linux-kernel - [RFC PATCH v3 15/15] context-tracking: Add a Kconfig to enable IPI deferral for NO_HZ

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Message-ID: <20241119153502.41361-16-vschneid@redhat.com>
Date: Tue, 19 Nov 2024 16:35:02 +0100
From: Valentin Schneider <vschneid@...hat.com>
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Subject: [RFC PATCH v3 15/15] context-tracking: Add a Kconfig to enable IPI deferral for NO_HZ_IDLE

With NO_HZ_IDLE, we get CONTEXT_TRACKING_IDLE, so we get these
transitions:

  ct_idle_enter()
    ct_kernel_exit()
      ct_state_inc_clear_work()

  ct_idle_exit()
    ct_kernel_enter()
      ct_work_flush()

With just CONTEXT_TRACKING_IDLE, ct_state_inc_clear_work() is just
ct_state_inc() and ct_work_flush() is a no-op. However, making them be
functional as if under CONTEXT_TRACKING_WORK would allow NO_HZ_IDLE to
leverage IPI deferral to keep idle CPUs idle longer.

Having this enabled for NO_HZ_IDLE is a different argument than for having
it for NO_HZ_FULL (power savings vs latency/performance), but the backing
mechanism is identical.

Add a default-no option to enable IPI deferral with NO_HZ_IDLE.

Signed-off-by: Valentin Schneider <vschneid@...hat.com>
---
 kernel/time/Kconfig | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 04efc2b605823..a3cb903a4e022 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -188,9 +188,23 @@ config CONTEXT_TRACKING_USER_FORCE
 
 config CONTEXT_TRACKING_WORK
 	bool
-	depends on HAVE_CONTEXT_TRACKING_WORK && CONTEXT_TRACKING_USER
+	depends on HAVE_CONTEXT_TRACKING_WORK && (CONTEXT_TRACKING_USER || CONTEXT_TRACKING_WORK_IDLE)
 	default y
 
+config CONTEXT_TRACKING_WORK_IDLE
+       bool
+       depends on HAVE_CONTEXT_TRACKING_WORK && CONTEXT_TRACKING_IDLE && !CONTEXT_TRACKING_USER
+       default n
+       help
+	 This option enables deferral of some IPIs when they are targeted at CPUs
+	 that are idle. This can help keep CPUs idle longer, but induces some
+	 extra overhead to idle <-> kernel transitions and to IPI sending.
+
+	 Say Y if the power improvements are worth more to you than the added
+	 overheads.
+
+	 Say N otherwise.
+
 config NO_HZ
 	bool "Old Idle dynticks config"
 	help
-- 
2.43.0