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SubjectRe: [PATCH v2 1/1] rcu/nocb: Add an option to ON/OFF an offloading from RT context
On Thu, May 12, 2022 at 03:11:19AM +0000, Joel Fernandes wrote:
> On Wed, May 11, 2022 at 10:57:03AM +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. For CONFIG_PREEMPT_RT kernel both RCU_BOOST and the
> > RCU_NOCB_CPU_CB_BOOST are active by default.
> >
> > This patch splits the CONFIG_RCU_BOOST config into two peaces:
> > a) boosting preempted RCU readers and the kthreads which are
> > directly responsible for driving expedited grace periods
> > forward;
> > b) boosting offloading-kthreads in a way that their scheduling
> > class are changed from SCHED_NORMAL to SCHED_FIFO.
> >
> > 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.
> >
> > v1 -> v2:
> > - fix the comment about the rcuc/rcub/rcuop;
> > - check the kthread_prio against zero value;
> > - by default the RCU_NOCB_CPU_CB_BOOST is ON for PREEMPT_RT.
> >
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
>
> Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Thank you both, and I will apply Joel's ack on the next rebase.

Thanx, Paul

> thanks,
>
> - Joel
>
>
> > ---
> > kernel/rcu/Kconfig | 14 ++++++++++++++
> > kernel/rcu/tree.c | 6 +++++-
> > kernel/rcu/tree_nocb.h | 3 ++-
> > 3 files changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> > index 27aab870ae4c..a4ed7b5e2b75 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 "Offload RCU callback from real-time kthread"
> > + depends on RCU_NOCB_CPU && RCU_BOOST
> > + default y if PREEMPT_RT
> > + 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..1c3852b1e0c8 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -154,7 +154,11 @@ 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 "rcuop"
> > + * real-time priority(enabling/disabling) is controlled by
> > + * the extra CONFIG_RCU_NOCB_CPU_CB_BOOST configuration.
> > + */
> > 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..fa8e4f82e60c 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) && kthread_prio)
> > 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
> >

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