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Date:   Fri, 24 Mar 2023 13:11:47 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Wysocki, Rafael J" <rafael.j.wysocki@intel.com>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, rostedt@goodmis.org, hch@lst.de,
        =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <mirq-linux@rere.qmqm.pl>,
        Dmitry Osipenko <dmitry.osipenko@collabora.com>
Subject: Re: [PATCH RFC rcu 03/19] srcu: Begin offloading srcu_struct fields
 to srcu_update
Message-ID: <a0a2f6a7-b623-454c-ac65-69043a9f9f4d@paulmck-laptop>
Reply-To: paulmck@kernel.org
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 <20230324001938.3443499-3-paulmck@kernel.org>
 <80166ee7-4ee3-bb2d-c715-a8180dddee31@intel.com>
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On Fri, Mar 24, 2023 at 08:10:31PM +0100, Wysocki, Rafael J wrote:
> On 3/24/2023 1:19 AM, Paul E. McKenney wrote:
> > The current srcu_struct structure is on the order of 200 bytes in size
> > (depending on architecture and .config), which is much better than the
> > old-style 26K bytes, but still all too inconvenient when one is trying
> > to achieve good cache locality on a fastpath involving SRCU readers.
> > 
> > However, only a few fields in srcu_struct are used by SRCU readers.
> > The remaining fields could be offloaded to a new srcu_update
> > structure, thus shrinking the srcu_struct structure down to a few
> > tens of bytes.  This commit begins this noble quest, a quest that is
> > complicated by open-coded initialization of the srcu_struct within the
> > srcu_notifier_head structure.  This complication is addressed by updating
> > the srcu_notifier_head structure's open coding, given that there does
> > not appear to be a straightforward way of abstracting that initialization.
> > 
> > This commit moves only the ->node pointer to srcu_update.  Later commits
> > will move additional fields.
> > 
> > [ paulmck: Fold in qiang1.zhang@intel.com's memory-leak fix. ]
> > 
> > Link: https://lore.kernel.org/all/20230320055751.4120251-1-qiang1.zhang@intel.com/
> > Suggested-by: Christoph Hellwig <hch@lst.de>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
> > Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
> > Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> 
> Fine with me.
> 
> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Thank you!  I will add this on my next rebase.

It is possible that this will be v6.5 rather than v6.4 material.

							Thanx, Paul

> > ---
> >   include/linux/notifier.h |  5 ++++-
> >   include/linux/srcutiny.h |  6 +++---
> >   include/linux/srcutree.h | 27 ++++++++++++++++++---------
> >   kernel/rcu/rcu.h         |  6 ++++--
> >   kernel/rcu/srcutree.c    | 28 +++++++++++++++++++---------
> >   5 files changed, 48 insertions(+), 24 deletions(-)
> > 
> > diff --git a/include/linux/notifier.h b/include/linux/notifier.h
> > index aef88c2d1173..2aba75145144 100644
> > --- a/include/linux/notifier.h
> > +++ b/include/linux/notifier.h
> > @@ -73,6 +73,9 @@ struct raw_notifier_head {
> >   struct srcu_notifier_head {
> >   	struct mutex mutex;
> > +#ifdef CONFIG_TREE_SRCU
> > +	struct srcu_usage srcuu;
> > +#endif
> >   	struct srcu_struct srcu;
> >   	struct notifier_block __rcu *head;
> >   };
> > @@ -107,7 +110,7 @@ extern void srcu_init_notifier_head(struct srcu_notifier_head *nh);
> >   	{							\
> >   		.mutex = __MUTEX_INITIALIZER(name.mutex),	\
> >   		.head = NULL,					\
> > -		.srcu = __SRCU_STRUCT_INIT(name.srcu, pcpu),	\
> > +		.srcu = __SRCU_STRUCT_INIT(name.srcu, name.srcuu, pcpu), \
> >   	}
> >   #define ATOMIC_NOTIFIER_HEAD(name)				\
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index 5aa5e0faf6a1..ebd72491af99 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -31,7 +31,7 @@ struct srcu_struct {
> >   void srcu_drive_gp(struct work_struct *wp);
> > -#define __SRCU_STRUCT_INIT(name, __ignored)				\
> > +#define __SRCU_STRUCT_INIT(name, __ignored, ___ignored)			\
> >   {									\
> >   	.srcu_wq = __SWAIT_QUEUE_HEAD_INITIALIZER(name.srcu_wq),	\
> >   	.srcu_cb_tail = &name.srcu_cb_head,				\
> > @@ -44,9 +44,9 @@ void srcu_drive_gp(struct work_struct *wp);
> >    * Tree SRCU, which needs some per-CPU data.
> >    */
> >   #define DEFINE_SRCU(name) \
> > -	struct srcu_struct name = __SRCU_STRUCT_INIT(name, name)
> > +	struct srcu_struct name = __SRCU_STRUCT_INIT(name, name, name)
> >   #define DEFINE_STATIC_SRCU(name) \
> > -	static struct srcu_struct name = __SRCU_STRUCT_INIT(name, name)
> > +	static struct srcu_struct name = __SRCU_STRUCT_INIT(name, name, name)
> >   void synchronize_srcu(struct srcu_struct *ssp);
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index 428480152375..2689e64024bb 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -57,11 +57,17 @@ struct srcu_node {
> >   	int grphi;				/* Biggest CPU for node. */
> >   };
> > +/*
> > + * Per-SRCU-domain structure, update-side data linked from srcu_struct.
> > + */
> > +struct srcu_usage {
> > +	struct srcu_node *node;			/* Combining tree. */
> > +};
> > +
> >   /*
> >    * Per-SRCU-domain structure, similar in function to rcu_state.
> >    */
> >   struct srcu_struct {
> > -	struct srcu_node *node;			/* Combining tree. */
> >   	struct srcu_node *level[RCU_NUM_LVLS + 1];
> >   						/* First node at each level. */
> >   	int srcu_size_state;			/* Small-to-big transition state. */
> > @@ -90,6 +96,7 @@ struct srcu_struct {
> >   	unsigned long reschedule_count;
> >   	struct delayed_work work;
> >   	struct lockdep_map dep_map;
> > +	struct srcu_usage *srcu_sup;		/* Update-side data. */
> >   };
> >   // Values for size state variable (->srcu_size_state).  Once the state
> > @@ -121,24 +128,24 @@ struct srcu_struct {
> >   #define SRCU_STATE_SCAN1	1
> >   #define SRCU_STATE_SCAN2	2
> > -#define __SRCU_STRUCT_INIT_COMMON(name)								\
> > +#define __SRCU_STRUCT_INIT_COMMON(name, usage_name)						\
> >   	.lock = __SPIN_LOCK_UNLOCKED(name.lock),						\
> >   	.srcu_gp_seq_needed = -1UL,								\
> >   	.work = __DELAYED_WORK_INITIALIZER(name.work, NULL, 0),					\
> > +	.srcu_sup = &usage_name,								\
> >   	__SRCU_DEP_MAP_INIT(name)
> > -#define __SRCU_STRUCT_INIT_MODULE(name)								\
> > +#define __SRCU_STRUCT_INIT_MODULE(name, usage_name)						\
> >   {												\
> > -	__SRCU_STRUCT_INIT_COMMON(name)								\
> > +	__SRCU_STRUCT_INIT_COMMON(name, usage_name)						\
> >   }
> > -#define __SRCU_STRUCT_INIT(name, pcpu_name)							\
> > +#define __SRCU_STRUCT_INIT(name, usage_name, pcpu_name)						\
> >   {												\
> >   	.sda = &pcpu_name,									\
> > -	__SRCU_STRUCT_INIT_COMMON(name)								\
> > +	__SRCU_STRUCT_INIT_COMMON(name, usage_name)						\
> >   }
> > -
> >   /*
> >    * Define and initialize a srcu struct at build time.
> >    * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it.
> > @@ -160,15 +167,17 @@ struct srcu_struct {
> >    */
> >   #ifdef MODULE
> >   # define __DEFINE_SRCU(name, is_static)								\
> > -	is_static struct srcu_struct name = __SRCU_STRUCT_INIT_MODULE(name);			\
> > +	static struct srcu_usage name##_srcu_usage;						\
> > +	is_static struct srcu_struct name = __SRCU_STRUCT_INIT_MODULE(name, name##_srcu_usage);	\
> >   	extern struct srcu_struct * const __srcu_struct_##name;					\
> >   	struct srcu_struct * const __srcu_struct_##name						\
> >   		__section("___srcu_struct_ptrs") = &name
> >   #else
> >   # define __DEFINE_SRCU(name, is_static)								\
> >   	static DEFINE_PER_CPU(struct srcu_data, name##_srcu_data);				\
> > +	static struct srcu_usage name##_srcu_usage;						\
> >   	is_static struct srcu_struct name =							\
> > -		__SRCU_STRUCT_INIT(name, name##_srcu_data)
> > +		__SRCU_STRUCT_INIT(name, name##_srcu_usage, name##_srcu_data)
> >   #endif
> >   #define DEFINE_SRCU(name)		__DEFINE_SRCU(name, /* not static */)
> >   #define DEFINE_STATIC_SRCU(name)	__DEFINE_SRCU(name, static)
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index a3adcf9a9919..4a1b9622598b 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -378,11 +378,13 @@ extern void rcu_init_geometry(void);
> >    * specified state structure (for SRCU) or the only rcu_state structure
> >    * (for RCU).
> >    */
> > -#define srcu_for_each_node_breadth_first(sp, rnp) \
> > +#define _rcu_for_each_node_breadth_first(sp, rnp) \
> >   	for ((rnp) = &(sp)->node[0]; \
> >   	     (rnp) < &(sp)->node[rcu_num_nodes]; (rnp)++)
> >   #define rcu_for_each_node_breadth_first(rnp) \
> > -	srcu_for_each_node_breadth_first(&rcu_state, rnp)
> > +	_rcu_for_each_node_breadth_first(&rcu_state, rnp)
> > +#define srcu_for_each_node_breadth_first(ssp, rnp) \
> > +	_rcu_for_each_node_breadth_first(ssp->srcu_sup, rnp)
> >   /*
> >    * Scan the leaves of the rcu_node hierarchy for the rcu_state structure.
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 7a6d9452a5d0..ad1d5ca42a99 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -173,12 +173,12 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
> >   	/* Initialize geometry if it has not already been initialized. */
> >   	rcu_init_geometry();
> > -	ssp->node = kcalloc(rcu_num_nodes, sizeof(*ssp->node), gfp_flags);
> > -	if (!ssp->node)
> > +	ssp->srcu_sup->node = kcalloc(rcu_num_nodes, sizeof(*ssp->srcu_sup->node), gfp_flags);
> > +	if (!ssp->srcu_sup->node)
> >   		return false;
> >   	/* Work out the overall tree geometry. */
> > -	ssp->level[0] = &ssp->node[0];
> > +	ssp->level[0] = &ssp->srcu_sup->node[0];
> >   	for (i = 1; i < rcu_num_lvls; i++)
> >   		ssp->level[i] = ssp->level[i - 1] + num_rcu_lvl[i - 1];
> >   	rcu_init_levelspread(levelspread, num_rcu_lvl);
> > @@ -195,7 +195,7 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
> >   		snp->srcu_gp_seq_needed_exp = SRCU_SNP_INIT_SEQ;
> >   		snp->grplo = -1;
> >   		snp->grphi = -1;
> > -		if (snp == &ssp->node[0]) {
> > +		if (snp == &ssp->srcu_sup->node[0]) {
> >   			/* Root node, special case. */
> >   			snp->srcu_parent = NULL;
> >   			continue;
> > @@ -236,8 +236,12 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
> >    */
> >   static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
> >   {
> > +	if (!is_static)
> > +		ssp->srcu_sup = kzalloc(sizeof(*ssp->srcu_sup), GFP_KERNEL);
> > +	if (!ssp->srcu_sup)
> > +		return -ENOMEM;
> >   	ssp->srcu_size_state = SRCU_SIZE_SMALL;
> > -	ssp->node = NULL;
> > +	ssp->srcu_sup->node = NULL;
> >   	mutex_init(&ssp->srcu_cb_mutex);
> >   	mutex_init(&ssp->srcu_gp_mutex);
> >   	ssp->srcu_idx = 0;
> > @@ -249,8 +253,11 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
> >   	ssp->sda_is_static = is_static;
> >   	if (!is_static)
> >   		ssp->sda = alloc_percpu(struct srcu_data);
> > -	if (!ssp->sda)
> > +	if (!ssp->sda) {
> > +		if (!is_static)
> > +			kfree(ssp->srcu_sup);
> >   		return -ENOMEM;
> > +	}
> >   	init_srcu_struct_data(ssp);
> >   	ssp->srcu_gp_seq_needed_exp = 0;
> >   	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
> > @@ -259,6 +266,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
> >   			if (!ssp->sda_is_static) {
> >   				free_percpu(ssp->sda);
> >   				ssp->sda = NULL;
> > +				kfree(ssp->srcu_sup);
> >   				return -ENOMEM;
> >   			}
> >   		} else {
> > @@ -656,13 +664,15 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
> >   			rcu_seq_current(&ssp->srcu_gp_seq), ssp->srcu_gp_seq_needed);
> >   		return; /* Caller forgot to stop doing call_srcu()? */
> >   	}
> > +	kfree(ssp->srcu_sup->node);
> > +	ssp->srcu_sup->node = NULL;
> > +	ssp->srcu_size_state = SRCU_SIZE_SMALL;
> >   	if (!ssp->sda_is_static) {
> >   		free_percpu(ssp->sda);
> >   		ssp->sda = NULL;
> > +		kfree(ssp->srcu_sup);
> > +		ssp->srcu_sup = NULL;
> >   	}
> > -	kfree(ssp->node);
> > -	ssp->node = NULL;
> > -	ssp->srcu_size_state = SRCU_SIZE_SMALL;
> >   }
> >   EXPORT_SYMBOL_GPL(cleanup_srcu_struct);