Re: [PATCH v5 net 1/3] rcu: add a helper to report consolidated flavor QS

[Yan Zhai]

Hi Paul,

On Tue, Mar 19, 2024 at 4:31 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Tue, Mar 19, 2024 at 01:44:34PM -0700, Yan Zhai wrote:
> > When under heavy load, network processing can run CPU-bound for many
> > tens of seconds. Even in preemptible kernels (non-RT kernel), this can
> > block RCU Tasks grace periods, which can cause trace-event removal to
> > take more than a minute, which is unacceptably long.
> >
> > This commit therefore creates a new helper function that passes through
> > both RCU and RCU-Tasks quiescent states every 100 milliseconds. This
> > hard-coded value suffices for current workloads.
> >
> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Reviewed-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
> > Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
>
> If you would like me to take this one via -rcu, I would be happy to take
> it.  If it would be easier for you to push these as a group though
> networking:
>
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>

Since the whole series aims at fixing net problems, going through net
is probably more consistent.
Also, thank you for your help through the series!

Yan

> > ---
> > v4->v5: adjusted kernel docs and commit message
> > v3->v4: kernel docs error
> >
> > ---
> >  include/linux/rcupdate.h | 31 +++++++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 16f519914415..17d7ed5f3ae6 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -247,6 +247,37 @@ do { \
> >       cond_resched(); \
> >  } while (0)
> >
> > +/**
> > + * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
> > + * @old_ts: jiffies at start of processing.
> > + *
> > + * This helper is for long-running softirq handlers, such as NAPI threads in
> > + * networking. The caller should initialize the variable passed in as @old_ts
> > + * at the beginning of the softirq handler. When invoked frequently, this macro
> > + * will invoke rcu_softirq_qs() every 100 milliseconds thereafter, which will
> > + * provide both RCU and RCU-Tasks quiescent states. Note that this macro
> > + * modifies its old_ts argument.
> > + *
> > + * Because regions of code that have disabled softirq act as RCU read-side
> > + * critical sections, this macro should be invoked with softirq (and
> > + * preemption) enabled.
> > + *
> > + * The macro is not needed when CONFIG_PREEMPT_RT is defined. RT kernels would
> > + * have more chance to invoke schedule() calls and provide necessary quiescent
> > + * states. As a contrast, calling cond_resched() only won't achieve the same
> > + * effect because cond_resched() does not provide RCU-Tasks quiescent states.
> > + */
> > +#define rcu_softirq_qs_periodic(old_ts) \
> > +do { \
> > +     if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> > +         time_after(jiffies, (old_ts) + HZ / 10)) { \
> > +             preempt_disable(); \
> > +             rcu_softirq_qs(); \
> > +             preempt_enable(); \
> > +             (old_ts) = jiffies; \
> > +     } \
> > +} while (0)
> > +
> >  /*
> >   * Infrastructure to implement the synchronize_() primitives in
> >   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> > --
> > 2.30.2
> >
> >