[LTP] [bug?] poor migrate_pages() performance on arm64

[John Garry]

+ cc'ing linuxarm@huawei.com

It seems that we're spending much time in cache invalidate.

When you say 4 nodes, does that mean memory on all 4 nodes?

Thanks,
John

On 12/12/2018 15:41, Jan Stancek wrote:
> Hi,
>
> I'm observing migrate_pages() taking quite long time on arm64
> system (Huawei TaiShan 2280, 4 nodes, 64 CPUs). I'm using 4.20.0-rc6,
> but it's reproducible with older kernels (4.14) as well.
>
> The test (see [1] below), is a trivial C application, that migrates
> current process from one node to another. More complicated example
> is also LTP's migrate_pages03, where this has been originally reported.
>
> It takes 2+ seconds to migrate process from one node to another:
>   # strace -f -t -T ./a.out
>   ...
>   [pid 13754] 10:17:13 migrate_pages(0, 8, [0x0000000000000002], [0x0000000000000001]) = 1 <0.058115>
>   [pid 13754] 10:17:13 migrate_pages(0, 8, [0x0000000000000001], [0x0000000000000002]) = 12 <2.348186>
>   [pid 13754] 10:17:16 migrate_pages(0, 8, [0x0000000000000002], [0x0000000000000001]) = 1 <0.057889>
>   [pid 13754] 10:17:16 migrate_pages(0, 8, [0x0000000000000001], [0x0000000000000002]) = 10 <2.194890>
>   ...
>
> This scales with number of children. For example with MAXCHILD 1000,
> it takes ~33 seconds:
>   # strace -f -t -T ./a.out
>   ...
>   [pid 13773] 10:17:55 migrate_pages(0, 8, [0x0000000000000001], [0x0000000000000002]) = 11 <33.615550>
>   [pid 13773] 10:18:29 migrate_pages(0, 8, [0x0000000000000002], [0x0000000000000001]) = 2 <5.460270>
>   ...
>
> It appears to be related to migration of shared pages, presumably
> executable code of glibc.
>
> If I run [1] without CAP_SYS_NICE, it completes very quickly:
>   # sudo -u nobody strace -f -t -T ./a.out
>   ...
>   [pid 14847] 10:24:57 migrate_pages(0, 8, [0x0000000000000001], [0x0000000000000002]) = 0 <0.000172>
>   [pid 14847] 10:24:57 migrate_pages(0, 8, [0x0000000000000002], [0x0000000000000001]) = 0 <0.000091>
>   [pid 14847] 10:24:57 migrate_pages(0, 8, [0x0000000000000001], [0x0000000000000002]) = 0 <0.000074>
>   [pid 14847] 10:24:57 migrate_pages(0, 8, [0x0000000000000002], [0x0000000000000001]) = 0 <0.000069>
>   ...
>
>
> Looking at perf, most of time is spent invalidating icache.
>
> -  100.00%     0.00%  a.out    [kernel.kallsyms]  [k] __sys_trace_return
>    - __sys_trace_return
>       - 100.00% __se_sys_migrate_pages
>            do_migrate_pages.part.9
>          - migrate_pages
>             - 99.92% rmap_walk
>                - 99.92% rmap_walk_file
>                   - 99.90% remove_migration_pte
>                      - 99.85% __sync_icache_dcache
>                           __flush_cache_user_range
>
> Percent│      nop
>        │      ubfx   x3, x3, #16, #4
>        │      mov    x2, #0x4                        // #4
>        │      lsl    x2, x2, x3
>        │      sub    x3, x2, #0x1
>        │      bic    x4, x0, x3
>   1.82 │      dc     cvau, x4
>        │      add    x4, x4, x2
>        │      cmp    x4, x1
>        │    → b.cc   0xffff00000809efc8  // b.lo, b.ul, fffff7f61067
>        │      dsb    ish
>        │      nop
>   0.07 │      nop
>        │      mrs    x3, ctr_el0
>        │      nop
>        │      and    x3, x3, #0xf
>        │      mov    x2, #0x4                        // #4
>        │      lsl    x2, x2, x3
>        │      sub    x3, x2, #0x1
>        │      bic    x3, x0, x3
>  96.17 │      ic     ivau, x3
>        │      add    x3, x3, x2
>        │      cmp    x3, x1
>        │    → b.cc   0xffff00000809f000  // b.lo, b.ul, fffff7f61067
>   0.10 │      dsb    ish
>        │      isb
>   1.85 │      mov    x0, #0x0                        // #0
>        │78: ← ret
>        │      mov    x0, #0xfffffffffffffff2         // #-14
>        │    ↑ b      78
>
> Regards,
> Jan
>
> [1]
> ----- 8< -----
> #include <signal.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/syscall.h>
>
> #define MAXCHILD 10
>
> int main(void)
> {
> 	long node1 = 1, node2 = 2;
> 	int i, child;
> 	int pids[MAXCHILD];
>
> 	for (i = 0; i < MAXCHILD; i++) {
> 		child = fork();
> 		if (child == 0) {
> 			sleep(600);
> 			exit(0);
> 		}
> 		pids[i] = child;
> 	}
>
> 	for (i = 0; i < 5; i++) {
> 		syscall(__NR_migrate_pages, 0, 8, &node1, &node2);
> 		syscall(__NR_migrate_pages, 0, 8, &node2, &node1);
> 	}
>
> 	for (i = 0; i < MAXCHILD; i++) {
> 		kill(pids[i], SIGKILL);
> 	}
>
> 	return 0;
> }
> ----- >8 -----
>
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