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Date:   Thu, 29 Aug 2019 18:13:12 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-kernel@vger.kernel.org, byungchul.park@lge.com,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-doc@vger.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        kernel-team@android.com
Subject: Re: [PATCH 4/5] rcu: Remove kfree_rcu() special casing and lazy
 handling
Message-ID: <20190829221312.GB183862@google.com>
References: <5d657e39.1c69fb81.54250.01e0@mx.google.com>
 <20190828214629.GZ26530@linux.ibm.com>
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All comments addressed on this one, thanks!

 - Joel

On Wed, Aug 28, 2019 at 02:46:29PM -0700, Paul E. McKenney wrote:
> On Tue, Aug 27, 2019 at 03:01:58PM -0400, Joel Fernandes (Google) wrote:
> > Remove kfree_rcu() special casing and lazy handling from RCU.
> > For Tiny RCU we fold the special handling into just Tiny RCU code.
> > 
> > Suggested-by: Paul E. McKenney <paulmck@linux.ibm.com>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> Nice!  Several comments inline below.
> 
> 							Thanx, Paul
> 
> > ---
> >  Documentation/RCU/stallwarn.txt | 13 +++------
> >  include/linux/rcu_segcblist.h   |  2 --
> >  include/trace/events/rcu.h      | 32 +++++++++-------------
> >  kernel/rcu/rcu.h                | 27 -------------------
> >  kernel/rcu/rcu_segcblist.c      | 25 +++--------------
> >  kernel/rcu/rcu_segcblist.h      | 25 ++---------------
> >  kernel/rcu/srcutree.c           |  4 +--
> >  kernel/rcu/tiny.c               | 29 +++++++++++++++++++-
> >  kernel/rcu/tree.c               | 48 ++++++++++++++++++++++++---------
> >  kernel/rcu/tree.h               |  1 -
> >  kernel/rcu/tree_plugin.h        | 42 +++++++----------------------
> >  kernel/rcu/tree_stall.h         |  6 ++---
> >  12 files changed, 97 insertions(+), 157 deletions(-)
> > 
> > diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
> > index f48f4621ccbc..2c02b8de01e7 100644
> > --- a/Documentation/RCU/stallwarn.txt
> > +++ b/Documentation/RCU/stallwarn.txt
> > @@ -225,18 +225,12 @@ an estimate of the total number of RCU callbacks queued across all CPUs
> >  In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
> >  for each CPU:
> >  
> > -	0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 Nonlazy posted: ..D
> > +	0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1
> >  
> >  The "last_accelerate:" prints the low-order 16 bits (in hex) of the
> >  jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
> >  from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
> > -rcu_prepare_for_idle().  The "Nonlazy posted:" indicates lazy-callback
> > -status, so that an "l" indicates that all callbacks were lazy at the start
> > -of the last idle period and an "L" indicates that there are currently
> > -no non-lazy callbacks (in both cases, "." is printed otherwise, as
> > -shown above) and "D" indicates that dyntick-idle processing is enabled
> > -("." is printed otherwise, for example, if disabled via the "nohz="
> > -kernel boot parameter).
> > +rcu_prepare_for_idle().
> > 
> >  If the grace period ends just as the stall warning starts printing,
> >  there will be a spurious stall-warning message, which will include
> > @@ -249,7 +243,8 @@ possible for a zero-jiffy stall to be flagged in this case, depending
> >  on how the stall warning and the grace-period initialization happen to
> >  interact.  Please note that it is not possible to entirely eliminate this
> >  sort of false positive without resorting to things like stop_machine(),
> > -which is overkill for this sort of problem.
> > +which is overkill for this sort of problem. "dyntick_enabled: 1" indicates that
> > +dyntick-idle processing is enabled.
> 
> OK, I'll bite...  Why not add the sentence to the earlier paragraph that
> used to discuss its "D" counterpart?
> 
> >  If all CPUs and tasks have passed through quiescent states, but the
> >  grace period has nevertheless failed to end, the stall-warning splat
> > diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
> > index 646759042333..b36afe7b22c9 100644
> > --- a/include/linux/rcu_segcblist.h
> > +++ b/include/linux/rcu_segcblist.h
> > @@ -22,7 +22,6 @@ struct rcu_cblist {
> >  	struct rcu_head *head;
> >  	struct rcu_head **tail;
> >  	long len;
> > -	long len_lazy;
> >  };
> >  
> >  #define RCU_CBLIST_INITIALIZER(n) { .head = NULL, .tail = &n.head }
> > @@ -73,7 +72,6 @@ struct rcu_segcblist {
> >  #else
> >  	long len;
> >  #endif
> > -	long len_lazy;
> >  	u8 enabled;
> >  	u8 offloaded;
> >  };
> > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > index 694bd040cf51..0dd3478597ee 100644
> > --- a/include/trace/events/rcu.h
> > +++ b/include/trace/events/rcu.h
> > @@ -474,16 +474,14 @@ TRACE_EVENT_RCU(rcu_dyntick,
> >   */
> >  TRACE_EVENT_RCU(rcu_callback,
> >  
> > -	TP_PROTO(const char *rcuname, struct rcu_head *rhp, long qlen_lazy,
> > -		 long qlen),
> > +	TP_PROTO(const char *rcuname, struct rcu_head *rhp, long qlen),
> >  
> > -	TP_ARGS(rcuname, rhp, qlen_lazy, qlen),
> > +	TP_ARGS(rcuname, rhp, qlen),
> >  
> >  	TP_STRUCT__entry(
> >  		__field(const char *, rcuname)
> >  		__field(void *, rhp)
> >  		__field(void *, func)
> > -		__field(long, qlen_lazy)
> >  		__field(long, qlen)
> >  	),
> >  
> > @@ -491,13 +489,12 @@ TRACE_EVENT_RCU(rcu_callback,
> >  		__entry->rcuname = rcuname;
> >  		__entry->rhp = rhp;
> >  		__entry->func = rhp->func;
> > -		__entry->qlen_lazy = qlen_lazy;
> >  		__entry->qlen = qlen;
> >  	),
> >  
> > -	TP_printk("%s rhp=%p func=%ps %ld/%ld",
> > +	TP_printk("%s rhp=%p func=%ps %ld",
> >  		  __entry->rcuname, __entry->rhp, __entry->func,
> > -		  __entry->qlen_lazy, __entry->qlen)
> > +		  __entry->qlen)
> >  );
> >  
> >  /*
> > @@ -511,15 +508,14 @@ TRACE_EVENT_RCU(rcu_callback,
> >  TRACE_EVENT_RCU(rcu_kfree_callback,
> >  
> >  	TP_PROTO(const char *rcuname, struct rcu_head *rhp, unsigned long offset,
> > -		 long qlen_lazy, long qlen),
> > +		 long qlen),
> >  
> > -	TP_ARGS(rcuname, rhp, offset, qlen_lazy, qlen),
> > +	TP_ARGS(rcuname, rhp, offset, qlen),
> >  
> >  	TP_STRUCT__entry(
> >  		__field(const char *, rcuname)
> >  		__field(void *, rhp)
> >  		__field(unsigned long, offset)
> > -		__field(long, qlen_lazy)
> >  		__field(long, qlen)
> >  	),
> >  
> > @@ -527,13 +523,12 @@ TRACE_EVENT_RCU(rcu_kfree_callback,
> >  		__entry->rcuname = rcuname;
> >  		__entry->rhp = rhp;
> >  		__entry->offset = offset;
> > -		__entry->qlen_lazy = qlen_lazy;
> >  		__entry->qlen = qlen;
> >  	),
> >  
> > -	TP_printk("%s rhp=%p func=%ld %ld/%ld",
> > +	TP_printk("%s rhp=%p func=%ld %ld",
> >  		  __entry->rcuname, __entry->rhp, __entry->offset,
> > -		  __entry->qlen_lazy, __entry->qlen)
> > +		  __entry->qlen)
> >  );
> >  
> >  /*
> > @@ -545,27 +540,24 @@ TRACE_EVENT_RCU(rcu_kfree_callback,
> >   */
> >  TRACE_EVENT_RCU(rcu_batch_start,
> >  
> > -	TP_PROTO(const char *rcuname, long qlen_lazy, long qlen, long blimit),
> > +	TP_PROTO(const char *rcuname, long qlen, long blimit),
> >  
> > -	TP_ARGS(rcuname, qlen_lazy, qlen, blimit),
> > +	TP_ARGS(rcuname, qlen, blimit),
> >  
> >  	TP_STRUCT__entry(
> >  		__field(const char *, rcuname)
> > -		__field(long, qlen_lazy)
> >  		__field(long, qlen)
> >  		__field(long, blimit)
> >  	),
> >  
> >  	TP_fast_assign(
> >  		__entry->rcuname = rcuname;
> > -		__entry->qlen_lazy = qlen_lazy;
> >  		__entry->qlen = qlen;
> >  		__entry->blimit = blimit;
> >  	),
> >  
> > -	TP_printk("%s CBs=%ld/%ld bl=%ld",
> > -		  __entry->rcuname, __entry->qlen_lazy, __entry->qlen,
> > -		  __entry->blimit)
> > +	TP_printk("%s CBs=%ld bl=%ld",
> > +		  __entry->rcuname, __entry->qlen, __entry->blimit)
> >  );
> >  
> >  /*
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index aeec70fda82c..3f5a30ca7ed7 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -198,33 +198,6 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
> >  }
> >  #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
> >  
> > -void kfree(const void *);
> > -
> > -/*
> > - * Reclaim the specified callback, either by invoking it (non-lazy case)
> > - * or freeing it directly (lazy case).  Return true if lazy, false otherwise.
> > - */
> > -static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
> > -{
> > -	rcu_callback_t f;
> > -	unsigned long offset = (unsigned long)head->func;
> > -
> > -	rcu_lock_acquire(&rcu_callback_map);
> > -	if (__is_kfree_rcu_offset(offset)) {
> > -		trace_rcu_invoke_kfree_callback(rn, head, offset);
> > -		kfree((void *)head - offset);
> > -		rcu_lock_release(&rcu_callback_map);
> > -		return true;
> > -	} else {
> > -		trace_rcu_invoke_callback(rn, head);
> > -		f = head->func;
> > -		WRITE_ONCE(head->func, (rcu_callback_t)0L);
> > -		f(head);
> > -		rcu_lock_release(&rcu_callback_map);
> > -		return false;
> > -	}
> > -}
> 
> Nice to see this one go!  ;-)
> 
> >  #ifdef CONFIG_RCU_STALL_COMMON
> >  
> >  extern int rcu_cpu_stall_ftrace_dump;
> > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> > index cbc87b804db9..5f4fd3b8777c 100644
> > --- a/kernel/rcu/rcu_segcblist.c
> > +++ b/kernel/rcu/rcu_segcblist.c
> > @@ -20,14 +20,10 @@ void rcu_cblist_init(struct rcu_cblist *rclp)
> >  	rclp->head = NULL;
> >  	rclp->tail = &rclp->head;
> >  	rclp->len = 0;
> > -	rclp->len_lazy = 0;
> >  }
> >  
> >  /*
> >   * Enqueue an rcu_head structure onto the specified callback list.
> > - * This function assumes that the callback is non-lazy because it
> > - * is intended for use by no-CBs CPUs, which do not distinguish
> > - * between lazy and non-lazy RCU callbacks.
> >   */
> >  void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp)
> >  {
> > @@ -54,7 +50,6 @@ void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
> >  	else
> >  		drclp->tail = &drclp->head;
> >  	drclp->len = srclp->len;
> > -	drclp->len_lazy = srclp->len_lazy;
> >  	if (!rhp) {
> >  		rcu_cblist_init(srclp);
> >  	} else {
> > @@ -62,16 +57,12 @@ void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
> >  		srclp->head = rhp;
> >  		srclp->tail = &rhp->next;
> >  		WRITE_ONCE(srclp->len, 1);
> > -		srclp->len_lazy = 0;
> >  	}
> >  }
> >  
> >  /*
> >   * Dequeue the oldest rcu_head structure from the specified callback
> > - * list.  This function assumes that the callback is non-lazy, but
> > - * the caller can later invoke rcu_cblist_dequeued_lazy() if it
> > - * finds otherwise (and if it cares about laziness).  This allows
> > - * different users to have different ways of determining laziness.
> > + * list.
> >   */
> >  struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp)
> >  {
> > @@ -161,7 +152,6 @@ void rcu_segcblist_init(struct rcu_segcblist *rsclp)
> >  	for (i = 0; i < RCU_CBLIST_NSEGS; i++)
> >  		rsclp->tails[i] = &rsclp->head;
> >  	rcu_segcblist_set_len(rsclp, 0);
> > -	rsclp->len_lazy = 0;
> >  	rsclp->enabled = 1;
> >  }
> >  
> > @@ -173,7 +163,6 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
> >  {
> >  	WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
> >  	WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
> > -	WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
> >  	rsclp->enabled = 0;
> >  }
> >  
> > @@ -253,11 +242,9 @@ bool rcu_segcblist_nextgp(struct rcu_segcblist *rsclp, unsigned long *lp)
> >   * absolutely not OK for it to ever miss posting a callback.
> >   */
> >  void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> > -			   struct rcu_head *rhp, bool lazy)
> > +			   struct rcu_head *rhp)
> >  {
> >  	rcu_segcblist_inc_len(rsclp);
> > -	if (lazy)
> > -		rsclp->len_lazy++;
> >  	smp_mb(); /* Ensure counts are updated before callback is enqueued. */
> >  	rhp->next = NULL;
> >  	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
> > @@ -275,15 +262,13 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> >   * period.  You have been warned.
> >   */
> >  bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
> > -			   struct rcu_head *rhp, bool lazy)
> > +			   struct rcu_head *rhp)
> >  {
> >  	int i;
> >  
> >  	if (rcu_segcblist_n_cbs(rsclp) == 0)
> >  		return false;
> >  	rcu_segcblist_inc_len(rsclp);
> > -	if (lazy)
> > -		rsclp->len_lazy++;
> >  	smp_mb(); /* Ensure counts are updated before callback is entrained. */
> >  	rhp->next = NULL;
> >  	for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
> > @@ -307,8 +292,6 @@ bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
> >  void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
> >  					       struct rcu_cblist *rclp)
> >  {
> > -	rclp->len_lazy += rsclp->len_lazy;
> > -	rsclp->len_lazy = 0;
> >  	rclp->len = rcu_segcblist_xchg_len(rsclp, 0);
> >  }
> >  
> > @@ -361,9 +344,7 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
> >  void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
> >  				struct rcu_cblist *rclp)
> >  {
> > -	rsclp->len_lazy += rclp->len_lazy;
> >  	rcu_segcblist_add_len(rsclp, rclp->len);
> > -	rclp->len_lazy = 0;
> >  	rclp->len = 0;
> >  }
> >  
> > diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> > index 815c2fdd3fcc..5c293afc07b8 100644
> > --- a/kernel/rcu/rcu_segcblist.h
> > +++ b/kernel/rcu/rcu_segcblist.h
> > @@ -15,15 +15,6 @@ static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
> >  	return READ_ONCE(rclp->len);
> >  }
> >  
> > -/*
> > - * Account for the fact that a previously dequeued callback turned out
> > - * to be marked as lazy.
> > - */
> > -static inline void rcu_cblist_dequeued_lazy(struct rcu_cblist *rclp)
> > -{
> > -	rclp->len_lazy--;
> > -}
> > -
> >  void rcu_cblist_init(struct rcu_cblist *rclp);
> >  void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
> >  void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
> > @@ -59,18 +50,6 @@ static inline long rcu_segcblist_n_cbs(struct rcu_segcblist *rsclp)
> >  #endif
> >  }
> >  
> > -/* Return number of lazy callbacks in segmented callback list. */
> > -static inline long rcu_segcblist_n_lazy_cbs(struct rcu_segcblist *rsclp)
> > -{
> > -	return rsclp->len_lazy;
> > -}
> > -
> > -/* Return number of lazy callbacks in segmented callback list. */
> > -static inline long rcu_segcblist_n_nonlazy_cbs(struct rcu_segcblist *rsclp)
> > -{
> > -	return rcu_segcblist_n_cbs(rsclp) - rsclp->len_lazy;
> > -}
> > -
> >  /*
> >   * Is the specified rcu_segcblist enabled, for example, not corresponding
> >   * to an offline CPU?
> > @@ -106,9 +85,9 @@ struct rcu_head *rcu_segcblist_first_cb(struct rcu_segcblist *rsclp);
> >  struct rcu_head *rcu_segcblist_first_pend_cb(struct rcu_segcblist *rsclp);
> >  bool rcu_segcblist_nextgp(struct rcu_segcblist *rsclp, unsigned long *lp);
> >  void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> > -			   struct rcu_head *rhp, bool lazy);
> > +			   struct rcu_head *rhp);
> >  bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
> > -			   struct rcu_head *rhp, bool lazy);
> > +			   struct rcu_head *rhp);
> >  void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
> >  				 struct rcu_cblist *rclp);
> >  void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 5dffade2d7cd..d0a9d5b69087 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -853,7 +853,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >  	local_irq_save(flags);
> >  	sdp = this_cpu_ptr(ssp->sda);
> >  	spin_lock_rcu_node(sdp);
> > -	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp, false);
> > +	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> >  	rcu_segcblist_advance(&sdp->srcu_cblist,
> >  			      rcu_seq_current(&ssp->srcu_gp_seq));
> >  	s = rcu_seq_snap(&ssp->srcu_gp_seq);
> > @@ -1052,7 +1052,7 @@ void srcu_barrier(struct srcu_struct *ssp)
> >  		sdp->srcu_barrier_head.func = srcu_barrier_cb;
> >  		debug_rcu_head_queue(&sdp->srcu_barrier_head);
> >  		if (!rcu_segcblist_entrain(&sdp->srcu_cblist,
> > -					   &sdp->srcu_barrier_head, 0)) {
> > +					   &sdp->srcu_barrier_head)) {
> >  			debug_rcu_head_unqueue(&sdp->srcu_barrier_head);
> >  			atomic_dec(&ssp->srcu_barrier_cpu_cnt);
> >  		}
> > diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> > index 477b4eb44af5..cc64dc8b3893 100644
> > --- a/kernel/rcu/tiny.c
> > +++ b/kernel/rcu/tiny.c
> > @@ -73,6 +73,33 @@ void rcu_sched_clock_irq(int user)
> >  	}
> >  }
> >  
> > +void kfree(const void *);
> > +
> > +/*
> > + * Reclaim the specified callback, either by invoking it for non-kfree cases or
> > + * freeing it directly (for kfree). Return true if kfreeing, false otherwise.
> > + */
> > +static inline bool rcu_reclaim_tiny(struct rcu_head *head)
> > +{
> > +	rcu_callback_t f;
> > +	unsigned long offset = (unsigned long)head->func;
> > +
> > +	rcu_lock_acquire(&rcu_callback_map);
> > +	if (__is_kfree_rcu_offset(offset)) {
> > +		trace_rcu_invoke_kfree_callback("", head, offset);
> > +		kfree((void *)head - offset);
> > +		rcu_lock_release(&rcu_callback_map);
> > +		return true;
> > +	}
> > +
> > +	trace_rcu_invoke_callback("", head);
> > +	f = head->func;
> > +	WRITE_ONCE(head->func, (rcu_callback_t)0L);
> > +	f(head);
> > +	rcu_lock_release(&rcu_callback_map);
> > +	return false;
> > +}
> 
> Even if it did show up here in the meantime.  ;-)
> 
> >  /* Invoke the RCU callbacks whose grace period has elapsed.  */
> >  static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused)
> >  {
> > @@ -100,7 +127,7 @@ static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused
> >  		prefetch(next);
> >  		debug_rcu_head_unqueue(list);
> >  		local_bh_disable();
> > -		__rcu_reclaim("", list);
> > +		rcu_reclaim_tiny(list);
> >  		local_bh_enable();
> >  		list = next;
> >  	}
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 64568f12641d..12c17e10f2b4 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2129,6 +2129,23 @@ int rcutree_dead_cpu(unsigned int cpu)
> >  	return 0;
> >  }
> >  
> > +/*
> > + * Reclaim the specified callback.
> > + */
> > +static inline void rcu_reclaim_tree(const char *rn, struct rcu_head *head)
> > +{
> > +	rcu_callback_t f;
> > +
> > +	rcu_lock_acquire(&rcu_callback_map);
> > +	trace_rcu_invoke_callback(rn, head);
> > +
> > +	f = head->func;
> > +	WRITE_ONCE(head->func, (rcu_callback_t)0L);
> > +	f(head);
> > +
> > +	rcu_lock_release(&rcu_callback_map);
> > +}
> 
> Might be worth inlining this one, as you did for kfree_rcu().
> 
> >  /*
> >   * Invoke any RCU callbacks that have made it to the end of their grace
> >   * period.  Thottle as specified by rdp->blimit.
> > @@ -2146,7 +2163,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
> >  	/* If no callbacks are ready, just return. */
> >  	if (!rcu_segcblist_ready_cbs(&rdp->cblist)) {
> >  		trace_rcu_batch_start(rcu_state.name,
> > -				      rcu_segcblist_n_lazy_cbs(&rdp->cblist),
> >  				      rcu_segcblist_n_cbs(&rdp->cblist), 0);
> >  		trace_rcu_batch_end(rcu_state.name, 0,
> >  				    !rcu_segcblist_empty(&rdp->cblist),
> > @@ -2168,7 +2184,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
> >  	if (unlikely(bl > 100))
> >  		tlimit = local_clock() + rcu_resched_ns;
> >  	trace_rcu_batch_start(rcu_state.name,
> > -			      rcu_segcblist_n_lazy_cbs(&rdp->cblist),
> >  			      rcu_segcblist_n_cbs(&rdp->cblist), bl);
> >  	rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
> >  	if (offloaded)
> > @@ -2181,8 +2196,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> >  	rhp = rcu_cblist_dequeue(&rcl);
> >  	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
> >  		debug_rcu_head_unqueue(rhp);
> > -		if (__rcu_reclaim(rcu_state.name, rhp))
> > -			rcu_cblist_dequeued_lazy(&rcl);
> > +		rcu_reclaim_tree(rcu_state.name, rhp);
> > +
> >  		/*
> >  		 * Stop only if limit reached and CPU has something to do.
> >  		 * Note: The rcl structure counts down from zero.
> > @@ -2585,7 +2600,7 @@ static void rcu_leak_callback(struct rcu_head *rhp)
> >   * is expected to specify a CPU.
> >   */
> >  static void
> > -__call_rcu(struct rcu_head *head, rcu_callback_t func, bool lazy)
> > +__call_rcu(struct rcu_head *head, rcu_callback_t func)
> >  {
> >  	unsigned long flags;
> >  	struct rcu_data *rdp;
> > @@ -2620,18 +2635,17 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func, bool lazy)
> >  		if (rcu_segcblist_empty(&rdp->cblist))
> >  			rcu_segcblist_init(&rdp->cblist);
> >  	}
> > +
> >  	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
> >  		return; // Enqueued onto ->nocb_bypass, so just leave.
> >  	/* If we get here, rcu_nocb_try_bypass() acquired ->nocb_lock. */
> > -	rcu_segcblist_enqueue(&rdp->cblist, head, lazy);
> > +	rcu_segcblist_enqueue(&rdp->cblist, head);
> >  	if (__is_kfree_rcu_offset((unsigned long)func))
> >  		trace_rcu_kfree_callback(rcu_state.name, head,
> >  					 (unsigned long)func,
> > -					 rcu_segcblist_n_lazy_cbs(&rdp->cblist),
> >  					 rcu_segcblist_n_cbs(&rdp->cblist));
> >  	else
> >  		trace_rcu_callback(rcu_state.name, head,
> > -				   rcu_segcblist_n_lazy_cbs(&rdp->cblist),
> >  				   rcu_segcblist_n_cbs(&rdp->cblist));
> >  
> >  	/* Go handle any RCU core processing required. */
> > @@ -2681,7 +2695,7 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func, bool lazy)
> >   */
> >  void call_rcu(struct rcu_head *head, rcu_callback_t func)
> >  {
> > -	__call_rcu(head, func, 0);
> > +	__call_rcu(head, func);
> >  }
> >  EXPORT_SYMBOL_GPL(call_rcu);
> >  
> > @@ -2755,10 +2769,18 @@ static void kfree_rcu_work(struct work_struct *work)
> >  	 * access is Ok.
> >  	 */
> >  	for (; head; head = next) {
> > +		unsigned long offset = (unsigned long)head->func;
> > +
> >  		next = head->next;
> > -		/* Could be possible to optimize with kfree_bulk in future */
> > +
> >  		debug_rcu_head_unqueue(head);
> > -		__rcu_reclaim(rcu_state.name, head);
> > +		rcu_lock_acquire(&rcu_callback_map);
> > +		trace_rcu_invoke_kfree_callback(rcu_state.name, head, offset);
> > +
> > +		/* Could be possible to optimize with kfree_bulk in future */
> > +		kfree((void *)head - offset);
> > +
> > +		rcu_lock_release(&rcu_callback_map);
> >  		cond_resched_tasks_rcu_qs();
> >  	}
> >  }
> > @@ -2840,7 +2862,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
> >   */
> >  void kfree_call_rcu_nobatch(struct rcu_head *head, rcu_callback_t func)
> >  {
> > -	__call_rcu(head, func, 1);
> > +	__call_rcu(head, func);
> >  }
> >  EXPORT_SYMBOL_GPL(kfree_call_rcu_nobatch);
> >  
> > @@ -3094,7 +3116,7 @@ static void rcu_barrier_func(void *unused)
> >  	debug_rcu_head_queue(&rdp->barrier_head);
> >  	rcu_nocb_lock(rdp);
> >  	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
> > -	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head, 0)) {
> > +	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
> >  		atomic_inc(&rcu_state.barrier_cpu_count);
> >  	} else {
> >  		debug_rcu_head_unqueue(&rdp->barrier_head);
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 055c31781d3a..15405420b40c 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -183,7 +183,6 @@ struct rcu_data {
> >  	bool rcu_urgent_qs;		/* GP old need light quiescent state. */
> >  	bool rcu_forced_tick;		/* Forced tick to provide QS. */
> >  #ifdef CONFIG_RCU_FAST_NO_HZ
> > -	bool all_lazy;			/* All CPU's CBs lazy at idle start? */
> >  	unsigned long last_accelerate;	/* Last jiffy CBs were accelerated. */
> >  	unsigned long last_advance_all;	/* Last jiffy CBs were all advanced. */
> >  	int tick_nohz_enabled_snap;	/* Previously seen value from sysfs. */
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 2defc7fe74c3..0d0fa615c5fa 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -1274,21 +1274,15 @@ static void rcu_prepare_for_idle(void)
> 
> We now have only one preprocessor symbol (as opposed to the erroneous
> "three proprocessor symbols" in my original).
> 
> >   *	number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
> >   *	system.  And if you are -that- concerned about energy efficiency,
> >   *	just power the system down and be done with it!
> > - * RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
> > - *	permitted to sleep in dyntick-idle mode with only lazy RCU
> > - *	callbacks pending.  Setting this too high can OOM your system.
> >   *
> >   * The values below work well in practice.  If future workloads require
> 
> "value below works"
> 
> >   * adjustment, they can be converted into kernel config parameters, though
> >   * making the state machine smarter might be a better option.
> >   */
> >  #define RCU_IDLE_GP_DELAY 4		/* Roughly one grace period. */
> > -#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ)	/* Roughly six seconds. */
> >  
> >  static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
> >  module_param(rcu_idle_gp_delay, int, 0644);
> > -static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
> > -module_param(rcu_idle_lazy_gp_delay, int, 0644);
> >  
> >  /*
> >   * Try to advance callbacks on the current CPU, but only if it has been
> > @@ -1327,8 +1321,7 @@ static bool __maybe_unused rcu_try_advance_all_cbs(void)
> >  /*
> >   * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
> >   * to invoke.  If the CPU has callbacks, try to advance them.  Tell the
> > - * caller to set the timeout based on whether or not there are non-lazy
> > - * callbacks.
> > + * caller about what to set the timeout.
> >   *
> >   * The caller must have disabled interrupts.
> >   */
> > @@ -1354,25 +1347,19 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
> >  	}
> >  	rdp->last_accelerate = jiffies;
> >  
> > -	/* Request timer delay depending on laziness, and round. */
> > -	rdp->all_lazy = !rcu_segcblist_n_nonlazy_cbs(&rdp->cblist);
> > -	if (rdp->all_lazy) {
> > -		dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
> > -	} else {
> > -		dj = round_up(rcu_idle_gp_delay + jiffies,
> > -			       rcu_idle_gp_delay) - jiffies;
> > -	}
> > +	/* Request timer and round. */
> > +	dj = round_up(rcu_idle_gp_delay + jiffies,
> > +		      rcu_idle_gp_delay) - jiffies;
> 
> I bet that this now fits onto one line.
> 
> > +
> >  	*nextevt = basemono + dj * TICK_NSEC;
> >  	return 0;
> >  }
> >  
> >  /*
> > - * Prepare a CPU for idle from an RCU perspective.  The first major task
> > - * is to sense whether nohz mode has been enabled or disabled via sysfs.
> > - * The second major task is to check to see if a non-lazy callback has
> > - * arrived at a CPU that previously had only lazy callbacks.  The third
> > - * major task is to accelerate (that is, assign grace-period numbers to)
> > - * any recently arrived callbacks.
> > + * Prepare a CPU for idle from an RCU perspective.  The first major task is to
> > + * sense whether nohz mode has been enabled or disabled via sysfs.  The second
> > + * major task is to accelerate (that is, assign grace-period numbers to) any
> > + * recently arrived callbacks.
> >   *
> >   * The caller must have disabled interrupts.
> >   */
> > @@ -1398,17 +1385,6 @@ static void rcu_prepare_for_idle(void)
> >  	if (!tne)
> >  		return;
> >  
> > -	/*
> > -	 * If a non-lazy callback arrived at a CPU having only lazy
> > -	 * callbacks, invoke RCU core for the side-effect of recalculating
> > -	 * idle duration on re-entry to idle.
> > -	 */
> > -	if (rdp->all_lazy && rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)) {
> > -		rdp->all_lazy = false;
> > -		invoke_rcu_core();
> > -		return;
> > -	}
> > -
> >  	/*
> >  	 * If we have not yet accelerated this jiffy, accelerate all
> >  	 * callbacks on this CPU.
> > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > index 841ab43f3e60..67530c23c708 100644
> > --- a/kernel/rcu/tree_stall.h
> > +++ b/kernel/rcu/tree_stall.h
> > @@ -263,11 +263,9 @@ static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
> >  {
> >  	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> >  
> > -	sprintf(cp, "last_accelerate: %04lx/%04lx, Nonlazy posted: %c%c%c",
> > +	sprintf(cp, "last_accelerate: %04lx/%04lx dyntick_enabled: %d",
> >  		rdp->last_accelerate & 0xffff, jiffies & 0xffff,
> > -		".l"[rdp->all_lazy],
> > -		".L"[!rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)],
> > -		".D"[!!rdp->tick_nohz_enabled_snap]);
> > +		!!rdp->tick_nohz_enabled_snap);
> >  }
> >  
> >  #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
> > -- 
> > 2.23.0.187.g17f5b7556c-goog
> >