Re: [PATCH] rcu/nocb: Add an option to ON/OFF an offloading from RT context

[Paul E. McKenney]

On Thu, May 05, 2022 at 12:16:41PM +0200, Uladzislau Rezki (Sony) wrote:
> Introduce a RCU_NOCB_CPU_CB_BOOST kernel option. So a user can
> decide if an offloading has to be done in a high-prio context or
> not. Please note an option depends on RCU_NOCB_CPU and RCU_BOOST
> parameters and by default it is off.
> 
> This patch splits the boosting preempted RCU readers and those
> kthreads which directly responsible for driving expedited grace
> periods forward with enabling/disabling the offloading from/to
> SCHED_FIFO/SCHED_OTHER contexts.
> 
> The main reason of such split is, for example on Android there
> are some workloads which require fast expedited grace period to
> be done whereas offloading in RT context can lead to starvation
> and hogging a CPU for a long time what is not acceptable for
> latency sensitive environment. For instance:
> 
> <snip>
> <...>-60 [006] d..1 2979.028717: rcu_batch_start: rcu_preempt CBs=34619 bl=270
> <snip>
> 
> invoking 34 619 callbacks will take time thus making other CFS
> tasks waiting in run-queue to be starved due to such behaviour.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>

All good points!

Some questions and comments below.

Adding Sebastian on CC for his perspective.

						Thanx, Paul

> ---
>  kernel/rcu/Kconfig     | 14 ++++++++++++++
>  kernel/rcu/tree.c      |  5 ++++-
>  kernel/rcu/tree_nocb.h |  3 ++-
>  3 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 27aab870ae4c..074630b94902 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -275,6 +275,20 @@ config RCU_NOCB_CPU_DEFAULT_ALL
>  	  Say Y here if you want offload all CPUs by default on boot.
>  	  Say N here if you are unsure.
>  
> +config RCU_NOCB_CPU_CB_BOOST
> +	bool "Perform offloading from real-time kthread"
> +	depends on RCU_NOCB_CPU && RCU_BOOST
> +	default n

I understand that you need this to default to "n" on your systems.
However, other groups already using callback offloading should not see
a sudden change.  I don't see an Android-specific defconfig file, but
perhaps something in drivers/android/Kconfig?

One easy way to make this work would be to invert the sense of this
Kconfig option ("RCU_NOCB_CB_NO_BOOST"?), continue having it default to
"n", but then select it somewhere in drivers/android/Kconfig.  But I
would not be surprised if there is a better way.

> +	help
> +	  Use this option to offload callbacks from the SCHED_FIFO context
> +	  to make the process faster. As a side effect of this approach is
> +	  a latency especially for the SCHED_OTHER tasks which will not be
> +	  able to preempt an offloading kthread. That latency depends on a
> +	  number of callbacks to be invoked.
> +
> +	  Say Y here if you want to set RT priority for offloading kthreads.
> +	  Say N here if you are unsure.
> +
>  config TASKS_TRACE_RCU_READ_MB
>  	bool "Tasks Trace RCU readers use memory barriers in user and idle"
>  	depends on RCU_EXPERT && TASKS_TRACE_RCU
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9dc4c4e82db6..d769a15bc0e3 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -154,7 +154,10 @@ static void sync_sched_exp_online_cleanup(int cpu);
>  static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
>  static bool rcu_rdp_is_offloaded(struct rcu_data *rdp);
>  
> -/* rcuc/rcub/rcuop kthread realtime priority */
> +/*
> + * rcuc/rcub/rcuop kthread realtime priority. The former
> + * depends on if CONFIG_RCU_NOCB_CPU_CB_BOOST is set.

Aren't the rcuo[ps] kthreads controlled by the RCU_NOCB_CPU_CB_BOOST
Kconfig option?  (As opposed to the "former", which is "rcuc".)

> + */
>  static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
>  module_param(kthread_prio, int, 0444);
>  
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 60cc92cc6655..a2823be9b1d0 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1315,8 +1315,9 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
>  	if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
>  		goto end;
>  
> -	if (kthread_prio)
> +	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_CB_BOOST))

Don't we need both non-zero kthread_prio and the proper setting of the
new Kconfig option before we run it at SCHED_FIFO?

Yes, we could rely on sched_setscheduler_nocheck() erroring out in
that case, but that sounds like an accident waiting to happen.

>  		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> +
>  	WRITE_ONCE(rdp->nocb_cb_kthread, t);
>  	WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
>  	return;
> -- 
> 2.30.2
>