linux-kernel - [PATCH -v4 4/5] x86,smp: keep spinlock delay values per hashed spinlock address

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Message-ID: <20130125141842.58010c25@annuminas.surriel.com>
Date:	Fri, 25 Jan 2013 14:18:42 -0500
From:	Rik van Riel <riel@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	aquini@...hat.com, walken@...gle.com, eric.dumazet@...il.com,
	lwoodman@...hat.com, knoel@...hat.com, chegu_vinod@...com,
	raghavendra.kt@...ux.vnet.ibm.com, mingo@...hat.com
Subject: [PATCH -v4 4/5] x86,smp: keep spinlock delay values per hashed
 spinlock address

From: Eric Dumazet <eric.dumazet@...il.com>

Eric Dumazet found a regression with the first version of the spinlock
backoff code, in a workload where multiple spinlocks were contended,
each having a different wait time.

This patch has multiple delay values per cpu, indexed on a hash
of the lock address, to avoid that problem.

Eric Dumazet wrote:

I did some tests with your patches with following configuration :

tc qdisc add dev eth0 root htb r2q 1000 default 3
(to force a contention on qdisc lock, even with a multi queue net
device)

and 24 concurrent "netperf -t UDP_STREAM -H other_machine -- -m 128"

Machine : 2 Intel(R) Xeon(R) CPU X5660  @ 2.80GHz
(24 threads), and a fast NIC (10Gbps)

Resulting in a 13 % regression (676 Mbits -> 595 Mbits)

In this workload we have at least two contended spinlocks, with
different delays. (spinlocks are not held for the same duration)

It clearly defeats your assumption of a single per cpu delay being OK :
Some cpus are spinning too long while the lock was released.

We might try to use a hash on lock address, and an array of 16 different
delays so that different spinlocks have a chance of not sharing the same
delay.

With following patch, I get 982 Mbits/s with same bench, so an increase
of 45 % instead of a 13 % regression.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: Rik van Riel <riel@...hat.com>
Acked-by: Rafael Aquini <aquini@...hat.com>
Reviewed-by: Michel Lespinasse <walken@...gle.com>

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 05f828b..3d30f6e 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -23,6 +23,7 @@
 #include <linux/interrupt.h>
 #include <linux/cpu.h>
 #include <linux/gfp.h>
+#include <linux/hash.h>
 
 #include <asm/mtrr.h>
 #include <asm/tlbflush.h>
@@ -134,12 +135,26 @@ static bool smp_no_nmi_ipi = false;
 #define DELAY_FIXED_1 (1<<DELAY_SHIFT)
 #define MIN_SPINLOCK_DELAY (1 * DELAY_FIXED_1)
 #define MAX_SPINLOCK_DELAY (16000 * DELAY_FIXED_1)
-DEFINE_PER_CPU(unsigned, spinlock_delay) = { MIN_SPINLOCK_DELAY };
+#define DELAY_HASH_SHIFT 6
+struct delay_entry {
+	u32 hash;
+	u32 delay;
+};
+static DEFINE_PER_CPU(struct delay_entry [1 << DELAY_HASH_SHIFT], spinlock_delay) = {
+	[0 ... (1 << DELAY_HASH_SHIFT) - 1] = {
+		.hash = 0,
+		.delay = MIN_SPINLOCK_DELAY,
+	},
+};
+
 void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
 {
 	__ticket_t head = inc.head, ticket = inc.tail;
 	__ticket_t waiters_ahead;
-	unsigned delay = __this_cpu_read(spinlock_delay);
+	u32 hash = hash32_ptr(lock);
+	u32 slot = hash_32(hash, DELAY_HASH_SHIFT);
+	struct delay_entry *ent = &__get_cpu_var(spinlock_delay[slot]);
+	u32 delay = (ent->hash == hash) ? ent->delay : MIN_SPINLOCK_DELAY;
 	unsigned loops = 1;
 
 	for (;;) {
@@ -171,11 +186,12 @@ void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
 			 * to get it back to a good value quickly.
 			 */
 			if (delay >= 2 * DELAY_FIXED_1)
-				delay -= max(delay/32, DELAY_FIXED_1);
+				delay -= max_t(u32, delay/32, DELAY_FIXED_1);
 			break;
 		}
 	}
-	__this_cpu_write(spinlock_delay, delay);
+	ent->hash = hash;
+	ent->delay = delay;
 }
 
 /*
--
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