Re: [RFC PATCH v3 2/2] arm64: tlb: Use the TLBI RANGE feature in arm64

Zhenyu Ye
2020-06-09T13:26:50Z

[-- Attachment #1: Type: text/plain, Size: 3038 bytes --]

Hi Catalin,

On 2020/5/18 20:21, Zhenyu Ye wrote:
> I will test the performance of your suggestion and then reply you again
> here.
>

I have sent the v4 of this series [1], and compared the performance of
these two different implement. The test code is in the attachment (directly
call the __flush_tlb_range()).

First, I tested the v4 on a machine whose cpus do not support tlb range.
Fortunately, the newly added judgment in loop has very little effect on
performance. When page nums are 256 (loop 256 times), the impact is less
than 0.5%:

[page num] [before change] [v4 change]
1 1457 1491
2 1911 1957
3 2382 2377
4 2827 2852
5 3282 3349
6 3763 3781
7 4295 4252
8 4716 4716
9 5186 5218
10 5618 5648
16 8427 8454
32 15938 15951
64 30890 30977
128 60802 60863
256 120826 121395
512 1508 1555

Then I tested them on a FPGA machine whose cpus support the tlb range
feature (this machine is not the same as above). Below is the test
data when the stride = PTE:

[page num] [before change] [v3 change] [v4 change]
1 16051 15094 13524
2 11366 11270 11146
3 11582 11536 12171
4 11694 11199 11101
5 12138 11506 12267
6 12290 11214 11105
7 12400 11448 12002
8 12837 11225 11097
9 14791 11529 12140
10 15461 11218 11087
16 18233 11192 11094
32 26983 11224 11079
64 43840 11237 11092
128 77754 11247 11098
256 145514 11223 11089
512 280932 11197 11111

We can see the v3 and v4 are very similar in this scene, and both
of them performance improved very much compared to current
implementation. When the page nums are 256, the performance is
improved by more than 10 times. And the TLBI RANGE instruction
cost less time than classic TLBI in all secenes on this machine,
even if the page num is small. (but this may be different on
different machines)

Everything performs will util now, but I added a new judgment of
stride in the v4:

if (cpus_have_const_cap(ARM64_HAS_TLBI_RANGE) &&
stride == PAGE_SIZE)
use tlbi range here...

So when the stride != PTE, then there will use the classic tlbi
instruction and flush the tlbs one by one, where the performance
becomes worse than v3:

[page num] [before change] [v3 change] [v4 change]
1 14047 11332 11611
2 11568 11255 11701
3 11664 11231 11759
4 12097 11204 12173
5 12229 11236 12374
6 12399 11203 12497
7 12802 11266 12914
8 14764 17098 14907
9 15370 17106 15551
10 16130 17103 16137
16 19029 17175 19194
32 27300 17097 27604
64 44172 17075 44609
128 77878 17176 78548
256 145185 12022 146063
512 279822 12029 279922

And as we can see, "handle the 2MB with a single classic TLBI"
costs the same time as a single TLBI RANGE instruction. So should
I remove the judgment of stride and only figure which to use based
on cpucaps in the loop? But if removes the judgment, the logic
will be the same as v3.(both of them only judge cpucaps)

Waiting for your suggestions...

Thanks,
Zhenyu

[-- Attachment #2: test-tlb-range-perf.c --]
[-- Type: text/plain, Size: 2931 bytes --]

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/delay.h>
#include <asm/tlb.h>
#include <linux/time.h>
#include <asm/current.h>
#include <linux/sched.h>
#include <linux/delay.h>
#include <linux/mm.h>

#define TESTTIMES 10000

void testRangePerf(void);

static int __init test_init(void)
{
printk("BEGIN TEST\n");

testRangePerf();

printk("END TEST\n");
return 0;
}

static void __exit test_exit(void)
{
return;
}

void testRangePerf(void)
{
int i, j;
struct timespec64 start, end;
struct task_struct *ts;
struct vm_area_struct *vma;

printk("BEGIN testRangePerf\n");

ts = current;
vma = ts->mm->mmap;

printk("vma->start: %lx, vma->end: %lx, ttl = 0, PAGE_SIZE = 0x%lx\n", vma->vm_start, vma->vm_end, PAGE_SIZE);
for (i = 1; i <= 10; i++) {
ktime_get_ts64(&start);
for (j = 0; j < TESTTIMES; j++) {
__flush_tlb_range(vma, vma->vm_start, vma->vm_start + PAGE_SIZE * i, PAGE_SIZE, false);
}
ktime_get_ts64(&end);
printk("test __flush_tlb_range with %04d pages, used time: %12lld ns\n", i,
((end.tv_sec - start.tv_sec) * 1000000000 + end.tv_nsec - start.tv_nsec) / TESTTIMES);
msleep(100);
}

for (i = 16; i <= 512; i+=i) {
ktime_get_ts64(&start);
for (j = 0; j < TESTTIMES; j++) {
__flush_tlb_range(vma, vma->vm_start, vma->vm_start + PAGE_SIZE * i, PAGE_SIZE, false);
}
ktime_get_ts64(&end);
printk("test __flush_tlb_range with %04d pages, used time: %12lld ns\n", i,
((end.tv_sec - start.tv_sec) * 1000000000 + end.tv_nsec - start.tv_nsec) / TESTTIMES);
msleep(100);
}

printk("vma->start: %lx, vma->end: %lx, ttl = 0, PAGE_SIZE = 0x%lx\n", vma->vm_start, vma->vm_end, PMD_SIZE);
for (i = 1; i <= 10; i++) {
ktime_get_ts64(&start);
for (j = 0; j < TESTTIMES; j++) {
__flush_tlb_range(vma, vma->vm_start, vma->vm_start + PMD_SIZE * i, PMD_SIZE, false);
}
ktime_get_ts64(&end);
printk("test __flush_tlb_range with %04d pages, used time: %12lld ns\n", i,
((end.tv_sec - start.tv_sec) * 1000000000 + end.tv_nsec - start.tv_nsec) / TESTTIMES);
msleep(100);
}

for (i = 16; i <= 512; i+=i) {
ktime_get_ts64(&start);
for (j = 0; j < TESTTIMES; j++) {
__flush_tlb_range(vma, vma->vm_start, vma->vm_start + PMD_SIZE * i, PMD_SIZE, false);
}
ktime_get_ts64(&end);
printk("test __flush_tlb_range with %04d pages, used time: %12lld ns\n", i,
((end.tv_sec - start.tv_sec) * 1000000000 + end.tv_nsec - start.tv_nsec) / TESTTIMES);
msleep(100);
}
}

module_init(test_init)
module_exit(test_exit)

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Eillon");
MODULE_DESCRIPTION("do TTL test");
MODULE_VERSION("1.0");