [c0000005b76d3db0] [c000000000102714]
> .cpu_startup_entry+0x554/0x580
> [   84.692909] [c0000005b76d3ed0] [c000000000043d68]
> .start_secondary+0x348/0x390
> [   84.692913] [c0000005b76d3f90] [c000000000008a6c]
> start_secondary_prolog+0x10/0x14
> 
> [   84.692918] ===============================
> [   84.692919] [ INFO: suspicious RCU usage. ]
> [   84.692920] 4.5.0-rc3-00765-g78b70d4-dirty #36 Tainted: G S
> [   84.692921] -------------------------------
> [   84.692922] include/trace/events/kmem.h:141 suspicious
> rcu_dereference_check() usage!
> [   84.692923]
> other info that might help us debug this:
> 
> [   84.692924]
> RCU used illegally from offline CPU!
> rcu_scheduler_active = 1, debug_locks = 1
> [   84.692925] no locks held by swapper/11/0.
> [   84.692926]
> stack backtrace:
> [   84.692928] CPU: 11 PID: 0 Comm: swapper/11 Tainted: G S
>   4.5.0-rc3-00765-g78b70d4-dirty #36
> [   84.692929] Call Trace:
> [   84.692933] [c0000005b76d38c0] [c0000000008c57ac]
> .dump_stack+0xe0/0x14c (unreliable)
> [   84.692936] [c0000005b76d3950] [c00000000010d0c8]
> .lockdep_rcu_suspicious+0x108/0x170
> [   84.692939] [c0000005b76d39e0] [c00000000029f1c0] .kfree+0x380/0x470
> [   84.692942] [c0000005b76d3a80] [c000000000056194] .destroy_context+0x44/0x100
> [   84.692946] [c0000005b76d3b00] [c0000000000936c0] .__mmdrop+0x60/0x150
> [   84.692948] [c0000005b76d3b90] [c0000000000e46e0] .idle_task_exit+0x120/0x130
> [   84.692951] [c0000005b76d3c20] [c000000000075a14]
> .pseries_mach_cpu_die+0x64/0x310
> [   84.692954] [c0000005b76d3cd0] [c00000000004401c] .cpu_die+0x3c/0x60
> [   84.692957] [c0000005b76d3d40] [c000000000018998]
> .arch_cpu_idle_dead+0x28/0x40
> [   84.692960] [c0000005b76d3db0] [c000000000102714]
> .cpu_startup_entry+0x554/0x580
> [   84.692963] [c0000005b76d3ed0] [c000000000043d68]
> .start_secondary+0x348/0x390
> [   84.692966] [c0000005b76d3f90] [c000000000008a6c]
> start_secondary_prolog+0x10/0x14
> 
> 
> 
> >
> > -- Steve
> >
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index acd522a91539..acfdbf353a0b 100644
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -14,8 +14,10 @@
> >   * See the file COPYING for more details.
> >   */
> >
> > +#include <linux/smp.h>
> >  #include <linux/errno.h>
> >  #include <linux/types.h>
> > +#include <linux/cpumask.h>
> >  #include <linux/rcupdate.h>
> >  #include <linux/tracepoint-defs.h>
> >
> > @@ -132,6 +134,9 @@ extern void syscall_unregfunc(void);
> >  		void *it_func;						\
> >  		void *__data;						\
> >  									\
> > +		if (!cpu_online(raw_smp_processor_id()))		\
> > +			return;						\
> > +									\
> >  		if (!(cond))						\
> >  			return;						\
> >  		prercu;							\
> >
>