Re: [PATCH v4 1/4] rcu: Reduce synchronize_rcu() latency

[Uladzislau Rezki <urezki@gmail.com>]

On Sat, Jan 13, 2024 at 12:09:51AM +0100, Frederic Weisbecker wrote:
> Le Thu, Jan 04, 2024 at 05:25:07PM +0100, Uladzislau Rezki (Sony) a écrit :
> > diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
> > index 9b0b52e1836f..4812c6249185 100644
> > --- a/kernel/rcu/Kconfig.debug
> > +++ b/kernel/rcu/Kconfig.debug
> > @@ -168,4 +168,16 @@ config RCU_STRICT_GRACE_PERIOD
> >  	  when looking for certain types of RCU usage bugs, for example,
> >  	  too-short RCU read-side critical sections.
> >  
> > +config RCU_SR_NORMAL_DEBUG_GP
> > +	bool "Debug synchronize_rcu() callers for a grace period completion"
> > +	depends on DEBUG_KERNEL && RCU_EXPERT
> > +	default n
> > +	help
> > +	  This option enables additional debugging for detecting a grace
> > +	  period incompletion for synchronize_rcu() users. If a GP is not
> > +	  fully passed for any user, the warning message is emitted.
> > +
> > +	  Say Y here if you want to enable such debugging
> > +	  Say N if you are unsure.
> 
> How about just reuse CONFIG_PROVE_RCU instead?
> 
Less extra CONFIG_* configuration we have the better approach is. I do
not mind, so we can reuse it. Thanks for this point :)

I see in some places indeed it is used as a debugging peace.

> > +
> >  endmenu # "RCU Debugging"
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 499803234176..b756c40e4960 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1422,6 +1422,106 @@ static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
> >  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> >  }
> >  
> > +/*
> > + * There are three lists for handling synchronize_rcu() users.
> > + * A first list corresponds to new coming users, second for users
> > + * which wait for a grace period and third is for which a grace
> > + * period is passed.
> > + */
> > +static struct sr_normal_state {
> > +	struct llist_head srs_next;	/* request a GP users. */
> > +	struct llist_head srs_wait;	/* wait for GP users. */
> > +	struct llist_head srs_done;	/* ready for GP users. */
> > +
> > +	/*
> > +	 * In order to add a batch of nodes to already
> > +	 * existing srs-done-list, a tail of srs-wait-list
> > +	 * is maintained.
> > +	 */
> > +	struct llist_node *srs_wait_tail;
> > +} sr;
> 
> "sr" is good enough for a function scope variable but not for a file scope one.
> 
> At least "sr_state" would be better. Or maybe you don't even need to name that
> struct and make instead:
> 
> struct {
>     ...
>     ...
> } sr_normal_state;
> 
It is moved by the following patch in the series under the "rcu_state" struct variable.

> 
> > +
> > +/* Disabled by default. */
> > +static int rcu_normal_wake_from_gp;
> > +module_param(rcu_normal_wake_from_gp, int, 0644);
> > +
> > +static void rcu_sr_normal_complete(struct llist_node *node)
> > +{
> > +	struct rcu_synchronize *rs = container_of(
> > +		(struct rcu_head *) node, struct rcu_synchronize, head);
> 
> Should there be some union in struct rcu_synchronize between struct rcu_head
> and struct llist_node?
> 
> Anyway it's stack allocated, they could even be separate fields.
> 
> > +	unsigned long oldstate = (unsigned long) rs->head.func;
> 
> Luckily struct callback_head layout allows such magic but if rcu_head
> and llist_node were separate, reviewers would be less hurt.
> 
> If stack space really matters, something like the below?
> 
> struct rcu_synchronize {
> 	union {
> 		struct rcu_head head;
> 		struct {
> 			struct llist_node node;
> 			unsigned long seq;
> 		}
> 	}
> 	struct completion completion;
> };
> 
> 
We can do that. I am not sure if should be a separate patch or as a big
change. I tend to separate it.

> > +
> > +	WARN_ONCE(IS_ENABLED(CONFIG_RCU_SR_NORMAL_DEBUG_GP) &&
> > +		!poll_state_synchronize_rcu(oldstate),
> > +		"A full grace period is not passed yet: %lu",
> > +		rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
> > +
> > +	/* Finally. */
> > +	complete(&rs->completion);
> > +}
> > +
> [...]
> > +
> > +/*
> > + * Helper function for rcu_gp_cleanup().
> > + */
> > +static void rcu_sr_normal_gp_cleanup(void)
> > +{
> > +	struct llist_node *head, *tail;
> > +
> > +	if (llist_empty(&sr.srs_wait))
> > +		return;
> > +
> > +	tail = READ_ONCE(sr.srs_wait_tail);
> 
> Is the READ_ONCE() needed?
> 
> A part from those boring details:
> 
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
>
Appreciate for the review. I will fix all the comments.

Thanks!

--
Uladzislau Rezki