Re: [PATCH v2] Binder: add TF_UPDATE_TXN to replace outdated txn

[Greg KH]

On Thu, May 19, 2022 at 11:34:54AM -0700, Li Li wrote:
> From: Li Li <dualli@xxxxxxxxxx>
> 
> When the target process is busy, incoming oneway transactions are
> queued in the async_todo list. If the clients continue sending extra
> oneway transactions while the target process is frozen, this queue can
> become too large to accommodate new transactions. That's why binder
> driver introduced ONEWAY_SPAM_DETECTION to detect this situation. It's
> helpful to debug the async binder buffer exhausting issue, but the
> issue itself isn't solved directly.
> 
> In real cases applications are designed to send oneway transactions
> repeatedly, delivering updated inforamtion to the target process.
> Typical examples are Wi-Fi signal strength and some real time sensor
> data. Even if the apps might only care about the lastet information,
> all outdated oneway transactions are still accumulated there until the
> frozen process is thawed later. For this kind of situations, there's
> no existing method to skip those outdated transactions and deliver the
> latest one only.
> 
> This patch introduces a new transaction flag TF_UPDATE_TXN. To use it,
> use apps can set this new flag along with TF_ONE_WAY. When such an
> oneway transaction is to be queued into the async_todo list of a frozen
> process, binder driver will check if any previous pending transactions
> can be superseded by comparing their code, flags and target node. If
> such an outdated pending transaction is found, the latest transaction
> will supersede that outdated one. This effectively prevents the async
> binder buffer running out and saves unnecessary binder read workloads.
> 
> Signed-off-by: Li Li <dualli@xxxxxxxxxx>
> ---
>  drivers/android/binder.c            | 85 ++++++++++++++++++++++++++++-
>  drivers/android/binder_trace.h      |  4 ++
>  include/uapi/linux/android/binder.h |  1 +
>  3 files changed, 87 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index f3b639e89dd8..bb968cf2f9ec 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -2594,6 +2594,56 @@ static int binder_fixup_parent(struct list_head *pf_head,
>  	return binder_add_fixup(pf_head, buffer_offset, bp->buffer, 0);
>  }
>  
> +/**
> + * binder_can_update_transaction() - Can a txn be superseded by an updated one?
> + * @t1: the pending async txn in the frozen process
> + * @t2: the new async txn to supersede the outdated pending one
> + *
> + * Return:  true if t2 can supersede t1
> + *          false if t2 can not supersede t1
> + */
> +static bool binder_can_update_transaction(struct binder_transaction *t1,
> +					  struct binder_transaction *t2)
> +{
> +	if ((t1->flags & t2->flags & (TF_ONE_WAY | TF_UPDATE_TXN)) !=
> +	    (TF_ONE_WAY | TF_UPDATE_TXN) || !t1->to_proc || !t2->to_proc)
> +		return false;
> +	if (t1->to_proc->tsk == t2->to_proc->tsk && t1->code == t2->code &&
> +	    t1->flags == t2->flags && t1->buffer->pid == t2->buffer->pid &&
> +	    t1->buffer->target_node->ptr == t2->buffer->target_node->ptr &&
> +	    t1->buffer->target_node->cookie == t2->buffer->target_node->cookie)
> +		return true;
> +	return false;
> +}
> +
> +/**
> + * binder_find_outdated_transaction_ilocked() - Find the outdated transaction
> + * @t:		 new async transaction
> + * @target_list: list to find outdated transaction
> + *
> + * Return: the outdated transaction if found
> + *         NULL if no outdated transacton can be found
> + *
> + * Requires the proc->inner_lock to be held.
> + */
> +static struct binder_transaction *
> +binder_find_outdated_transaction_ilocked(struct binder_transaction *t,
> +					 struct list_head *target_list)
> +{
> +	struct binder_work *w;
> +
> +	list_for_each_entry(w, target_list, entry) {
> +		struct binder_transaction *t_queued;
> +
> +		if (w->type != BINDER_WORK_TRANSACTION)
> +			continue;
> +		t_queued = container_of(w, struct binder_transaction, work);
> +		if (binder_can_update_transaction(t_queued, t))
> +			return t_queued;
> +	}
> +	return NULL;
> +}
> +
>  /**
>   * binder_proc_transaction() - sends a transaction to a process and wakes it up
>   * @t:		transaction to send
> @@ -2619,6 +2669,7 @@ static int binder_proc_transaction(struct binder_transaction *t,
>  	struct binder_node *node = t->buffer->target_node;
>  	bool oneway = !!(t->flags & TF_ONE_WAY);
>  	bool pending_async = false;
> +	struct binder_transaction *t_outdated = NULL;
>  
>  	BUG_ON(!node);
>  	binder_node_lock(node);
> @@ -2646,12 +2697,24 @@ static int binder_proc_transaction(struct binder_transaction *t,
>  	if (!thread && !pending_async)
>  		thread = binder_select_thread_ilocked(proc);
>  
> -	if (thread)
> +	if (thread) {
>  		binder_enqueue_thread_work_ilocked(thread, &t->work);
> -	else if (!pending_async)
> +	} else if (!pending_async) {
>  		binder_enqueue_work_ilocked(&t->work, &proc->todo);
> -	else
> +	} else {
> +		if ((t->flags & TF_UPDATE_TXN) && proc->is_frozen) {
> +			t_outdated = binder_find_outdated_transaction_ilocked(t,
> +									      &node->async_todo);
> +			if (t_outdated) {
> +				binder_debug(BINDER_DEBUG_TRANSACTION,
> +					     "txn %d supersedes %d\n",
> +					     t->debug_id, t_outdated->debug_id);
> +				list_del_init(&t_outdated->work.entry);
> +				proc->outstanding_txns--;
> +			}
> +		}
>  		binder_enqueue_work_ilocked(&t->work, &node->async_todo);
> +	}
>  
>  	if (!pending_async)
>  		binder_wakeup_thread_ilocked(proc, thread, !oneway /* sync */);
> @@ -2660,6 +2723,22 @@ static int binder_proc_transaction(struct binder_transaction *t,
>  	binder_inner_proc_unlock(proc);
>  	binder_node_unlock(node);
>  
> +	/*
> +	 * To reduce potential contention, free the outdated transaction and
> +	 * buffer after releasing the locks.
> +	 */
> +	if (t_outdated) {
> +		struct binder_buffer *buffer = t_outdated->buffer;
> +
> +		t_outdated->buffer = NULL;
> +		buffer->transaction = NULL;
> +		trace_binder_transaction_update_buffer_release(buffer);
> +		binder_transaction_buffer_release(proc, NULL, buffer, 0, 0);
> +		binder_alloc_free_buf(&proc->alloc, buffer);
> +		kfree(t_outdated);
> +		binder_stats_deleted(BINDER_STAT_TRANSACTION);
> +	}
> +
>  	return 0;
>  }
>  
> diff --git a/drivers/android/binder_trace.h b/drivers/android/binder_trace.h
> index 8eeccdc64724..8cc07e6a4273 100644
> --- a/drivers/android/binder_trace.h
> +++ b/drivers/android/binder_trace.h
> @@ -311,6 +311,10 @@ DEFINE_EVENT(binder_buffer_class, binder_transaction_failed_buffer_release,
>  	TP_PROTO(struct binder_buffer *buffer),
>  	TP_ARGS(buffer));
>  
> +DEFINE_EVENT(binder_buffer_class, binder_transaction_update_buffer_release,
> +	     TP_PROTO(struct binder_buffer *buffer),
> +	     TP_ARGS(buffer));
> +
>  TRACE_EVENT(binder_update_page_range,
>  	TP_PROTO(struct binder_alloc *alloc, bool allocate,
>  		 void __user *start, void __user *end),
> diff --git a/include/uapi/linux/android/binder.h b/include/uapi/linux/android/binder.h
> index 11157fae8a8e..0b291b1d29d1 100644
> --- a/include/uapi/linux/android/binder.h
> +++ b/include/uapi/linux/android/binder.h
> @@ -271,6 +271,7 @@ enum transaction_flags {
>  	TF_STATUS_CODE	= 0x08,	/* contents are a 32-bit status code */
>  	TF_ACCEPT_FDS	= 0x10,	/* allow replies with file descriptors */
>  	TF_CLEAR_BUF	= 0x20,	/* clear buffer on txn complete */
> +	TF_UPDATE_TXN	= 0x40,	/* update the outdated pending async txn */
>  };
>  
>  struct binder_transaction_data {
> -- 
> 2.36.1.124.g0e6072fb45-goog
> 
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Hi,

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