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

Joel Fernandes
2020-03-24T00:06:44Z

On Thu, Mar 19, 2020 at 07:49:43PM -0700, Paul E. McKenney wrote:
[...]
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit e26a234c1205bf02b62b62cd7f15f8086fc0b13b
> Author: Paul E. McKenney <paulmck@kernel.org>
> 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@efficios.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> 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?

thanks,

- Joel

> +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
> +}
> +
> +/* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
> +static void rcu_dynticks_task_trace_exit(void)
> +{
> +#ifdef CONFIG_TASKS_RCU_TRACE
> + if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
> + current->trc_reader_special.b.need_mb = false;
> +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
> +}