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

Mateusz Guzik
2024-11-27T06:20:14Z

On Wed, Nov 27, 2024 at 7:13 AM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> On Wed, Nov 27, 2024 at 6:48 AM Bharata B Rao <bharata@amd.com> wrote:
> >
> > Recently we discussed the scalability issues while running large
> > instances of FIO with buffered IO option on NVME block devices here:
> >
> > https://lore.kernel.org/linux-mm/d2841226-e27b-4d3d-a578-63587a3aa4f3@amd.com/
> >
> > One of the suggestions Chris Mason gave (during private discussions) was
> > to enable large folios in block buffered IO path as that could
> > improve the scalability problems and improve the lock contention
> > scenarios.
> >
>
> I have no basis to comment on the idea.
>
> However, it is pretty apparent whatever the situation it is being
> heavily disfigured by lock contention in blkdev_llseek:
>
> > perf-lock contention output
> > ---------------------------
> > The lock contention data doesn't look all that conclusive but for 30% rwmixwrite
> > mix it looks like this:
> >
> > perf-lock contention default
> > contended total wait max wait avg wait type caller
> >
> > 1337359017 64.69 h 769.04 us 174.14 us spinlock rwsem_wake.isra.0+0x42
> > 0xffffffff903f60a3 native_queued_spin_lock_slowpath+0x1f3
> > 0xffffffff903f537c _raw_spin_lock_irqsave+0x5c
> > 0xffffffff8f39e7d2 rwsem_wake.isra.0+0x42
> > 0xffffffff8f39e88f up_write+0x4f
> > 0xffffffff8f9d598e blkdev_llseek+0x4e
> > 0xffffffff8f703322 ksys_lseek+0x72
> > 0xffffffff8f7033a8 __x64_sys_lseek+0x18
> > 0xffffffff8f20b983 x64_sys_call+0x1fb3
> > 2665573 64.38 h 1.98 s 86.95 ms rwsem:W blkdev_llseek+0x31
> > 0xffffffff903f15bc rwsem_down_write_slowpath+0x36c
> > 0xffffffff903f18fb down_write+0x5b
> > 0xffffffff8f9d5971 blkdev_llseek+0x31
> > 0xffffffff8f703322 ksys_lseek+0x72
> > 0xffffffff8f7033a8 __x64_sys_lseek+0x18
> > 0xffffffff8f20b983 x64_sys_call+0x1fb3
> > 0xffffffff903dce5e do_syscall_64+0x7e
> > 0xffffffff9040012b entry_SYSCALL_64_after_hwframe+0x76
>
> Admittedly I'm not familiar with this code, but at a quick glance the
> lock can be just straight up removed here?
>
> 534 static loff_t blkdev_llseek(struct file *file, loff_t offset, int whence)
> 535 {
> 536 │ struct inode *bd_inode = bdev_file_inode(file);
> 537 │ loff_t retval;
> 538 │
> 539 │ inode_lock(bd_inode);
> 540 │ retval = fixed_size_llseek(file, offset, whence,
> i_size_read(bd_inode));
> 541 │ inode_unlock(bd_inode);
> 542 │ return retval;
> 543 }
>
> At best it stabilizes the size for the duration of the call. Sounds
> like it helps nothing since if the size can change, the file offset
> will still be altered as if there was no locking?
>
> Suppose this cannot be avoided to grab the size for whatever reason.
>
> While the above fio invocation did not work for me, I ran some crapper
> which I had in my shell history and according to strace:
> [pid 271829] lseek(7, 0, SEEK_SET) = 0
> [pid 271829] lseek(7, 0, SEEK_SET) = 0
> [pid 271830] lseek(7, 0, SEEK_SET) = 0
>
> ... the lseeks just rewind to the beginning, *definitely* not needing
> to know the size. One would have to check but this is most likely the
> case in your test as well.
>
> And for that there is 0 need to grab the size, and consequently the inode lock.

That is to say bare minimum this needs to be benchmarked before/after
with the lock removed from the picture, like so:

diff --git a/block/fops.c b/block/fops.c
index 2d01c9007681..7f9e9e2f9081 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -534,12 +534,8 @@ const struct address_space_operations def_blk_aops = {
static loff_t blkdev_llseek(struct file *file, loff_t offset, int whence)
{
struct inode *bd_inode = bdev_file_inode(file);
- loff_t retval;

- inode_lock(bd_inode);
- retval = fixed_size_llseek(file, offset, whence, i_size_read(bd_inode));
- inode_unlock(bd_inode);
- return retval;
+ return fixed_size_llseek(file, offset, whence, i_size_read(bd_inode));
}

static int blkdev_fsync(struct file *filp, loff_t start, loff_t end,

To be aborted if it blows up (but I don't see why it would).

--
Mateusz Guzik <mjguzik gmail.com>