Re: [RFC PATCH 0/1] Large folios in block buffered IO path

Bharata B Rao
2024-12-03T05:02:04Z

On 02-Dec-24 3:38 PM, Mateusz Guzik wrote:
> On Mon, Dec 2, 2024 at 10:37 AM Bharata B Rao <bharata@amd.com> wrote:
>>
>> On 28-Nov-24 10:01 AM, Mateusz Guzik wrote:
>>
>>> WIlly mentioned the folio wait queue hash table could be grown, you
>>> can find it in mm/filemap.c:
>>> 1062 #define PAGE_WAIT_TABLE_BITS 8
>>> 1063 #define PAGE_WAIT_TABLE_SIZE (1 << PAGE_WAIT_TABLE_BITS)
>>> 1064 static wait_queue_head_t folio_wait_table[PAGE_WAIT_TABLE_SIZE]
>>> __cacheline_aligned;
>>> 1065
>>> 1066 static wait_queue_head_t *folio_waitqueue(struct folio *folio)
>>> 1067 {
>>> 1068 │ return &folio_wait_table[hash_ptr(folio, PAGE_WAIT_TABLE_BITS)];
>>> 1069 }
>>>
>>> Can you collect off cpu time? offcputime-bpfcc -K > /tmp/out
>>
>> Flamegraph for "perf record --off-cpu -F 99 -a -g --all-kernel
>> --kernel-callchains -- sleep 120" is attached.
>>
>> Off-cpu samples were collected for 120s at around 45th minute run of the
>> FIO benchmark that actually runs for 1hr. This run was with kernel that
>> had your inode_lock fix but no changes to PAGE_WAIT_TABLE_BITS.
>>
>> Hopefully this captures the representative sample of the scalability
>> issue with folio lock.

Here is the data from offcputime-bpfcc -K run with inode_lock fix and no
change to PAGE_WAIT_TABLE_BITS. This data was captured for the entire
duration of FIO run (1hr). Since the data is huge, I am pasting a few
relevant entries.

The first entry in the offcputime records

finish_task_switch.isra.0
schedule
irqentry_exit_to_user_mode
irqentry_exit
sysvec_reschedule_ipi
asm_sysvec_reschedule_ipi
- fio (33790)
2

There are thousands of entries for read and write paths of FIO and I
have shown only the first and last entries for the same here.

First entry for FIO read path that waits on folio_lock

finish_task_switch.isra.0
schedule
io_schedule
folio_wait_bit_common
filemap_get_pages
filemap_read
blkdev_read_iter
vfs_read
ksys_read
__x64_sys_read
x64_sys_call
do_syscall_64
entry_SYSCALL_64_after_hwframe
- fio (34143)
3381769535

Last entry for FIO read path that waits on folio_lock

finish_task_switch.isra.0
schedule
io_schedule
folio_wait_bit_common
filemap_get_pages
filemap_read
blkdev_read_iter
vfs_read
ksys_read
__x64_sys_read
x64_sys_call
do_syscall_64
entry_SYSCALL_64_after_hwframe
- fio (34171)
3516224519

First entry for FIO write path that waits on folio_lock

finish_task_switch.isra.0
schedule
io_schedule
folio_wait_bit_common
__filemap_get_folio
iomap_get_folio
iomap_write_begin
iomap_file_buffered_write
blkdev_write_iter
vfs_write
ksys_write
__x64_sys_write
x64_sys_call
do_syscall_64
entry_SYSCALL_64_after_hwframe
- fio (33842)
48900

Last entry for FIO write path that waits on folio_lock

finish_task_switch.isra.0
schedule
io_schedule
folio_wait_bit_common
__filemap_get_folio
iomap_get_folio
iomap_write_begin
iomap_file_buffered_write
blkdev_write_iter
vfs_write
ksys_write
__x64_sys_write
x64_sys_call
do_syscall_64
entry_SYSCALL_64_after_hwframe
- fio (34187)
1815993

The last entry in the offcputime records

finish_task_switch.isra.0
schedule
futex_wait_queue
__futex_wait
futex_wait
do_futex
__x64_sys_futex
x64_sys_call
do_syscall_64
entry_SYSCALL_64_after_hwframe
- multipathd (6308)
3698877753

Regards,
Bharata.