Re: [PATCH 09/24] rcu/tree: cache specified number of objects

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, May 01, 2020 at 02:27:49PM -0700, Paul E. McKenney wrote:
> On Tue, Apr 28, 2020 at 10:58:48PM +0200, Uladzislau Rezki (Sony) wrote:
> > Cache some extra objects per-CPU. During reclaim process
> > some pages are cached instead of releasing by linking them
> > into the list. Such approach provides O(1) access time to
> > the cache.
> > 
> > That reduces number of requests to the page allocator, also
> > that makes it more helpful if a low memory condition occurs.
> > 
> > A parameter reflecting the minimum allowed pages to be
> > cached per one CPU is propagated via sysfs, it is read
> > only, the name is "rcu_min_cached_objs".
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  kernel/rcu/tree.c | 64 ++++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 60 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 89e9ca3f4e3e..d8975819b1c9 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -178,6 +178,14 @@ module_param(gp_init_delay, int, 0444);
> >  static int gp_cleanup_delay;
> >  module_param(gp_cleanup_delay, int, 0444);
> >  
> > +/*
> > + * This rcu parameter is read-only, but can be write also.
> 
> You mean that although the parameter is read-only, you see no reason
> why it could not be converted to writeable?
> 
I added just a note. If it is writable, then we can change the size of the
per-CPU cache dynamically, i.e. "echo 5 > /sys/.../rcu_min_cached_objs"
would cache 5 pages. But i do not have a strong opinion if it should be
writable.

>
> If it was writeable, and a given CPU had the maximum numbr of cached
> objects, the rcu_min_cached_objs value was decreased, but that CPU never
> saw another kfree_rcu(), would the number of cached objects change?
> 
No. It works the way: unqueue the page from cache in the kfree_rcu(),
whereas "rcu work" will put it back if number of objects < rcu_min_cached_objs,
if >= will free the page.

>
> (Just curious, not asking for a change in functionality.)
> 
> > + * It reflects the minimum allowed number of objects which
> > + * can be cached per-CPU. Object size is equal to one page.
> > + */
> > +int rcu_min_cached_objs = 2;
> > +module_param(rcu_min_cached_objs, int, 0444);
> > +
> >  /* Retrieve RCU kthreads priority for rcutorture */
> >  int rcu_get_gp_kthreads_prio(void)
> >  {
> > @@ -2887,7 +2895,6 @@ struct kfree_rcu_cpu_work {
> >   * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
> >   * @head: List of kfree_rcu() objects not yet waiting for a grace period
> >   * @bhead: Bulk-List of kfree_rcu() objects not yet waiting for a grace period
> > - * @bcached: Keeps at most one object for later reuse when build chain blocks
> >   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
> >   * @lock: Synchronize access to this structure
> >   * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
> > @@ -2902,7 +2909,6 @@ struct kfree_rcu_cpu_work {
> >  struct kfree_rcu_cpu {
> >  	struct rcu_head *head;
> >  	struct kfree_rcu_bulk_data *bhead;
> > -	struct kfree_rcu_bulk_data *bcached;
> >  	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> >  	raw_spinlock_t lock;
> >  	struct delayed_work monitor_work;
> > @@ -2910,6 +2916,15 @@ struct kfree_rcu_cpu {
> >  	bool initialized;
> >  	// Number of objects for which GP not started
> >  	int count;
> > +
> > +	/*
> > +	 * Number of cached objects which are queued into
> > +	 * the lock-less list. This cache is used by the
> > +	 * kvfree_call_rcu() function and as of now its
> > +	 * size is static.
> > +	 */
> > +	struct llist_head bkvcache;
> > +	int nr_bkv_objs;
> >  };
> >  
> >  static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
> > @@ -2946,6 +2961,31 @@ krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
> >  	local_irq_restore(flags);
> >  }
> >  
> > +static inline struct kfree_rcu_bulk_data *
> > +get_cached_bnode(struct kfree_rcu_cpu *krcp)
> > +{
> > +	if (!krcp->nr_bkv_objs)
> > +		return NULL;
> > +
> > +	krcp->nr_bkv_objs--;
> > +	return (struct kfree_rcu_bulk_data *)
> > +		llist_del_first(&krcp->bkvcache);
> > +}
> > +
> > +static inline bool
> > +put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > +	struct kfree_rcu_bulk_data *bnode)
> > +{
> > +	/* Check the limit. */
> > +	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > +		return false;
> > +
> > +	llist_add((struct llist_node *) bnode, &krcp->bkvcache);
> > +	krcp->nr_bkv_objs++;
> > +	return true;
> > +
> > +}
> > +
> >  /*
> >   * This function is invoked in workqueue context after a grace period.
> >   * It frees all the objects queued on ->bhead_free or ->head_free.
> > @@ -2981,7 +3021,12 @@ static void kfree_rcu_work(struct work_struct *work)
> >  		kfree_bulk(bhead->nr_records, bhead->records);
> >  		rcu_lock_release(&rcu_callback_map);
> >  
> > -		if (cmpxchg(&krcp->bcached, NULL, bhead))
> > +		krcp = krc_this_cpu_lock(&flags);
> 
> Presumably the list can also be accessed without holding this lock,
> because otherwise we shouldn't need llist...
> 
Hm... We increase the number of elements in cache, therefore it is not
lockless. From the other hand i used llist_head to maintain the cache
because it is single linked list, we do not need "*prev" link. Also
we do not need to init the list.

But i can change it to list_head. Please let me know if i need :)

--
Vlad Rezki