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Date: Tue, 3 Nov 2020 13:03:16 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
"Paul E . McKenney" <paulmck@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Michal Hocko <mhocko@...e.com>,
Thomas Gleixner <tglx@...utronix.de>,
"Theodore Y . Ts'o" <tytso@....edu>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 15/16] rcu/tree: Allocate a page when caller is
preemptible
Hi Vlad,
Few minor nits:
On Thu, Oct 29, 2020 at 05:50:18PM +0100, Uladzislau Rezki (Sony) wrote:
> Given that CONFIG_PREEMPT_COUNT is unconditionally enabled by the
> earlier commits in this series, the preemptible() macro now properly
> detects preempt-disable code regions even in kernels built with
> CONFIG_PREEMPT_NONE.
>
> This commit therefore uses preemptible() to determine whether allocation
> is possible at all for double-argument kvfree_rcu(). If !preemptible(),
> then allocation is not possible, and kvfree_rcu() falls back to using
> the less cache-friendly rcu_head approach. Even when preemptible(),
> the caller might be involved in reclaim, so the GFP_ flags used by
> double-argument kvfree_rcu() must avoid invoking reclaim processing.
>
> Note that single-argument kvfree_rcu() must be invoked in sleepable
> contexts, and that its fallback is the relatively high latency
> synchronize_rcu(). Single-argument kvfree_rcu() therefore uses
> GFP_KERNEL|__GFP_RETRY_MAYFAIL to allow limited sleeping within the
> memory allocator.
>
> [ paulmck: Add add_ptr_to_bulk_krc_lock header comment per Michal Hocko. ]
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
> kernel/rcu/tree.c | 48 ++++++++++++++++++++++++++++++-----------------
> 1 file changed, 31 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f2da2a1cc716..3f9b016a44dc 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3406,37 +3406,55 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
> }
> }
>
> +// Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock()
> +// state specified by flags. If can_sleep is true, the caller must
> +// be schedulable and not be holding any locks or mutexes that might be
> +// acquired by the memory allocator or anything that it might invoke.
> +// If !can_sleep, then if !preemptible() no allocation will be undertaken,
> +// otherwise the allocation will use GFP_ATOMIC to avoid the remainder of
> +// the aforementioned deadlock possibilities. Returns true if ptr was
> +// successfully recorded, else the caller must use a fallback.
> static inline bool
> -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> +add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
> + unsigned long *flags, void *ptr, bool can_sleep)
> {
> struct kvfree_rcu_bulk_data *bnode;
> + bool can_alloc_page = preemptible();
> + gfp_t gfp = (can_sleep ? GFP_KERNEL | __GFP_RETRY_MAYFAIL :
> + GFP_ATOMIC) | __GFP_NOWARN;
> int idx;
>
> - if (unlikely(!krcp->initialized))
> + *krcp = krc_this_cpu_lock(flags);
> + if (unlikely(!(*krcp)->initialized))
> return false;
>
> - lockdep_assert_held(&krcp->lock);
> idx = !!is_vmalloc_addr(ptr);
>
> /* Check if a new block is required. */
Maybe convert this comment also to //... like the new ones you added (and the
ones below).
> - if (!krcp->bkvhead[idx] ||
> - krcp->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> - bnode = get_cached_bnode(krcp);
> - /* Switch to emergency path. */
> + if (!(*krcp)->bkvhead[idx] ||
> + (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> + bnode = get_cached_bnode(*krcp);
> + if (!bnode && can_alloc_page) {
I think you can directly put preemptible() here with a comment saying
allocate only if preemptible and get rid of can_alloc_page.
> + krc_this_cpu_unlock(*krcp, *flags);
> + bnode = (struct kvfree_rcu_bulk_data *)
> + __get_free_page(gfp);
> + *krcp = krc_this_cpu_lock(flags);
> + }
> +
I think the "Switch to emergency path" comment should stay here before the
if().
thanks,
- Joel
> if (!bnode)
> return false;
>
> /* Initialize the new block. */
> bnode->nr_records = 0;
> - bnode->next = krcp->bkvhead[idx];
> + bnode->next = (*krcp)->bkvhead[idx];
>
> /* Attach it to the head. */
> - krcp->bkvhead[idx] = bnode;
> + (*krcp)->bkvhead[idx] = bnode;
> }
>
> /* Finally insert. */
> - krcp->bkvhead[idx]->records
> - [krcp->bkvhead[idx]->nr_records++] = ptr;
> + (*krcp)->bkvhead[idx]->records
> + [(*krcp)->bkvhead[idx]->nr_records++] = ptr;
>
> return true;
> }
> @@ -3474,20 +3492,16 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> ptr = (unsigned long *) func;
> }
>
> - krcp = krc_this_cpu_lock(&flags);
> -
> // 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;
> + return;
> }
>
> - success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
> + success = add_ptr_to_bulk_krc_lock(&krcp, &flags, ptr, !head);
> if (!success) {
> run_page_cache_worker(krcp);
>
> --
> 2.20.1
>
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