Re: 10% regression in qperf tcp latency after introducing commit "4a61bf7f9b18 random: defer fast pool mixing to worker"

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Hey again Sherry,

On Thu, Sep 22, 2022 at 12:32:49AM +0200, Jason A. Donenfeld wrote:
> That leads me to suspect that queue_work_on() might actually not be as
> cheap as I assumed? If so, is that surprising to anybody else? And what
> should we do about this?

Sultan (CC'd) suggested I look at the much less expensive softirq
tasklet for this, which matches the use case pretty much entirely as
well. Can you try out this patch below and see if it resolves the
performance regression?

Thanks,
Jason

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 520a385c7dab..ad17b36cf977 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -918,13 +918,16 @@ EXPORT_SYMBOL_GPL(unregister_random_vmfork_notifier);
 #endif

 struct fast_pool {
-	struct work_struct mix;
+	struct tasklet_struct mix;
 	unsigned long pool[4];
 	unsigned long last;
 	unsigned int count;
 };

+static void mix_interrupt_randomness(struct tasklet_struct *work);
+
 static DEFINE_PER_CPU(struct fast_pool, irq_randomness) = {
+	.mix = { .use_callback = true, .callback = mix_interrupt_randomness },
 #ifdef CONFIG_64BIT
 #define FASTMIX_PERM SIPHASH_PERMUTATION
 	.pool = { SIPHASH_CONST_0, SIPHASH_CONST_1, SIPHASH_CONST_2, SIPHASH_CONST_3 }
@@ -973,7 +976,7 @@ int __cold random_online_cpu(unsigned int cpu)
 }
 #endif

-static void mix_interrupt_randomness(struct work_struct *work)
+static void mix_interrupt_randomness(struct tasklet_struct *work)
 {
 	struct fast_pool *fast_pool = container_of(work, struct fast_pool, mix);
 	/*
@@ -1027,10 +1030,8 @@ void add_interrupt_randomness(int irq)
 	if (new_count < 1024 && !time_is_before_jiffies(fast_pool->last + HZ))
 		return;

-	if (unlikely(!fast_pool->mix.func))
-		INIT_WORK(&fast_pool->mix, mix_interrupt_randomness);
 	fast_pool->count |= MIX_INFLIGHT;
-	queue_work_on(raw_smp_processor_id(), system_highpri_wq, &fast_pool->mix);
+	tasklet_hi_schedule(&fast_pool->mix);
 }
 EXPORT_SYMBOL_GPL(add_interrupt_randomness);