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63 lines
2.6 KiB
Diff
63 lines
2.6 KiB
Diff
From: Julia Cartwright <julia@ni.com>
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Date: Tue, 15 Dec 2020 15:16:47 +0100
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Subject: [PATCH 3/5] rcu: Enable rcu_normal_after_boot unconditionally for RT
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Expedited RCU grace periods send IPIs to all non-idle CPUs, and thus can
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disrupt time-critical code in real-time applications. However, there
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is a portion of boot-time processing (presumably before any real-time
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applications have started) where expedited RCU grace periods are the only
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option. And so it is that experience with the -rt patchset indicates that
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PREEMPT_RT systems should always set the rcupdate.rcu_normal_after_boot
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kernel boot parameter.
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This commit therefore makes the post-boot application environment safe
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for real-time applications by making PREEMPT_RT systems disable the
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rcupdate.rcu_normal_after_boot kernel boot parameter and acting as
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if this parameter had been set. This means that post-boot calls to
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synchronize_rcu_expedited() will be treated as if they were instead
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calls to synchronize_rcu(), thus preventing the IPIs, and thus avoiding
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disrupting real-time applications.
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Suggested-by: Luiz Capitulino <lcapitulino@redhat.com>
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Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
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Signed-off-by: Julia Cartwright <julia@ni.com>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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[ paulmck: Update kernel-parameters.txt accordingly. ]
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Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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---
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Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
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kernel/rcu/update.c | 4 +++-
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2 files changed, 10 insertions(+), 1 deletion(-)
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--- a/Documentation/admin-guide/kernel-parameters.txt
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+++ b/Documentation/admin-guide/kernel-parameters.txt
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@@ -4474,6 +4474,13 @@
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only normal grace-period primitives. No effect
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on CONFIG_TINY_RCU kernels.
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+ But note that CONFIG_PREEMPT_RT=y kernels enables
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+ this kernel boot parameter, forcibly setting
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+ it to the value one, that is, converting any
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+ post-boot attempt at an expedited RCU grace
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+ period to instead use normal non-expedited
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+ grace-period processing.
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+
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rcupdate.rcu_task_ipi_delay= [KNL]
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Set time in jiffies during which RCU tasks will
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avoid sending IPIs, starting with the beginning
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--- a/kernel/rcu/update.c
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+++ b/kernel/rcu/update.c
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@@ -56,8 +56,10 @@
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#ifndef CONFIG_TINY_RCU
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module_param(rcu_expedited, int, 0);
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module_param(rcu_normal, int, 0);
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-static int rcu_normal_after_boot;
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+static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
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+#ifndef CONFIG_PREEMPT_RT
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module_param(rcu_normal_after_boot, int, 0);
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+#endif
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#endif /* #ifndef CONFIG_TINY_RCU */
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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