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

Paul E. McKenney
2020-05-04T15:24:40Z

On Mon, May 04, 2020 at 02:43:23PM +0200, Uladzislau Rezki wrote:
> 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 to make sure I understand... If someone writes a smaller number to
the sysfs variable, the per-CPU caches will be decreased at that point,
immediately during that sysfs write? Or are you saying something else?

> > (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 :)

Hmmm... Maybe it is time for a non-atomic singly linked list? In the RCU
callback processing, the operations were open-coded, but they have been
pushed into include/linux/rcu_segcblist.h and kernel/rcu/rcu_segcblist.*.

Maybe some non-atomic/protected/whatever macros in the llist.h file?
Or maybe just open-code the singly linked list? (Probably not the
best choice, though.) Add comments stating that the atomic properties
of the llist functions aren't neded? Something else?

The comments would be a good start. Just to take pity on people seeing
the potential for concurrency and wondering how the concurrent accesses
actually happen. ;-)

Thanx, Paul