linux-kernel - Re: [PATCH tip/core/rcu 55/55] powerpc: Work around tracing from dyntick-idle mode

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Date:	Wed, 7 Sep 2011 06:44:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org,
	anton@...ba.org, paulus@...ba.org
Subject: Re: [PATCH tip/core/rcu 55/55] powerpc: Work around tracing from
 dyntick-idle mode

On Wed, Sep 07, 2011 at 07:00:22AM -0300, Benjamin Herrenschmidt wrote:
> On Tue, 2011-09-06 at 11:00 -0700, Paul E. McKenney wrote:
> > PowerPC LPAR's __trace_hcall_exit() can invoke event tracing at a
> > point where RCU has been told that the CPU is in dyntick-idle mode.
> > Because event tracing uses RCU, this can result in failures.
> > 
> > A correct fix would arrange for RCU to be told about dyntick-idle
> > mode after tracing had completed, however, this will require some care
> > because it appears that __trace_hcall_exit() can also be called from
> > non-dyntick-idle mode.
> 
> This obviously needs to be fixed properly. hcall tracing is very useful
> and if I understand your patch properly, it just comments it out :-)

That is exactly what it does, and I completely agree that this patch
is nothing but a short-term work-around to allow my RCU tests to find
other bugs.

> I'm not sure what the best approach is, maybe have the hcall tracing
> test for the dyntick-idle mode and skip tracing in that case ?

Another approach would be to update Frederic Weisbecker's patch at:

	https://lkml.org/lkml/2011/8/20/83

so that powerpc does tick_nohz_enter_idle(false), and then uses
rcu_enter_nohz() explicitly just after doing the hcall tracing.
If pseries is the only powerpc architecture requiring this, then
the argument to tick_nohz_enter_idle() could depend on the powerpc
sub-architecture.

The same thing would be needed for tick_nohz_exit_idle() and
rcu_exit_nohz(): powerpc would need to invoke rcu_exit_nohz() after
gaining control from the hypervisor but before doing its first tracing,
and then it would need the idle loop to to tick_nohz_exit_idle(false).
Again, if pseries is the only powerpc architecture requiring this,
the argument to tick_nohz_exit_idle() could depend on the architecture.

Would this approach work?

							Thanx, Paul

> Cheers,
> Ben.
> 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: anton@...ba.org
> > Cc: benh@...nel.crashing.org
> > Cc: paulus@...ba.org
> > ---
> >  arch/powerpc/platforms/pseries/lpar.c |    6 ++++++
> >  1 files changed, 6 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
> > index 39e6e0a..668f300 100644
> > --- a/arch/powerpc/platforms/pseries/lpar.c
> > +++ b/arch/powerpc/platforms/pseries/lpar.c
> > @@ -715,12 +715,14 @@ EXPORT_SYMBOL(arch_free_page);
> >  /* NB: reg/unreg are called while guarded with the tracepoints_mutex */
> >  extern long hcall_tracepoint_refcount;
> >  
> > +#if 0 /* work around buggy use of RCU from dyntick-idle mode */
> >  /* 
> >   * Since the tracing code might execute hcalls we need to guard against
> >   * recursion. One example of this are spinlocks calling H_YIELD on
> >   * shared processor partitions.
> >   */
> >  static DEFINE_PER_CPU(unsigned int, hcall_trace_depth);
> > +#endif /* #if 0 work around buggy use of RCU from dyntick-idle mode */
> >  
> >  void hcall_tracepoint_regfunc(void)
> >  {
> > @@ -734,6 +736,7 @@ void hcall_tracepoint_unregfunc(void)
> >  
> >  void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
> >  {
> > +#if 0 /* work around buggy use of RCU from dyntick-idle mode */
> >  	unsigned long flags;
> >  	unsigned int *depth;
> >  
> > @@ -750,11 +753,13 @@ void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
> >  
> >  out:
> >  	local_irq_restore(flags);
> > +#endif /* #if 0 work around buggy use of RCU from dyntick-idle mode */
> >  }
> >  
> >  void __trace_hcall_exit(long opcode, unsigned long retval,
> >  			unsigned long *retbuf)
> >  {
> > +#if 0 /* work around buggy use of RCU from dyntick-idle mode */
> >  	unsigned long flags;
> >  	unsigned int *depth;
> >  
> > @@ -771,6 +776,7 @@ void __trace_hcall_exit(long opcode, unsigned long retval,
> >  
> >  out:
> >  	local_irq_restore(flags);
> > +#endif /* #if 0 work around buggy use of RCU from dyntick-idle mode */
> >  }
> >  #endif
> >  
> 
> 
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