Re: [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to sample state of locked-down task

[Paul E. McKenney]

On Mon, Mar 23, 2020 at 08:15:49PM -0400, Joel Fernandes wrote:
> On Mon, Mar 23, 2020 at 08:06:39PM -0400, Joel Fernandes wrote:
> > On Thu, Mar 19, 2020 at 07:49:43PM -0700, Paul E. McKenney wrote:
> > [...] 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > commit e26a234c1205bf02b62b62cd7f15f8086fc0b13b
> > > Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > Date:   Thu Mar 19 15:33:12 2020 -0700
> > > 
> > >     rcu-tasks: Avoid IPIing userspace/idle tasks if kernel is so built
> > >     
> > >     Systems running CPU-bound real-time task do not want IPIs sent to CPUs
> > >     executing nohz_full userspace tasks.  Battery-powered systems don't
> > >     want IPIs sent to idle CPUs in low-power mode.  Unfortunately, RCU tasks
> > >     trace can and will send such IPIs in some cases.
> > >     
> > >     Both of these situations occur only when the target CPU is in RCU
> > >     dyntick-idle mode, in other words, when RCU is not watching the
> > >     target CPU.  This suggests that CPUs in dyntick-idle mode should use
> > >     memory barriers in outermost invocations of rcu_read_lock_trace()
> > >     and rcu_read_unlock_trace(), which would allow the RCU tasks trace
> > >     grace period to directly read out the target CPU's read-side state.
> > >     One challenge is that RCU tasks trace is not targeting a specific
> > >     CPU, but rather a task.  And that task could switch from one CPU to
> > >     another at any time.
> > >     
> > >     This commit therefore uses try_invoke_on_locked_down_task()
> > >     and checks for task_curr() in trc_inspect_reader_notrunning().
> > >     When this condition holds, the target task is running and cannot move.
> > >     If CONFIG_TASKS_TRACE_RCU_READ_MB=y, the new rcu_dynticks_zero_in_eqs()
> > >     function can be used to check if the specified integer (in this case,
> > >     t->trc_reader_nesting) is zero while the target CPU remains in that same
> > >     dyntick-idle sojourn.  If so, the target task is in a quiescent state.
> > >     If not, trc_read_check_handler() must indicate failure so that the
> > >     grace-period kthread can take appropriate action or retry after an
> > >     appropriate delay, as the case may be.
> > >     
> > >     With this change, given CONFIG_TASKS_TRACE_RCU_READ_MB=y, if a given
> > >     CPU remains idle or a given task continues executing in nohz_full mode,
> > >     the RCU tasks trace grace-period kthread will detect this without the
> > >     need to send an IPI.
> > >     
> > >     Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > 
> > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > > index e1089fd..296f926 100644
> > > --- a/kernel/rcu/rcu.h
> > > +++ b/kernel/rcu/rcu.h
> > > @@ -501,6 +501,7 @@ void srcutorture_get_gp_data(enum rcutorture_type test_type,
> > >  #endif
> > >  
> > >  #ifdef CONFIG_TINY_RCU
> > > +static inline bool rcu_dynticks_zero_in_eqs(int cpu, int *vp) { return false; }
> > >  static inline unsigned long rcu_get_gp_seq(void) { return 0; }
> > >  static inline unsigned long rcu_exp_batches_completed(void) { return 0; }
> > >  static inline unsigned long
> > > @@ -510,6 +511,7 @@ static inline void show_rcu_gp_kthreads(void) { }
> > >  static inline int rcu_get_gp_kthreads_prio(void) { return 0; }
> > >  static inline void rcu_fwd_progress_check(unsigned long j) { }
> > >  #else /* #ifdef CONFIG_TINY_RCU */
> > > +bool rcu_dynticks_zero_in_eqs(int cpu, int *vp);
> > >  unsigned long rcu_get_gp_seq(void);
> > >  unsigned long rcu_exp_batches_completed(void);
> > >  unsigned long srcu_batches_completed(struct srcu_struct *sp);
> > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > index d31ed74..36f03d3 100644
> > > --- a/kernel/rcu/tasks.h
> > > +++ b/kernel/rcu/tasks.h
> > > @@ -802,22 +802,38 @@ static void trc_read_check_handler(void *t_in)
> > >  /* Callback function for scheduler to check non-running) task.  */
> > >  static bool trc_inspect_reader_notrunning(struct task_struct *t, void *arg)
> > 
> > This function name is a bit confusing. The task could be running when this
> > function is called. Below you are detecting that the task is running, by
> > calling task_curr().
> > 
> > Maybe just trc_inspect_reader() is better?
> > 
> > [..]
> > 
> > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > > index 44edd0a..43991a4 100644
> > > --- a/kernel/rcu/tree.h
> > > +++ b/kernel/rcu/tree.h
> > > @@ -455,6 +455,8 @@ static void rcu_bind_gp_kthread(void);
> > >  static bool rcu_nohz_full_cpu(void);
> > >  static void rcu_dynticks_task_enter(void);
> > >  static void rcu_dynticks_task_exit(void);
> > > +static void rcu_dynticks_task_trace_enter(void);
> > > +static void rcu_dynticks_task_trace_exit(void);
> > >  
> > >  /* Forward declarations for tree_stall.h */
> > >  static void record_gp_stall_check_time(void);
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 9355536..f4a344e 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -2553,3 +2553,21 @@ static void rcu_dynticks_task_exit(void)
> > >  	WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
> > >  #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
> > >  }
> > > +
> > > +/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
> > > +static void rcu_dynticks_task_trace_enter(void)
> > > +{
> > > +#ifdef CONFIG_TASKS_RCU_TRACE
> > > +	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
> > > +		current->trc_reader_special.b.need_mb = true;
> > 
> > If this is every called from middle of a reader section (that is we
> > transition from IPI-mode to using heavier reader-sections), then is a memory
> > barrier needed here just to protect the reader section that already started?
> 
> Forgot to add:
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Applied, thank you!

							Thanx, Paul