Re: [REGRESSION]: mmap performance regression starting with k-6.1

[David Wang <00107082@163.com>]

Hi, 

Just contribute some information I recently collected for this thread:

I run some profiler, and it shows fundamental difference between 6.0 and 6.1:


v6.0:
```
__x64_sys_munmap(60.544% 6474/10693)
    __vm_munmap(98.749% 6393/6474)
        __do_munmap(97.982% 6264/6393)
            __split_vma(53.975% 3381/6264)
                vm_area_dup(59.036% 1996/3381)
                __vma_adjust(32.121% 1086/3381)
                anon_vma_clone(5.915% 200/3381)
                vma_dup_policy(0.769% 26/3381)
            unmap_region(16.699% 1046/6264)
            find_vma(10.361% 649/6264)
            remove_vma(7.822% 490/6264)
            percpu_counter_add_batch(2.011% 126/6264)
            __vma_rb_erase(1.405% 88/6264)
            userfaultfd_unmap_prep(0.798% 50/6264)
            downgrade_write(0.511% 32/6264)
```

v6.1:
```
__x64_sys_munmap(68.024% 24741/36371)
    __vm_munmap(99.681% 24662/24741)
        do_mas_munmap(99.015% 24419/24662)
            do_mas_align_munmap(98.243% 23990/24419)
                __split_vma(58.966% 14146/23990)
                    __vma_adjust(83.755% 11848/14146)
                    vm_area_dup(13.191% 1866/14146)
                    anon_vma_clone(2.050% 290/14146)
                    vma_dup_policy(0.254% 36/14146)
                mas_store_prealloc(11.709% 2809/23990)
                mas_preallocate(9.579% 2298/23990)
                unmap_region(5.523% 1325/23990)
```

v6.1 introduce the maple tree data structure, and mmap/munmap performance started to degrade since.
Base on the observation, I tested two commit:
9832fb87834e2bd925d30020962c81b05948fa7b GOOD (Same as v6.0, about 20seonds) (This is before "Patch series "Introducing the Maple Tree")
11f9a21ab65542189372b7d64bb2d2937dfdc9dc BAD  (about 51seconds) (This one is somewhere middle in the path series for maple tree.)
While with v6.1, the test run about 56 seconds

For v6.7, profiler show further fundimental changes, some vmi stuff, and preformance is worse (~70 seconds).
```
__x64_sys_munmap(63.873% 30725/48103)
    __vm_munmap(99.456% 30558/30725)
        do_vmi_munmap(97.670% 29846/30558)
            do_vmi_align_munmap(97.196% 29009/29846)
                __split_vma(63.701% 18479/29009)
                    vma_complete(34.417% 6360/18479)
                    vm_area_dup(33.681% 6224/18479)
                    mas_preallocate(11.835% 2187/18479)
                    down_write(5.173% 956/18479)
                    up_write(3.815% 705/18479)
                    asm_sysvec_apic_timer_interrupt(1.153% 213/18479)
                    anon_vma_clone(0.974% 180/18479)
                    vma_adjust_trans_huge(0.622% 115/18479)
                    mas_next_slot(0.498% 92/18479)
                    vma_dup_policy(0.465% 86/18479)
                    vma_prepare(0.357% 66/18479)
                    srso_return_thunk(0.336% 62/18479)
                    mas_find(0.114% 21/18479)
                unmap_region.constprop.0(12.196% 3538/29009)
                mas_store_gfp(10.548% 3060/29009)
                __call_rcu_common.constprop.0(1.992% 578/29009)
```

I use following test code, and timed it
```
#define MAXN 1024
struct { void* addr; size_t n; } maps[MAXN];
int main() {
	int i, n, k, r;
	void *p;
	for (i=0; i<MAXN; i++) {
		n = 1024*((rand()%32)+1);
		p = mmap(NULL, n, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
		if (p == MAP_FAILED) {
			perror("fail to mmap");
			return -1;
		}
		maps[i].addr = p; 
		maps[i].n = n;

	}
	for (i=0; i<10000000; i++) {
		k = rand()%MAXN;
		r = munmap(maps[k].addr, maps[k].n);
		if (r) {
			perror("fail to munmap");
			return -1;
		}
		n = 1024*((rand()%32)+1);
		p = mmap(NULL, n, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
		if (p == MAP_FAILED) {
			perror("fail to mmap");
			return -1;
		}
		maps[k].addr = p; 
		maps[k].n = n;
	}
	for (i=0; i<MAXN; i++) munmap(maps[i].addr, maps[i].n);
	return 0;
}
```

Thanks
David Wang