[PATCH v2] random: set fast pool count to zero in cpuhp teardown

[Jason A. Donenfeld]

Rather than having to use expensive atomics, which were visibly the most
expensive thing in the entire irq handler, simply take care of the
extreme edge case of resetting count to 0 in the cpuhp teardown handler,
after no more interrupts will arrive on that CPU. This simplifies the
code a bit and lets us use vanilla variables rather than atomics, and
performance should be improved.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Cc: Jonathan Neuschäfer <j.neuschaefer@xxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
Sebastian -

v2 moves the teardown to CPUHP_OFFLINE…CPUHP_BRINGUP_CPU, per our
discussion.

This was *way* simpler than I had anticipated, and I wish it
were done this way originally. If this looks good to you and you ack it,
I'll wind up merging this commit into the previous one in my branch,
since the intermediate atomic_t stage isn't really that interesting
any more, now that I've seen the light. Please take a look and let me
know what you think.

-Jason

 drivers/char/random.c      | 33 ++++++++++++++++++---------------
 include/linux/cpuhotplug.h |  1 +
 include/linux/random.h     |  2 ++
 kernel/cpu.c               |  6 ++++++
 4 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index bf6e8627b74e..df5aef93da34 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1179,7 +1179,7 @@ struct fast_pool {
 	unsigned long pool[16 / sizeof(long)];
 	struct work_struct mix;
 	unsigned long last;
-	atomic_t count;
+	unsigned int count;
 	u16 reg_idx;
 };
 
@@ -1215,6 +1215,19 @@ static void fast_mix(u32 pool[4])
 
 static DEFINE_PER_CPU(struct fast_pool, irq_randomness);
 
+int random_dead_cpu(unsigned int cpu)
+{
+	/*
+	 * Set the count to zero after offlining the CPU for two
+	 * reasons: 1) so that all new accumulated irqs are fresh
+	 * when it comes back online, and 2) so that its worker is
+	 * permitted to schedule again when it comes back online,
+	 * since the MIX_INFLIGHT flag will be cleared.
+	 */
+	per_cpu_ptr(&irq_randomness, cpu)->count = 0;
+	return 0;
+}
+
 static u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
 {
 	u32 *ptr = (u32 *)regs;
@@ -1239,15 +1252,6 @@ static void mix_interrupt_randomness(struct work_struct *work)
 	local_irq_disable();
 	if (fast_pool != this_cpu_ptr(&irq_randomness)) {
 		local_irq_enable();
-		/*
-		 * If we are unlucky enough to have been moved to another CPU,
-		 * during CPU hotplug while the CPU was shutdown then we set
-		 * our count to zero atomically so that when the CPU comes
-		 * back online, it can enqueue work again. The _release here
-		 * pairs with the atomic_inc_return_acquire in
-		 * add_interrupt_randomness().
-		 */
-		atomic_set_release(&fast_pool->count, 0);
 		return;
 	}
 
@@ -1256,7 +1260,7 @@ static void mix_interrupt_randomness(struct work_struct *work)
 	 * consistent view, before we reenable irqs again.
 	 */
 	memcpy(pool, fast_pool->pool, sizeof(pool));
-	atomic_set(&fast_pool->count, 0);
+	fast_pool->count = 0;
 	fast_pool->last = jiffies;
 	local_irq_enable();
 
@@ -1288,14 +1292,13 @@ void add_interrupt_randomness(int irq)
 	}
 
 	fast_mix((u32 *)fast_pool->pool);
-	/* The _acquire here pairs with the atomic_set_release in mix_interrupt_randomness(). */
-	new_count = (unsigned int)atomic_inc_return_acquire(&fast_pool->count);
+	new_count = ++fast_pool->count;
 
 	if (unlikely(crng_init == 0)) {
 		if (new_count >= 64 &&
 		    crng_pre_init_inject(fast_pool->pool, sizeof(fast_pool->pool),
 					 true, true) > 0) {
-			atomic_set(&fast_pool->count, 0);
+			fast_pool->count = 0;
 			fast_pool->last = now;
 			if (spin_trylock(&input_pool.lock)) {
 				_mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
@@ -1313,7 +1316,7 @@ void add_interrupt_randomness(int irq)
 
 	if (unlikely(!fast_pool->mix.func))
 		INIT_WORK(&fast_pool->mix, mix_interrupt_randomness);
-	atomic_or(MIX_INFLIGHT, &fast_pool->count);
+	fast_pool->count |= MIX_INFLIGHT;
 	queue_work_on(raw_smp_processor_id(), system_highpri_wq, &fast_pool->mix);
 }
 EXPORT_SYMBOL_GPL(add_interrupt_randomness);
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 411a428ace4d..38294af566e4 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -127,6 +127,7 @@ enum cpuhp_state {
 	CPUHP_MM_ZSWP_POOL_PREPARE,
 	CPUHP_KVM_PPC_BOOK3S_PREPARE,
 	CPUHP_ZCOMP_PREPARE,
+	CPUHP_RANDOM_PREPARE,
 	CPUHP_TIMERS_PREPARE,
 	CPUHP_MIPS_SOC_PREPARE,
 	CPUHP_BP_PREPARE_DYN,
diff --git a/include/linux/random.h b/include/linux/random.h
index d7354de9351e..fd8c354bae8e 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -35,6 +35,8 @@ extern void add_interrupt_randomness(int irq) __latent_entropy;
 extern void add_hwgenerator_randomness(const void *buffer, size_t count,
 				       size_t entropy);
 
+extern int random_dead_cpu(unsigned int cpu);
+
 extern void get_random_bytes(void *buf, size_t nbytes);
 extern int wait_for_random_bytes(void);
 extern int __init rand_initialize(void);
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 407a2568f35e..f83ae4ae7275 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -34,6 +34,7 @@
 #include <linux/scs.h>
 #include <linux/percpu-rwsem.h>
 #include <linux/cpuset.h>
+#include <linux/random.h>
 
 #include <trace/events/power.h>
 #define CREATE_TRACE_POINTS
@@ -1689,6 +1690,11 @@ static struct cpuhp_step cpuhp_hp_states[] = {
 		.startup.single		= rcutree_prepare_cpu,
 		.teardown.single	= rcutree_dead_cpu,
 	},
+	[CPUHP_RANDOM_PREPARE] = {
+		.name			= "random:prepare",
+		.startup.single		= NULL,
+		.teardown.single	= random_dead_cpu,
+	},
 	/*
 	 * On the tear-down path, timers_dead_cpu() must be invoked
 	 * before blk_mq_queue_reinit_notify() from notify_dead(),
-- 
2.35.0