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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id a6sm2280044lji.29.2020.05.25.14.48.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 25 May 2020 14:48:25 -0700 (PDT) From: "Uladzislau Rezki (Sony)" To: LKML , linux-mm@kvack.org Cc: Andrew Morton , "Paul E . McKenney" , "Theodore Y . Ts'o" , Matthew Wilcox , Joel Fernandes , RCU , Uladzislau Rezki , Oleksiy Avramchenko Subject: [PATCH v2 14/16] rcu: Support reclaim for head-less object Date: Mon, 25 May 2020 23:47:58 +0200 Message-Id: <20200525214800.93072-15-urezki@gmail.com> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200525214800.93072-1-urezki@gmail.com> References: <20200525214800.93072-1-urezki@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org Update the kvfree_call_rcu() function with head-less support. This allows RCU to reclaim objects without an embedded rcu_head. tree-RCU: We introduce two chains of arrays to store SLAB-backed and vmalloc pointers, each. Storage in either of these arrays does not require embedding an rcu_head within the object. Maintaining the arrays may become impossible due to high memory pressure. For such cases there is an emergency path. Objects with rcu_head inside are just queued on a backup rcu_head list. Later on that list is drained. As for the head-less variant, as the current context can sleep, the following emergency measures are applied: a) Synchronously wait until a grace period has elapsed. b) Call kvfree(). tiny-RCU: For double argument calls, there are no new changes in behavior. For single argument call, kvfree() is directly inlined on the current stack after a synchronize_rcu() call. Note that for tiny-RCU, any call to synchronize_rcu() is actually a quiescent state, therefore it does nothing. Reviewed-by: Joel Fernandes (Google) Signed-off-by: Uladzislau Rezki (Sony) Signed-off-by: Joel Fernandes (Google) Co-developed-by: Joel Fernandes (Google) --- include/linux/rcutiny.h | 18 ++++++++++++++++- kernel/rcu/tree.c | 45 +++++++++++++++++++++++++++++++++++++++-- 2 files changed, 60 insertions(+), 3 deletions(-) diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h index 00bbd0e328c8..e7e4ec5938af 100644 --- a/include/linux/rcutiny.h +++ b/include/linux/rcutiny.h @@ -31,9 +31,25 @@ static inline void synchronize_rcu_expedited(void) synchronize_rcu(); } +/* + * Add one more declaration of kvfree() here. It is + * not so straight forward to just include + * where it is defined due to getting many compile + * errors caused by that include. + */ +extern void kvfree(const void *addr); + static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) { - call_rcu(head, func); + if (head) { + call_rcu(head, func); + return; + } + + // kvfree_rcu(one_arg) call. + might_sleep(); + synchronize_rcu(); + kvfree((void *) func); } void rcu_qs(void); diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index da29e6078392..2459830a3851 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3180,6 +3180,13 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr) if (IS_ENABLED(CONFIG_PREEMPT_RT)) return false; + /* + * NOTE: For one argument of kvfree_rcu() we can + * drop the lock and get the page in sleepable + * context. That would allow to maintain an array + * for the CONFIG_PREEMPT_RT as well if no cached + * pages are available. + */ bnode = (struct kvfree_rcu_bulk_data *) __get_free_page(GFP_NOWAIT | __GFP_NOWARN); } @@ -3219,16 +3226,33 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) { unsigned long flags; struct kfree_rcu_cpu *krcp; + bool success; void *ptr; + if (head) { + ptr = (void *) head - (unsigned long) func; + } else { + /* + * Please note there is a limitation for the head-less + * variant, that is why there is a clear rule for such + * objects: it can be used from might_sleep() context + * only. For other places please embed an rcu_head to + * your data. + */ + might_sleep(); + ptr = (unsigned long *) func; + } + krcp = krc_this_cpu_lock(&flags); - ptr = (void *)head - (unsigned long)func; // Queue the object but don't yet schedule the batch. if (debug_rcu_head_queue(ptr)) { // Probable double kfree_rcu(), just leak. WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n", __func__, head); + + // Mark as success and leave. + success = true; goto unlock_return; } @@ -3236,10 +3260,16 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) * Under high memory pressure GFP_NOWAIT can fail, * in that case the emergency path is maintained. */ - if (unlikely(!kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr))) { + success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr); + if (!success) { + if (head == NULL) + // Inline if kvfree_rcu(one_arg) call. + goto unlock_return; + head->func = func; head->next = krcp->head; krcp->head = head; + success = true; } WRITE_ONCE(krcp->count, krcp->count + 1); @@ -3253,6 +3283,17 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) unlock_return: krc_this_cpu_unlock(krcp, flags); + + /* + * Inline kvfree() after synchronize_rcu(). We can do + * it from might_sleep() context only, so the current + * CPU can pass the QS state. + */ + if (!success) { + debug_rcu_head_unqueue((struct rcu_head *) ptr); + synchronize_rcu(); + kvfree(ptr); + } } EXPORT_SYMBOL_GPL(kvfree_call_rcu); -- 2.20.1