linux-kernel - Re: [PATCH v4] rcu/nocb: Add an option to offload all CPUs on boot

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20220418190417.GW4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 18 Apr 2022 12:04:17 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        rushikesh.s.kadam@...el.com, vineethrp@...il.com, urezki@...il.com
Subject: Re: [PATCH v4] rcu/nocb: Add an option to offload all CPUs on boot

On Mon, Apr 18, 2022 at 05:54:03PM +0000, Joel Fernandes (Google) wrote:
> From: Joel Fernandes <joel@...lfernandes.org>
> 
> On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> which ends up not offloading any CPU. This patch removes a dependency
> from the bootloader having to know about RCU and about how to provide
> the mask.
> 
> With the new option enabled, all CPUs will be offloaded on boot unless
> rcu_nocbs= or rcu_nohz_full= kernel parameters provide a CPU list.
> 
> Signed-off-by: Joel Fernandes <joel@...lfernandes.org>

Much better, queued for review and testing, thank you!

I resolved a (trivial) conflict with -rcu's "dev" branch.  Could you
please check the resolultion below in case my notion of "trivial" and/or
"resolved" is excessively optimistic?

						Thanx, Paul

------------------------------------------------------------------------

commit 21b595e4233e6885bdea2819c206a470cc207ea5
Author: Joel Fernandes <joel@...lfernandes.org>
Date:   Mon Apr 18 17:54:03 2022 +0000

    rcu/nocb: Add an option to offload all CPUs on boot
    
    On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
    which ends up not offloading any CPU. This patch removes a dependency
    from the bootloader having to know about RCU and about how to provide
    the mask.
    
    With the new option enabled, all CPUs will be offloaded on boot unless
    rcu_nocbs= or rcu_nohz_full= kernel parameters provide a CPU list.
    
    [ paulmck: Forward-port to -rcu "dev" branch. ]
    
    Signed-off-by: Joel Fernandes <joel@...lfernandes.org>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 789ef586009b..1e82ecb7a649 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3572,6 +3572,9 @@
 			just as if they had also been called out in the
 			rcu_nocbs= boot parameter.
 
+			Note that this argument takes precedence over
+			the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
+
 	noiotrap	[SH] Disables trapped I/O port accesses.
 
 	noirqdebug	[X86-32] Disables the code which attempts to detect and
@@ -4475,6 +4478,9 @@
 			no-callback mode from boot but the mode may be
 			toggled at runtime via cpusets.
 
+			Note that this argument takes precedence over
+			the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
+
 	rcu_nocb_poll	[KNL]
 			Rather than requiring that offloaded CPUs
 			(specified by rcu_nocbs= above) explicitly
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 1c630e573548..27aab870ae4c 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -262,6 +262,19 @@ config RCU_NOCB_CPU
 	  Say Y here if you need reduced OS jitter, despite added overhead.
 	  Say N here if you are unsure.
 
+config RCU_NOCB_CPU_DEFAULT_ALL
+	bool "Offload RCU callback processing from all CPUs by default"
+	depends on RCU_NOCB_CPU
+	default n
+	help
+	  Use this option to offload callback processing from all CPUs
+	  by default, in the absence of the rcu_nocbs or nohz_full boot
+	  parameter. This also avoids the need to use any boot parameters
+	  to achieve the effect of offloading all CPUs on boot.
+
+	  Say Y here if you want offload all CPUs by default on boot.
+	  Say N here if you are unsure.
+
 config TASKS_TRACE_RCU_READ_MB
 	bool "Tasks Trace RCU readers use memory barriers in user and idle"
 	depends on RCU_EXPERT && TASKS_TRACE_RCU
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 46694e13398a..ef29220c805f 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1155,11 +1155,21 @@ void __init rcu_init_nohz(void)
 {
 	int cpu;
 	bool need_rcu_nocb_mask = false;
+	bool offload_all = false;
 	struct rcu_data *rdp;
 
+#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
+	if (!rcu_nocb_is_setup) {
+		need_rcu_nocb_mask = true;
+		offload_all = true;
+	}
+#endif
+
 #if defined(CONFIG_NO_HZ_FULL)
-	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
 		need_rcu_nocb_mask = true;
+		offload_all = false; /* NO_HZ_FULL has its own mask. */
+	}
 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
 
 	if (need_rcu_nocb_mask) {
@@ -1180,6 +1190,9 @@ void __init rcu_init_nohz(void)
 		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
 
+	if (offload_all)
+		cpumask_setall(rcu_nocb_mask);
+
 	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
 		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
 		cpumask_and(rcu_nocb_mask, cpu_possible_mask,