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 -----
