linux-kernel - RE: [PATCH] rcu: Only check tasks blocked on leaf rcu_nodes for PREEMPT

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Date:   Fri, 22 Jul 2022 01:45:35 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "frederic@...nel.org" <frederic@...nel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu: Only check tasks blocked on leaf rcu_nodes for
 PREEMPT_RCU

On Fri, Jul 22, 2022 at 08:52:13AM +0800, Zqiang wrote:
> In PREEMPT_RCU kernel, for multi-node rcu tree, if the RCU read
> critical section is preempted, the current task are only queued
> leaf rcu_node blkd list, for single-node rcu tree, the root node
> is also leaf node.
> 
> This commit add rcu_is_leaf_node() to filter out checks for non-leaf
> rcu_node.
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
>
>First, thank you for digging into this!
>
> ---
>  kernel/rcu/tree_plugin.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index b2219577fbe2..a9df11ec65af 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -693,6 +693,8 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
>  
>  	RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
>  	raw_lockdep_assert_held_rcu_node(rnp);
> +	if (!rcu_is_leaf_node(rnp))
> +		goto end;
>  	if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
>  		dump_blkd_tasks(rnp, 10);
>  	if (rcu_preempt_has_tasks(rnp) &&

>So you are adding a simple check for all rcu_node structures
>that removes two simple checks for non-leaf rcu_node structures.
>
>Assuming that the costs of all these checks is roughly the same (but
>please feel free to actually measure them on real hardware), what what
>fraction of the rcu_node structures must be non-leaf for this change to
>be a win?  Then for what configurations using default fanouts is this
>fraction exceeded?
>
>The default fanout is 16 from non-leaf to leaf and 64 from non-leaf
>to non-leaf (32 for non-leaf to non-leaf on 32-bit systems).

Thanks for reminding, I only considered the logic in the code, 
not the actual real usage scenarios. I will do some tests on the
actual hardware. 😊

Thanks
Zqiang

>
>Hey, you wanted some algebra practice anyway.  ;-)

> @@ -703,6 +705,7 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
>  		trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
>  						rnp->gp_seq, t->pid);
>  	}
> +end:
>  	WARN_ON_ONCE(rnp->qsmask);
>  }
>  
> @@ -1178,7 +1181,8 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
>   */
>  static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
>  {
> -	rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
> +	if (rcu_is_leaf_node(rnp))
> +		rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
>
>And here you are adding a check on all nodes to eliminate an addition
>and a store on non-leaf nodes.  Same questions as above, with the same
>invitation to actually measure the costs of these operations.
>
>						Thanx, Paul
>
>  }
>  
>  /*
> -- 
> 2.25.1
>