Re: Hugepage program taking forever to exit

[Jens Axboe <axboe@kernel.dk>]

On 9/10/24 2:17 PM, Yu Zhao wrote:
> On Tue, Sep 10, 2024 at 12:21?PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> Hi,
>>
>> Investigating another issue, I wrote the following simple program that allocates
>> and faults in 500 1GB huge pages, and then registers them with io_uring. Each
>> step is timed:
>>
>> Got 500 huge pages (each 1024MB) in 0 msec
>> Faulted in 500 huge pages in 38632 msec
>> Registered 500 pages in 867 msec
>>
>> and as expected, faulting in the pages takes (by far) the longest. From
>> the above, you'd also expect the total runtime to be around ~39 seconds.
>> But it is not... In fact it takes 82 seconds in total for this program
>> to have exited. Looking at why, I see:
>>
>> [<0>] __wait_rcu_gp+0x12b/0x160
>> [<0>] synchronize_rcu_normal.part.0+0x2a/0x30
>> [<0>] hugetlb_vmemmap_restore_folios+0x22/0xe0
>> [<0>] update_and_free_pages_bulk+0x4c/0x220
>> [<0>] return_unused_surplus_pages+0x80/0xa0
>> [<0>] hugetlb_acct_memory.part.0+0x2dd/0x3b0
>> [<0>] hugetlb_vm_op_close+0x160/0x180
>> [<0>] remove_vma+0x20/0x60
>> [<0>] exit_mmap+0x199/0x340
>> [<0>] mmput+0x49/0x110
>> [<0>] do_exit+0x261/0x9b0
>> [<0>] do_group_exit+0x2c/0x80
>> [<0>] __x64_sys_exit_group+0x14/0x20
>> [<0>] x64_sys_call+0x714/0x720
>> [<0>] do_syscall_64+0x5b/0x160
>> [<0>] entry_SYSCALL_64_after_hwframe+0x4b/0x53
>>
>> and yes, it does look like the program is mostly idle for most of the
>> time while returning these huge pages. It's also telling us exactly why
>> we're just sitting idle - RCU grace period.
>>
>> The below quick change means the runtime of the program is pretty much
>> just the time it takes to execute the parts of it, as you can see from
>> the full output after the change:
>>
>> axboe@r7525 ~> time sudo ./reg-huge
>> Got 500 huge pages (each 1024MB) in 0 msec
>> Faulted in 500 huge pages in 38632 msec
>> Registered 500 pages in 867 msec
>>
>> ________________________________________________________
>> Executed in   39.53 secs      fish           external
>>    usr time    4.88 millis  238.00 micros    4.64 millis
>>    sys time    0.00 millis    0.00 micros    0.00 millis
>>
>> where 38632+876 == 39.51s.
>>
>> Looks like this was introduced by:
>>
>> commit bd225530a4c717714722c3731442b78954c765b3
>> Author: Yu Zhao <yuzhao@google.com>
>> Date:   Thu Jun 27 16:27:05 2024 -0600
> 
> Fixes are in git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-stable
> 
> c2a967f6ab0e mm/hugetlb_vmemmap: don't synchronize_rcu() without HVO
> c0f398c3b2cf mm/hugetlb_vmemmap: batch HVO work when demoting
> 
> Additional improvements from mm-stable that may or may not help your test case:
> 
> e98337d11bbd mm/contig_alloc: support __GFP_COMP
> 463586e9ff39 mm/cma: add cma_{alloc,free}_folio()
> cf54f310d0d3 mm/hugetlb: use __GFP_COMP for gigantic folios

I didn't pick patches, just pulled all of mm-stable - yep that fixes it
for me, no longer spending 60+% of the runtime during exit. I trust
these are heading to stable?

-- 
Jens Axboe