linux-kernel - Re: [PATCH] use x86 cpu park to speedup smp

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Date:   Wed, 08 Dec 2021 16:57:07 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     paulmck@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "Schander, Johanna 'Mimoja' Amelie" <mimoja@...zon.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        hewenliang4@...wei.com, hushiyuan@...wei.com,
        luolongjun@...wei.com, hejingxian <hejingxian@...wei.com>
Subject: Re: [PATCH] use x86 cpu park to speedup smp_init in kexec situation

On Wed, 2021-12-08 at 15:10 +0000, David Woodhouse wrote:
> @@ -4266,13 +4266,13 @@ void rcu_cpu_starting(unsigned int cpu)
>                 rcu_disable_urgency_upon_qs(rdp);
>                 /* Report QS -after- changing ->qsmaskinitnext! */
>                 rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
> +               /* Er, why didn't we drop the lock here? */
> -       } else {
> -               raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>         }
> 

Oh, I see... how about this straw man then...

From 083c8fb2656e9fc60a17c9bfd538fcee4c5ebacc Mon Sep 17 00:00:00 2001
From: David Woodhouse <dwmw@...zon.co.uk>
Date: Tue, 16 Feb 2021 15:04:34 +0000
Subject: [PATCH 1/4] rcu: Expand locking around rcu_cpu_starting() to cover
 rnp->ofl_seq bump

To allow architectures to bring APs online in parallel, we need only one
of them to be going through rcu_cpu_starting() at a time. Expand the
coverage of the existing per-node lock to cover the manipulation of
rnp->ofl_seq too.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 kernel/rcu/tree.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ef8d36f580fc..544198c674f2 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4246,11 +4246,11 @@ void rcu_cpu_starting(unsigned int cpu)
 
 	rnp = rdp->mynode;
 	mask = rdp->grpmask;
+	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
 	WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
 	rcu_dynticks_eqs_online();
 	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
-	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
 	newcpu = !(rnp->expmaskinitnext & mask);
 	rnp->expmaskinitnext |= mask;
@@ -4261,6 +4261,11 @@ void rcu_cpu_starting(unsigned int cpu)
 	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
 	rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
 
+	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
+	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
+	WARN_ON_ONCE(rnp->ofl_seq & 0x1);
+	smp_mb(); /* Ensure RCU read-side usage follows above initialization. */
+
 	/* An incoming CPU should never be blocking a grace period. */
 	if (WARN_ON_ONCE(rnp->qsmask & mask)) { /* RCU waiting on incoming CPU? */
 		rcu_disable_urgency_upon_qs(rdp);
@@ -4269,10 +4274,6 @@ void rcu_cpu_starting(unsigned int cpu)
 	} else {
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
-	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
-	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
-	WARN_ON_ONCE(rnp->ofl_seq & 0x1);
-	smp_mb(); /* Ensure RCU read-side usage follows above initialization. */
 }
 
 /*
-- 
2.31.1


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