Date: Wed, 28 Jun 2023 10:57:26 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: daniel@...earbox.net, andrii@...nel.org, void@...ifault.com,
	houtao@...weicloud.com, tj@...nel.org, rcu@...r.kernel.org,
	netdev@...r.kernel.org, bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v3 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

On Tue, Jun 27, 2023 at 06:56:33PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> Introduce bpf_mem_[cache_]free_rcu() similar to kfree_rcu().
> Unlike bpf_mem_[cache_]free() that links objects for immediate reuse into
> per-cpu free list the _rcu() flavor waits for RCU grace period and then moves
> objects into free_by_rcu_ttrace list where they are waiting for RCU
> task trace grace period to be freed into slab.
> 
> The life cycle of objects:
> alloc: dequeue free_llist
> free: enqeueu free_llist
> free_rcu: enqueue free_by_rcu -> waiting_for_gp
> free_llist above high watermark -> free_by_rcu_ttrace
> after RCU GP waiting_for_gp -> free_by_rcu_ttrace
> free_by_rcu_ttrace -> waiting_for_gp_ttrace -> slab
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
>  include/linux/bpf_mem_alloc.h |   2 +
>  kernel/bpf/memalloc.c         | 129 +++++++++++++++++++++++++++++++++-
>  2 files changed, 128 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bpf_mem_alloc.h b/include/linux/bpf_mem_alloc.h
> index 3929be5743f4..d644bbb298af 100644
> --- a/include/linux/bpf_mem_alloc.h
> +++ b/include/linux/bpf_mem_alloc.h
> @@ -27,10 +27,12 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma);
>  /* kmalloc/kfree equivalent: */
>  void *bpf_mem_alloc(struct bpf_mem_alloc *ma, size_t size);
>  void bpf_mem_free(struct bpf_mem_alloc *ma, void *ptr);
> +void bpf_mem_free_rcu(struct bpf_mem_alloc *ma, void *ptr);
>  
>  /* kmem_cache_alloc/free equivalent: */
>  void *bpf_mem_cache_alloc(struct bpf_mem_alloc *ma);
>  void bpf_mem_cache_free(struct bpf_mem_alloc *ma, void *ptr);
> +void bpf_mem_cache_free_rcu(struct bpf_mem_alloc *ma, void *ptr);
>  void bpf_mem_cache_raw_free(void *ptr);
>  void *bpf_mem_cache_alloc_flags(struct bpf_mem_alloc *ma, gfp_t flags);
>  
> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> index 40524d9454c7..3081d06a434c 100644
> --- a/kernel/bpf/memalloc.c
> +++ b/kernel/bpf/memalloc.c
> @@ -101,6 +101,15 @@ struct bpf_mem_cache {
>  	bool draining;
>  	struct bpf_mem_cache *tgt;
>  
> +	/* list of objects to be freed after RCU GP */
> +	struct llist_head free_by_rcu;
> +	struct llist_node *free_by_rcu_tail;
> +	struct llist_head waiting_for_gp;
> +	struct llist_node *waiting_for_gp_tail;
> +	struct rcu_head rcu;
> +	atomic_t call_rcu_in_progress;
> +	struct llist_head free_llist_extra_rcu;
> +
>  	/* list of objects to be freed after RCU tasks trace GP */
>  	struct llist_head free_by_rcu_ttrace;
>  	struct llist_head waiting_for_gp_ttrace;
> @@ -344,6 +353,69 @@ static void free_bulk(struct bpf_mem_cache *c)
>  	do_call_rcu_ttrace(tgt);
>  }
>  
> +static void __free_by_rcu(struct rcu_head *head)
> +{
> +	struct bpf_mem_cache *c = container_of(head, struct bpf_mem_cache, rcu);
> +	struct bpf_mem_cache *tgt = c->tgt;
> +	struct llist_node *llnode;
> +
> +	llnode = llist_del_all(&c->waiting_for_gp);
> +	if (!llnode)
> +		goto out;
> +
> +	llist_add_batch(llnode, c->waiting_for_gp_tail, &tgt->free_by_rcu_ttrace);
> +
> +	/* Objects went through regular RCU GP. Send them to RCU tasks trace */
> +	do_call_rcu_ttrace(tgt);
> +out:
> +	atomic_set(&c->call_rcu_in_progress, 0);
> +}
> +
> +static void check_free_by_rcu(struct bpf_mem_cache *c)
> +{
> +	struct llist_node *llnode, *t;
> +	unsigned long flags;
> +
> +	/* drain free_llist_extra_rcu */
> +	if (unlikely(!llist_empty(&c->free_llist_extra_rcu))) {
> +		inc_active(c, &flags);
> +		llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist_extra_rcu))
> +			if (__llist_add(llnode, &c->free_by_rcu))
> +				c->free_by_rcu_tail = llnode;
> +		dec_active(c, flags);
> +	}
> +
> +	if (llist_empty(&c->free_by_rcu))
> +		return;
> +
> +	if (atomic_xchg(&c->call_rcu_in_progress, 1)) {
> +		/*
> +		 * Instead of kmalloc-ing new rcu_head and triggering 10k
> +		 * call_rcu() to hit rcutree.qhimark and force RCU to notice
> +		 * the overload just ask RCU to hurry up. There could be many
> +		 * objects in free_by_rcu list.
> +		 * This hint reduces memory consumption for an artifical
> +		 * benchmark from 2 Gbyte to 150 Mbyte.
> +		 */
> +		rcu_request_urgent_qs_task(current);

I have been going back and forth on whether rcu_request_urgent_qs_task()
needs to throttle calls to itself, for example, to pay attention to only
one invocation per jiffy.  The theory here is that RCU's state machine
normally only advances about once per jiffy anyway.

The main risk of *not* throttling is if several CPUs were to invoke
rcu_request_urgent_qs_task() in tight loops while those same CPUs were
undergoing interrupt storms, which would result in heavy lock contention
in __rcu_irq_enter_check_tick().  This is not exactly a common-case
scenario, but on the other hand, if you are having this degree of trouble,
should RCU really be adding lock contention to your troubles?

Thoughts?

							Thanx, Paul

> +		return;
> +	}
> +
> +	WARN_ON_ONCE(!llist_empty(&c->waiting_for_gp));
> +
> +	inc_active(c, &flags);
> +	WRITE_ONCE(c->waiting_for_gp.first, __llist_del_all(&c->free_by_rcu));
> +	c->waiting_for_gp_tail = c->free_by_rcu_tail;
> +	dec_active(c, flags);
> +
> +	if (unlikely(READ_ONCE(c->draining))) {
> +		free_all(llist_del_all(&c->waiting_for_gp), !!c->percpu_size);
> +		atomic_set(&c->call_rcu_in_progress, 0);
> +	} else {
> +		call_rcu_hurry(&c->rcu, __free_by_rcu);
> +	}
> +}
> +
>  static void bpf_mem_refill(struct irq_work *work)
>  {
>  	struct bpf_mem_cache *c = container_of(work, struct bpf_mem_cache, refill_work);
> @@ -358,6 +430,8 @@ static void bpf_mem_refill(struct irq_work *work)
>  		alloc_bulk(c, c->batch, NUMA_NO_NODE);
>  	else if (cnt > c->high_watermark)
>  		free_bulk(c);
> +
> +	check_free_by_rcu(c);
>  }
>  
>  static void notrace irq_work_raise(struct bpf_mem_cache *c)
> @@ -486,6 +560,9 @@ static void drain_mem_cache(struct bpf_mem_cache *c)
>  	free_all(llist_del_all(&c->waiting_for_gp_ttrace), percpu);
>  	free_all(__llist_del_all(&c->free_llist), percpu);
>  	free_all(__llist_del_all(&c->free_llist_extra), percpu);
> +	free_all(__llist_del_all(&c->free_by_rcu), percpu);
> +	free_all(__llist_del_all(&c->free_llist_extra_rcu), percpu);
> +	free_all(llist_del_all(&c->waiting_for_gp), percpu);
>  }
>  
>  static void free_mem_alloc_no_barrier(struct bpf_mem_alloc *ma)
> @@ -498,8 +575,8 @@ static void free_mem_alloc_no_barrier(struct bpf_mem_alloc *ma)
>  
>  static void free_mem_alloc(struct bpf_mem_alloc *ma)
>  {
> -	/* waiting_for_gp_ttrace lists was drained, but __free_rcu might
> -	 * still execute. Wait for it now before we freeing percpu caches.
> +	/* waiting_for_gp[_ttrace] lists were drained, but RCU callbacks
> +	 * might still execute. Wait for them.
>  	 *
>  	 * rcu_barrier_tasks_trace() doesn't imply synchronize_rcu_tasks_trace(),
>  	 * but rcu_barrier_tasks_trace() and rcu_barrier() below are only used
> @@ -508,7 +585,8 @@ static void free_mem_alloc(struct bpf_mem_alloc *ma)
>  	 * rcu_trace_implies_rcu_gp(), it will be OK to skip rcu_barrier() by
>  	 * using rcu_trace_implies_rcu_gp() as well.
>  	 */
> -	rcu_barrier_tasks_trace();
> +	rcu_barrier(); /* wait for __free_by_rcu */
> +	rcu_barrier_tasks_trace(); /* wait for __free_rcu */
>  	if (!rcu_trace_implies_rcu_gp())
>  		rcu_barrier();
>  	free_mem_alloc_no_barrier(ma);
> @@ -561,6 +639,7 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
>  			irq_work_sync(&c->refill_work);
>  			drain_mem_cache(c);
>  			rcu_in_progress += atomic_read(&c->call_rcu_ttrace_in_progress);
> +			rcu_in_progress += atomic_read(&c->call_rcu_in_progress);
>  		}
>  		/* objcg is the same across cpus */
>  		if (c->objcg)
> @@ -577,6 +656,7 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
>  				irq_work_sync(&c->refill_work);
>  				drain_mem_cache(c);
>  				rcu_in_progress += atomic_read(&c->call_rcu_ttrace_in_progress);
> +				rcu_in_progress += atomic_read(&c->call_rcu_in_progress);
>  			}
>  		}
>  		if (c->objcg)
> @@ -661,6 +741,27 @@ static void notrace unit_free(struct bpf_mem_cache *c, void *ptr)
>  		irq_work_raise(c);
>  }
>  
> +static void notrace unit_free_rcu(struct bpf_mem_cache *c, void *ptr)
> +{
> +	struct llist_node *llnode = ptr - LLIST_NODE_SZ;
> +	unsigned long flags;
> +
> +	c->tgt = *(struct bpf_mem_cache **)llnode;
> +
> +	local_irq_save(flags);
> +	if (local_inc_return(&c->active) == 1) {
> +		if (__llist_add(llnode, &c->free_by_rcu))
> +			c->free_by_rcu_tail = llnode;
> +	} else {
> +		llist_add(llnode, &c->free_llist_extra_rcu);
> +	}
> +	local_dec(&c->active);
> +	local_irq_restore(flags);
> +
> +	if (!atomic_read(&c->call_rcu_in_progress))
> +		irq_work_raise(c);
> +}
> +
>  /* Called from BPF program or from sys_bpf syscall.
>   * In both cases migration is disabled.
>   */
> @@ -694,6 +795,20 @@ void notrace bpf_mem_free(struct bpf_mem_alloc *ma, void *ptr)
>  	unit_free(this_cpu_ptr(ma->caches)->cache + idx, ptr);
>  }
>  
> +void notrace bpf_mem_free_rcu(struct bpf_mem_alloc *ma, void *ptr)
> +{
> +	int idx;
> +
> +	if (!ptr)
> +		return;
> +
> +	idx = bpf_mem_cache_idx(ksize(ptr - LLIST_NODE_SZ));
> +	if (idx < 0)
> +		return;
> +
> +	unit_free_rcu(this_cpu_ptr(ma->caches)->cache + idx, ptr);
> +}
> +
>  void notrace *bpf_mem_cache_alloc(struct bpf_mem_alloc *ma)
>  {
>  	void *ret;
> @@ -710,6 +825,14 @@ void notrace bpf_mem_cache_free(struct bpf_mem_alloc *ma, void *ptr)
>  	unit_free(this_cpu_ptr(ma->cache), ptr);
>  }
>  
> +void notrace bpf_mem_cache_free_rcu(struct bpf_mem_alloc *ma, void *ptr)
> +{
> +	if (!ptr)
> +		return;
> +
> +	unit_free_rcu(this_cpu_ptr(ma->cache), ptr);
> +}
> +
>  /* Directly does a kfree() without putting 'ptr' back to the free_llist
>   * for reuse and without waiting for a rcu_tasks_trace gp.
>   * The caller must first go through the rcu_tasks_trace gp for 'ptr'
> -- 
> 2.34.1
> 

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