回复: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()

["Zhang, Qiang" <Qiang.Zhang@xxxxxxxxxxxxx>]

>________________________________________
>发件人: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
>发送时间: 2021年1月21日 0:21
>收件人: LKML; RCU; Paul E . McKenney; Michael Ellerman
>抄送: Andrew Morton; Daniel Axtens; Frederic Weisbecker; Neeraj >Upadhyay; Joel Fernandes; Peter Zijlstra; Michal Hocko; Thomas >Gleixner; Theodore Y . Ts'o; Sebastian Andrzej Siewior; Uladzislau >Rezki; Oleksiy Avramchenko
>主题: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
>
>Since the page is obtained in a fully preemptible context, dropping
>the lock can lead to migration onto another CPU. As a result a prev.
>bnode of that CPU may be underutilised, because a decision has been
>made for a CPU that was run out of free slots to store a pointer.
>
>migrate_disable/enable() are now independent of RT, use it in order
>to prevent any migration during a page request for a specific CPU it
>is requested for.


Hello Rezki

The critical migrate_disable/enable() area is not allowed to block, under RT and non RT.  
There is such a description in preempt.h 


* Notes on the implementation.
 *
 * The implementation is particularly tricky since existing code patterns
 * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
 * This means that it cannot use cpus_read_lock() to serialize against hotplug,
 * nor can it easily migrate itself into a pending affinity mask change on
 * migrate_enable().


How about the following changes:

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e7a226abff0d..2aa19537ac7c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3488,12 +3488,10 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
                        (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
                bnode = get_cached_bnode(*krcp);
                if (!bnode && can_alloc) {
-                       migrate_disable();
                        krc_this_cpu_unlock(*krcp, *flags);
                        bnode = (struct kvfree_rcu_bulk_data *)
                                __get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
-                       *krcp = krc_this_cpu_lock(flags);
-                       migrate_enable();
+                       raw_spin_lock_irqsave(&(*krcp)->lock, *flags);
                }
 
                if (!bnode)


Thanks
Qiang



>
>Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
>---
> kernel/rcu/tree.c | 2 ++
>1 file changed, 2 insertions(+)
>
>diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>index 454809514c91..cad36074366d 100644
>--- a/kernel/rcu/tree.c
>+++ b/kernel/rcu/tree.c
>@@ -3489,10 +3489,12 @@ add_ptr_to_bulk_krc_lock(struct >kfree_rcu_cpu **krcp,
>                        (*krcp)->bkvhead[idx]->nr_records == >KVFREE_BULK_MAX_ENTR) {
>                bnode = get_cached_bnode(*krcp);
>                if (!bnode && can_alloc) {
>+                       migrate_disable();
>                        krc_this_cpu_unlock(*krcp, *flags);
>                        bnode = (struct kvfree_rcu_bulk_data *)
>                                __get_free_page(GFP_KERNEL | >__GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
>                       *krcp = krc_this_cpu_lock(flags);
>+                       migrate_enable();
>                }
>
>                if (!bnode)
>--
>2.20.1