Re: [RFC PATCH 1/3] mm, numa: rework do_pages_move

Michal Hocko
2018-01-03T09:52:17Z

On Wed 03-01-18 15:06:49, Anshuman Khandual wrote:
> On 01/03/2018 02:28 PM, Michal Hocko wrote:
> > On Wed 03-01-18 14:12:17, Anshuman Khandual wrote:
> >> On 12/08/2017 09:45 PM, Michal Hocko wrote:
[...]
> >>> @@ -1593,79 +1556,80 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> >>> const int __user *nodes,
> >>> int __user *status, int flags)
> >>> {
> >>> - struct page_to_node *pm;
> >>> - unsigned long chunk_nr_pages;
> >>> - unsigned long chunk_start;
> >>> - int err;
> >>> -
> >>> - err = -ENOMEM;
> >>> - pm = (struct page_to_node *)__get_free_page(GFP_KERNEL);
> >>> - if (!pm)
> >>> - goto out;
> >>> + int chunk_node = NUMA_NO_NODE;
> >>> + LIST_HEAD(pagelist);
> >>> + int chunk_start, i;
> >>> + int err = 0, err1;
> >>
> >> err init might not be required, its getting assigned to -EFAULT right away.
> >
> > No, nr_pages might be 0 AFAICS.
>
> Right but there is another err = 0 after the for loop.

No we have
out_flush:
/* Make sure we do not overwrite the existing error */
err1 = do_move_pages_to_node(mm, &pagelist, current_node);
if (!err1)
err1 = store_status(status, start, current_node, i - start);
if (!err)
err = err1;

This is obviously not an act of beauty and probably a subject to a
cleanup but I just wanted this thing to be working first. Further
cleanups can go on top.

> > [...]
> >>> + if (chunk_node == NUMA_NO_NODE) {
> >>> + chunk_node = node;
> >>> + chunk_start = i;
> >>> + } else if (node != chunk_node) {
> >>> + err = do_move_pages_to_node(mm, &pagelist, chunk_node);
> >>> + if (err)
> >>> + goto out;
> >>> + err = store_status(status, chunk_start, chunk_node, i - chunk_start);
> >>> + if (err)
> >>> + goto out;
> >>> + chunk_start = i;
> >>> + chunk_node = node;
> >>> }
>
> [...]
>
> >>> + err = do_move_pages_to_node(mm, &pagelist, chunk_node);
> >>> + if (err)
> >>> + goto out;
> >>> + if (i > chunk_start) {
> >>> + err = store_status(status, chunk_start, chunk_node, i - chunk_start);
> >>> + if (err)
> >>> + goto out;
> >>> + }
> >>> + chunk_node = NUMA_NO_NODE;
> >>
> >> This block of code is bit confusing.
> >
> > I believe this is easier to grasp when looking at the resulting code.
> >>
> >> 1) Why attempt to migrate when just one page could not be isolated ?
> >> 2) 'i' is always greater than chunk_start except the starting page
> >> 3) Why reset chunk_node as NUMA_NO_NODE ?
> >
> > This is all about flushing the pending state on an error and
> > distinguising a fresh batch.
>
> Okay. Will test it out on a multi node system once I get hold of one.

Thanks. I have been testing this specific code path with the following
simple test program and numactl -m0. The code is rather crude so I've
always modified it manually to test different scenarios (this one keeps
every 1k page on the node node to test batching.
---
#include <sys/mman.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <numaif.h>

int main()
{
unsigned long nr_pages = 10000;
size_t length = nr_pages << 12, i;
unsigned char *addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
void *addrs[nr_pages];
int nodes[nr_pages];
int status[nr_pages];
char cmd[128];
char ch;

if (addr == MAP_FAILED)
return 1;

madvise(addr, length, MADV_NOHUGEPAGE);

for (i = 0; i < length; i += 4096)
addr[i] = 1;
for (i = 0; i < nr_pages; i++)
{
addrs[i] = &addr[i * 4096];
if (i%1024)
nodes[i] = 1;
else
nodes[i] = 0;
status[i] = 0;
}
snprintf(cmd, sizeof(cmd)-1, "grep %lx /proc/%d/numa_maps", addr, getpid());
system(cmd);
snprintf(cmd, sizeof(cmd)-1, "grep %lx -A20 /proc/%d/smaps", addr, getpid());
system(cmd);
read(0, &ch, 1);
if (move_pages(0, nr_pages, addrs, nodes, status, MPOL_MF_MOVE)) {
printf("move_pages: err:%d\n", errno);
}
snprintf(cmd, sizeof(cmd)-1, "grep %lx /proc/%d/numa_maps", addr, getpid());
system(cmd);
snprintf(cmd, sizeof(cmd)-1, "grep %lx -A20 /proc/%d/smaps", addr, getpid());
system(cmd);
return 0;
}

---

--
Michal Hocko
SUSE Labs