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Message-ID: <20241119153502.41361-5-vschneid@redhat.com>
Date: Tue, 19 Nov 2024 16:34:51 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org,
kvm@...r.kernel.org,
linux-mm@...ck.org,
bpf@...r.kernel.org,
x86@...nel.org,
rcu@...r.kernel.org,
linux-kselftest@...r.kernel.org
Cc: "Paul E . McKenney" <paulmck@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Jonathan Corbet <corbet@....net>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
Joel Fernandes <joel@...lfernandes.org>,
Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang.zhang1211@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Uladzislau Rezki <urezki@...il.com>,
Christoph Hellwig <hch@...radead.org>,
Lorenzo Stoakes <lstoakes@...il.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Jason Baron <jbaron@...mai.com>,
Kees Cook <keescook@...omium.org>,
Sami Tolvanen <samitolvanen@...gle.com>,
Ard Biesheuvel <ardb@...nel.org>,
Nicholas Piggin <npiggin@...il.com>,
Juerg Haefliger <juerg.haefliger@...onical.com>,
Nicolas Saenz Julienne <nsaenz@...nel.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Nadav Amit <namit@...are.com>,
Dan Carpenter <error27@...il.com>,
Chuang Wang <nashuiliang@...il.com>,
Yang Jihong <yangjihong1@...wei.com>,
Petr Mladek <pmladek@...e.com>,
"Jason A. Donenfeld" <Jason@...c4.com>,
Song Liu <song@...nel.org>,
Julian Pidancet <julian.pidancet@...cle.com>,
Tom Lendacky <thomas.lendacky@....com>,
Dionna Glaze <dionnaglaze@...gle.com>,
Thomas Weißschuh <linux@...ssschuh.net>,
Juri Lelli <juri.lelli@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Yair Podemsky <ypodemsk@...hat.com>,
Daniel Wagner <dwagner@...e.de>,
Petr Tesarik <ptesarik@...e.com>
Subject: [RFC PATCH v3 04/15] rcu: Add a small-width RCU watching counter debug option
A later commit will reduce the size of the RCU watching counter to free up
some bits for another purpose. Paul suggested adding a config option to
test the extreme case where the counter is reduced to its minimum usable
width for rcutorture to poke at, so do that.
Make it only configurable under RCU_EXPERT. While at it, add a comment to
explain the layout of context_tracking->state.
Link: http://lore.kernel.org/r/4c2cb573-168f-4806-b1d9-164e8276e66a@paulmck-laptop
Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Valentin Schneider <vschneid@...hat.com>
---
include/linux/context_tracking_state.h | 44 ++++++++++++++++++++++----
kernel/rcu/Kconfig.debug | 14 ++++++++
2 files changed, 51 insertions(+), 7 deletions(-)
diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index 7b8433d5a8efe..0b81248aa03e2 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -18,12 +18,6 @@ enum ctx_state {
CT_STATE_MAX = 4,
};
-/* Odd value for watching, else even. */
-#define CT_RCU_WATCHING CT_STATE_MAX
-
-#define CT_STATE_MASK (CT_STATE_MAX - 1)
-#define CT_RCU_WATCHING_MASK (~CT_STATE_MASK)
-
struct context_tracking {
#ifdef CONFIG_CONTEXT_TRACKING_USER
/*
@@ -44,9 +38,45 @@ struct context_tracking {
#endif
};
+/*
+ * We cram two different things within the same atomic variable:
+ *
+ * CT_RCU_WATCHING_START CT_STATE_START
+ * | |
+ * v v
+ * MSB [ RCU watching counter ][ context_state ] LSB
+ * ^ ^
+ * | |
+ * CT_RCU_WATCHING_END CT_STATE_END
+ *
+ * Bits are used from the LSB upwards, so unused bits (if any) will always be in
+ * upper bits of the variable.
+ */
#ifdef CONFIG_CONTEXT_TRACKING
+#define CT_SIZE (sizeof(((struct context_tracking *)0)->state) * BITS_PER_BYTE)
+
+#define CT_STATE_WIDTH bits_per(CT_STATE_MAX - 1)
+#define CT_STATE_START 0
+#define CT_STATE_END (CT_STATE_START + CT_STATE_WIDTH - 1)
+
+#define CT_RCU_WATCHING_MAX_WIDTH (CT_SIZE - CT_STATE_WIDTH)
+#define CT_RCU_WATCHING_WIDTH (IS_ENABLED(CONFIG_RCU_DYNTICKS_TORTURE) ? 2 : CT_RCU_WATCHING_MAX_WIDTH)
+#define CT_RCU_WATCHING_START (CT_STATE_END + 1)
+#define CT_RCU_WATCHING_END (CT_RCU_WATCHING_START + CT_RCU_WATCHING_WIDTH - 1)
+#define CT_RCU_WATCHING BIT(CT_RCU_WATCHING_START)
+
+#define CT_STATE_MASK GENMASK(CT_STATE_END, CT_STATE_START)
+#define CT_RCU_WATCHING_MASK GENMASK(CT_RCU_WATCHING_END, CT_RCU_WATCHING_START)
+
+#define CT_UNUSED_WIDTH (CT_RCU_WATCHING_MAX_WIDTH - CT_RCU_WATCHING_WIDTH)
+
+static_assert(CT_STATE_WIDTH +
+ CT_RCU_WATCHING_WIDTH +
+ CT_UNUSED_WIDTH ==
+ CT_SIZE);
+
DECLARE_PER_CPU(struct context_tracking, context_tracking);
-#endif
+#endif /* CONFIG_CONTEXT_TRACKING */
#ifdef CONFIG_CONTEXT_TRACKING_USER
static __always_inline int __ct_state(void)
diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
index 9b0b52e1836fa..8dc505d841f8d 100644
--- a/kernel/rcu/Kconfig.debug
+++ b/kernel/rcu/Kconfig.debug
@@ -168,4 +168,18 @@ config RCU_STRICT_GRACE_PERIOD
when looking for certain types of RCU usage bugs, for example,
too-short RCU read-side critical sections.
+
+config RCU_DYNTICKS_TORTURE
+ bool "Minimize RCU dynticks counter size"
+ depends on RCU_EXPERT
+ default n
+ help
+ This option controls the width of the dynticks counter.
+
+ Lower values will make overflows more frequent, which will increase
+ the likelihood of extending grace-periods. This option sets the width
+ to its minimum usable value.
+
+ This has no value for production and is only for testing.
+
endmenu # "RCU Debugging"
--
2.43.0
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