Re: [PATCH rcu 04/20] srcu: Begin offloading srcu_struct fields to srcu_update

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Mar 30, 2023 at 03:47:10PM -0700, Paul E. McKenney wrote:
> The current srcu_struct structure is on the order of 200 bytes in size
> (depending on architecture and .config), which is much better than the
> old-style 26K bytes, but still all too inconvenient when one is trying
> to achieve good cache locality on a fastpath involving SRCU readers.
> 
> However, only a few fields in srcu_struct are used by SRCU readers.
> The remaining fields could be offloaded to a new srcu_update
> structure, thus shrinking the srcu_struct structure down to a few
> tens of bytes.  This commit begins this noble quest, a quest that is
> complicated by open-coded initialization of the srcu_struct within the
> srcu_notifier_head structure.  This complication is addressed by updating
> the srcu_notifier_head structure's open coding, given that there does
> not appear to be a straightforward way of abstracting that initialization.
> 
> This commit moves only the ->node pointer to srcu_update.  Later commits
> will move additional fields.
> 
> [ paulmck: Fold in qiang1.zhang@intel.com's memory-leak fix. ]
> 
> Link: https://lore.kernel.org/all/20230320055751.4120251-1-qiang1.zhang@intel.com/
> Suggested-by: Christoph Hellwig <hch@lst.de>

[..]
> @@ -236,8 +236,12 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
>   */
>  static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
>  {
> +	if (!is_static)
> +		ssp->srcu_sup = kzalloc(sizeof(*ssp->srcu_sup), GFP_KERNEL);
> +	if (!ssp->srcu_sup)
> +		return -ENOMEM;
>  	ssp->srcu_size_state = SRCU_SIZE_SMALL;
> -	ssp->node = NULL;
> +	ssp->srcu_sup->node = NULL;
>  	mutex_init(&ssp->srcu_cb_mutex);
>  	mutex_init(&ssp->srcu_gp_mutex);
>  	ssp->srcu_idx = 0;
> @@ -249,8 +253,11 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
>  	ssp->sda_is_static = is_static;
>  	if (!is_static)
>  		ssp->sda = alloc_percpu(struct srcu_data);
> -	if (!ssp->sda)
> +	if (!ssp->sda) {
> +		if (!is_static)
> +			kfree(ssp->srcu_sup);
>  		return -ENOMEM;
> +	}
>  	init_srcu_struct_data(ssp);
>  	ssp->srcu_gp_seq_needed_exp = 0;
>  	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
> @@ -259,6 +266,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)

[1] Here there is an if (!init_srcu_struct_nodes(...)) that the diff does not show.

>  			if (!ssp->sda_is_static) {
>  				free_percpu(ssp->sda);
>  				ssp->sda = NULL;
> +				kfree(ssp->srcu_sup);
>  				return -ENOMEM;
>  			}
>  		} else {


Just a comment about the original code with reference to [1].

Here if allocations in init_srcu_struct_nodes() fail, it will return false
and execute the "if (!ssp->sda_is_is_static)" bit.

So if the allocation in [1] fails, then if sda_is_static is true, we return
-ENOMEM, however  if sda_is_static is false, we do the following:

        ssp->srcu_sup->srcu_ssp = ssp;
        smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed, 0); /* Init done. */
        return 0;

Is that really correct?

In other words, if init_srcu_struct_nodes() returns false, then passing along
the return value of init_srcu_struct_nodes() back to the caller of
init_srcu_struct_fields() depends on whether is_static = true or false. That
seems a bit wrong to me, init_srcu_struct_fields() should always return
-ENOMEM  when init_srcu_struct_nodes() fails to allocate memory IMHO, whether
is_static is true or not.

Sorry if I missed something subtle, and if the code is correct to begin with.
Also I feel the return paths could be made better to also fix the above issue
I mentioned. How about the following diff on top of the series, would it
work?

Thanks!

---8<-----------------------

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index a887cfc89894..1975d06986fa 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -255,29 +255,30 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	ssp->srcu_sup->sda_is_static = is_static;
 	if (!is_static)
 		ssp->sda = alloc_percpu(struct srcu_data);
-	if (!ssp->sda) {
-		if (!is_static)
-			kfree(ssp->srcu_sup);
-		return -ENOMEM;
-	}
+	if (!ssp->sda)
+		goto err_free_sup;
 	init_srcu_struct_data(ssp);
 	ssp->srcu_sup->srcu_gp_seq_needed_exp = 0;
 	ssp->srcu_sup->srcu_last_gp_end = ktime_get_mono_fast_ns();
 	if (READ_ONCE(ssp->srcu_sup->srcu_size_state) == SRCU_SIZE_SMALL && SRCU_SIZING_IS_INIT()) {
-		if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC)) {
-			if (!ssp->srcu_sup->sda_is_static) {
-				free_percpu(ssp->sda);
-				ssp->sda = NULL;
-				kfree(ssp->srcu_sup);
-				return -ENOMEM;
-			}
-		} else {
+		if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC))
+			goto err_free_sda;
+		else
 			WRITE_ONCE(ssp->srcu_sup->srcu_size_state, SRCU_SIZE_BIG);
-		}
 	}
 	ssp->srcu_sup->srcu_ssp = ssp;
 	smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed, 0); /* Init done. */
 	return 0;
+
+err_free_sda:
+	if (!is_static) {
+		free_percpu(ssp->sda);
+		ssp->sda = NULL;
+	}
+err_free_sup:
+	if (!is_static)
+		kfree(ssp->srcu_sup);
+	return -ENOMEM;
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC