linux-kernel - [PATCH V2 1/3] timer: Allocate per-cpu tvec

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Date:	Tue, 31 Mar 2015 20:49:00 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: [PATCH V2 1/3] timer: Allocate per-cpu tvec_base's statically

From: Peter Zijlstra <peterz@...radead.org>

Memory for tvec_base is allocated separately for boot CPU (statically)
and non-boot CPUs (dynamically).

The reason is because __TIMER_INITIALIZER() needs to set ->base to a
valid pointer (because we've made NULL special, hint: lock_timer_base())
and we cannot get a compile time pointer to per-cpu entries because we
don't know where we'll map the section, even for the boot cpu.

This can be simplified a bit by statically allocating per-cpu memory.
The only disadvantage is that memory for one of the structures will stay
unused, i.e. for the boot CPU, which uses boot_tvec_bases.

This will also guarantee that tvec_base is cacheline aligned. Even
though tvec_base has ____cacheline_aligned stuck on, kzalloc_node() does
not actually respect that (but guarantees a minimum u64 alignment).

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linaro-kernel@...ts.linaro.org
Cc: Ingo Molnar <mingo@...hat.com>
Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/time/timer.c | 48 +++++++++++++++++++-----------------------------
 1 file changed, 19 insertions(+), 29 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 2d3f5c504939..f3cc653f876c 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -90,8 +90,19 @@ struct tvec_base {
 	struct tvec tv5;
 } ____cacheline_aligned;
 
+/*
+ * __TIMER_INITIALIZER() needs to set ->base to a valid pointer (because we've
+ * made NULL special, hint: lock_timer_base()) and we cannot get a compile time
+ * pointer to per-cpu entries because we don't know where we'll map the section,
+ * even for the boot cpu.
+ *
+ * And so we use boot_tvec_bases for boot CPU and per-cpu __tvec_bases for the
+ * rest of them.
+ */
 struct tvec_base boot_tvec_bases;
 EXPORT_SYMBOL(boot_tvec_bases);
+static DEFINE_PER_CPU(struct tvec_base, __tvec_bases);
+
 static DEFINE_PER_CPU(struct tvec_base *, tvec_bases) = &boot_tvec_bases;
 
 /* Functions below help us manage 'deferrable' flag */
@@ -1534,46 +1545,25 @@ EXPORT_SYMBOL(schedule_timeout_uninterruptible);
 
 static int init_timers_cpu(int cpu)
 {
-	int j;
-	struct tvec_base *base;
+	struct tvec_base *base = per_cpu(tvec_bases, cpu);
 	static char tvec_base_done[NR_CPUS];
+	int j;
 
 	if (!tvec_base_done[cpu]) {
-		static char boot_done;
+		static char boot_cpu_skipped;
 
-		if (boot_done) {
-			/*
-			 * The APs use this path later in boot
-			 */
-			base = kzalloc_node(sizeof(*base), GFP_KERNEL,
-					    cpu_to_node(cpu));
-			if (!base)
-				return -ENOMEM;
-
-			/* Make sure tvec_base has TIMER_FLAG_MASK bits free */
-			if (WARN_ON(base != tbase_get_base(base))) {
-				kfree(base);
-				return -ENOMEM;
-			}
-			per_cpu(tvec_bases, cpu) = base;
+		if (!boot_cpu_skipped) {
+			boot_cpu_skipped = 1; /* skip the boot cpu */
 		} else {
-			/*
-			 * This is for the boot CPU - we use compile-time
-			 * static initialisation because per-cpu memory isn't
-			 * ready yet and because the memory allocators are not
-			 * initialised either.
-			 */
-			boot_done = 1;
-			base = &boot_tvec_bases;
+			base = per_cpu_ptr(&__tvec_bases, cpu);
+			per_cpu(tvec_bases, cpu) = base;
 		}
+
 		spin_lock_init(&base->lock);
 		tvec_base_done[cpu] = 1;
 		base->cpu = cpu;
-	} else {
-		base = per_cpu(tvec_bases, cpu);
 	}
 
-
 	for (j = 0; j < TVN_SIZE; j++) {
 		INIT_LIST_HEAD(base->tv5.vec + j);
 		INIT_LIST_HEAD(base->tv4.vec + j);
-- 
2.3.0.rc0.44.ga94655d

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