Re: [linux-next:master] [mm] 1111d46b5c: stress-ng.pthread.ops_per_sec -84.3% regression

Yin Fengwei
2023-12-21T01:08:17Z

>>>>
>>>> https://github.com/lattera/glibc/blob/master/nptl/allocatestack.c#L384
>>>> https://github.com/lattera/glibc/blob/master/nptl/pthread_create.c#L576
>>>
>>> It explains the problem. The madvise() does have some extra overhead
>>> for handling THP (splitting pmd, deferred split queue, etc).
>>>
>>>>
>>>> Another thing is whether it's worthy to make stack use THP? It may be
>>>> useful for some apps which need large stack size?
>>>
>>> Kernel actually doesn't apply THP to stack (see
>>> vma_is_temporary_stack()). But kernel can't know whether the VMA is
>>> stack or not by checking VM_GROWSDOWN | VM_GROWSUP flags. So if glibc
>>> doesn't set the proper flags to tell kernel the area is stack, kernel
>>> just treats it as normal anonymous area. So glibc should set up stack
>>> properly IMHO.
>>
>> If I read the code correctly, nptl allocates stack by the below code:
>>
>> mem = __mmap (NULL, size, (guardsize == 0) ? prot : PROT_NONE,
>>                          MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
>>
>> See
>> https://github.com/lattera/glibc/blob/master/nptl/allocatestack.c#L563
>>
>> The MAP_STACK is used, but it is a no-op on Linux. So the alternative
>> is to make MAP_STACK useful on Linux instead of changing glibc. But
>> the blast radius seems much wider.
> Yes. MAP_STACK is also mentioned in manpage of mmap. I did test to
> filter out of the MAP_STACK mapping based on this patch. The regression
> in stress-ng.pthread was gone. I suppose this is kind of safe because
> the madvise call is only applied to glibc allocated stack.

The patch I tested against stress-ng.pthread:

diff --git a/mm/mmap.c b/mm/mmap.c
index b78e83d351d2..1fd510aef82e 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1829,7 +1829,8 @@ get_unmapped_area(struct file *file, unsigned long
addr, unsigned long len,
*/
pgoff = 0;
get_area = shmem_get_unmapped_area;
- } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
+ } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+ !(flags & MAP_STACK)) {
/* Ensures that larger anonymous mappings are THP
aligned. */
get_area = thp_get_unmapped_area;
}

>
>
> But what I am not sure was whether it's worthy to do such kind of change
> as the regression only is seen obviously in micro-benchmark. No evidence
> showed the other regressionsin this report is related with madvise. At
> least from the perf statstics. Need to check more on stream/ramspeed.
> Thanks.
>
>
> Regards
> Yin, Fengwei
>
>>
>>>
>>>>
>>>>
>>>> Regards
>>>> Yin, Fengwei