Re: [PATCH RFC tip/core/rcu 02/14] rcu/nocb: Add bypass callback queueing

[Joel Fernandes]

On Tue, Aug 06, 2019 at 08:03:13PM -0400, Joel Fernandes wrote:
> On Fri, Aug 02, 2019 at 08:14:49AM -0700, Paul E. McKenney wrote:
> > Use of the rcu_data structure's segmented ->cblist for no-CBs CPUs
> > takes advantage of unrelated grace periods, thus reducing the memory
> > footprint in the face of floods of call_rcu() invocations.  However,
> > the ->cblist field is a more-complex rcu_segcblist structure which must
> > be protected via locking.  Even though there are only three entities
> > which can acquire this lock (the CPU invoking call_rcu(), the no-CBs
> > grace-period kthread, and the no-CBs callbacks kthread), the contention
> > on this lock is excessive under heavy stress.
> > 
> > This commit therefore greatly reduces contention by provisioning
> > an rcu_cblist structure field named ->nocb_bypass within the
> > rcu_data structure.  Each no-CBs CPU is permitted only a limited
> > number of enqueues onto the ->cblist per jiffy, controlled by a new
> > nocb_nobypass_lim_per_jiffy kernel boot parameter that defaults to
> > about 16 enqueues per millisecond (16 * 1000 / HZ).  When that limit is
> > exceeded, the CPU instead enqueues onto the new ->nocb_bypass.
> 
> Looks quite interesting. I am guessing the not-no-CB (regular) enqueues don't
> need to use the same technique because both enqueues / callback execution are
> happening on same CPU..
> 
> Still looking through patch but I understood the basic idea. Some nits below:
> 
> [snip]
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 2c3e9068671c..e4df86db8137 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -200,18 +200,26 @@ struct rcu_data {
> >  	atomic_t nocb_lock_contended;	/* Contention experienced. */
> >  	int nocb_defer_wakeup;		/* Defer wakeup of nocb_kthread. */
> >  	struct timer_list nocb_timer;	/* Enforce finite deferral. */
> > +	unsigned long nocb_gp_adv_time;	/* Last call_rcu() CB adv (jiffies). */
> > +
> > +	/* The following fields are used by call_rcu, hence own cacheline. */
> > +	raw_spinlock_t nocb_bypass_lock ____cacheline_internodealigned_in_smp;
> > +	struct rcu_cblist nocb_bypass;	/* Lock-contention-bypass CB list. */
> > +	unsigned long nocb_bypass_first; /* Time (jiffies) of first enqueue. */
> > +	unsigned long nocb_nobypass_last; /* Last ->cblist enqueue (jiffies). */
> > +	int nocb_nobypass_count;	/* # ->cblist enqueues at ^^^ time. */
> 
> Can these and below fields be ifdef'd out if !CONFIG_RCU_NOCB_CPU so as to
> keep the size of struct smaller for benefit of systems that don't use NOCB?
> 
> 
> >  
> >  	/* The following fields are used by GP kthread, hence own cacheline. */
> >  	raw_spinlock_t nocb_gp_lock ____cacheline_internodealigned_in_smp;
> > -	bool nocb_gp_sleep;
> > -					/* Is the nocb GP thread asleep? */
> > +	struct timer_list nocb_bypass_timer; /* Force nocb_bypass flush. */
> > +	bool nocb_gp_sleep;		/* Is the nocb GP thread asleep? */
> 
> And these too, I think.

Please ignore this comment, I missed that these were already ifdef'd out
since it did not appear in the diff.

thanks!