linux-kernel - Re: [PATCH v2] bit_spinlock: introduce smp_cond_load

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Date:   Tue, 30 Oct 2018 13:52:13 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Will Deacon" <will.deacon@....com>,
        <linux-kernel@...r.kernel.org>, Miao Xie <miaoxie@...wei.com>,
        Chao Yu <chao@...nel.org>
Subject: Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed

Hi,

On 2018/10/30 14:04, Gao Xiang wrote:
> It is better to use wrapped smp_cond_load_relaxed
> instead of open-coded busy waiting for bit_spinlock.
> 
> Signed-off-by: Gao Xiang <gaoxiang25@...wei.com>
> ---
> 
> change log v2:
>  - fix the incorrect expression !(VAL >> (bitnum & (BITS_PER_LONG-1)))
>  - the test result is described in the following reply.
> 
> Thanks,
> Gao Xiang


Simple test script:
#include <linux/kthread.h>
#include <linux/bit_spinlock.h>
#include <linux/module.h>

unsigned long global_lock;

int test_thread(void *data)
{
	unsigned long thread_id = (unsigned long)data;
	int i;
	u64 start = ktime_get_ns();

	for (i = 0; i < 500000; ++i) {
		bit_spin_lock(0, &global_lock);
		__asm__("yield");
		bit_spin_unlock(0, &global_lock);
	}
	pr_err("Thread id: %lu time: %llu\n", thread_id, ktime_get_ns() - start);

	do_exit(0);
}


static int __init bitspinlock_test_module_init(void)
{
	int i;

	for (i = 0; i < 8; ++i) {
		if (IS_ERR(kthread_run(test_thread, (void *)(unsigned long)i, "thread-%d", i)))
			pr_err("fail to create thread %d\n", i);
	}

	return 0;
}

static void __exit bitspinlock_test_module_exit(void)
{
}

module_init(bitspinlock_test_module_init);
module_exit(bitspinlock_test_module_exit);
MODULE_LICENSE("GPL");


...and tested in the following ARM server environment:

Processor: HI1616 (https://en.wikichip.org/wiki/hisilicon/hi16xx/hi1616)
Board: HiSilicon D05 Development Board (http://open-estuary.org/d05/)
Memory: 512GB
Host OS: Ubuntu 18.04.1 LTS (Ubuntu 4.15.0-29.31-generic 4.15.18)
QEMU KVM OS: Linux 4.19 + buildroot
QEMU KVM cmdline: qemu-system-aarch64 -enable-kvm -cpu host -smp 4 -m 256M -kernel Image -M virt,kernel_irqchip=on -nographic -hda rootfs.ext2 -append 'root=/dev/vda console=ttyAMA0 earlycon=pl011,0x9000000' -serial mon:stdio -net none

Without this patch:
  Thread 0   Thread 1   Thread 2   Thread 3   Thread 4   Thread 5   Thread 6   Thread 7
1 1283709480 1271869280  454742480 1173673820 1145643640 1118846920  774616920 1144146140
2  643580180  625143860  576841700  322982340  649987880  585749000  529178880  373374780
3  672307220  847315000  880801860 1039502040  667086380 1033939940 1035381120 1046898300
4  568635580  440547020  737000380  910040880  804543740  712314280  868896880  867049000
5  749107320  726397720  776134480  611970100  756721040  753449440  711691300  609343300

With this patch:
  Thread 0   Thread 1   Thread 2   Thread 3   Thread 4   Thread 5   Thread 6   Thread 7
1  170327620  196322160  169434180   74723860  178145600  178873460  143843260   70998780
2  166415220  129649200  166161240  175241520  155474460  112811860  157003140  150087420
3  511420780  117655640  598641860  596213720  462888760  430838600  554346300  428035120 
4  174520240  156311800  120274280   87465380  172781400  136118620  163728340   63026360
5  153677940  202786860  183626500  140721300  150311360  161266840  168154340  107247460

Thanks,
Gao Xiang