Re: Readahead regressed with c1f6925e1091("mm: put readahead pages in cache earlier") on multicore arm64 platforms

[Hsin-Yi Wang <hsinyi@chromium.org>]

On Wed, Oct 6, 2021 at 9:12 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Oct 06, 2021 at 09:07:56PM +0800, Hsin-Yi Wang wrote:
> > On Wed, Oct 6, 2021 at 7:21 PM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > On Wed, Oct 06, 2021 at 05:25:23PM +0800, Hsin-Yi Wang wrote:
> > > > Hi Matthew,
> > > >
> > > > We tested that the performance of readahead is regressed on multicore
> > > > arm64 platforms running on the 5.10 kernel.
> > > > - The platform we used: 8 cores (4x a53(small), 4x a73(big)) arm64 platform
> > > > - The command we used: ureadahead $FILE ($FILE is a 1MB+ pack file,
> > > > note that if the file size is small, it's not obvious to see the
> > > > regression)
> > > >
> > > > After we revert the commit c1f6925e1091("mm: put readahead pages in
> > > > cache earlier"), the readahead performance is back:
> > > > - time ureadahead $FILE:
> > > >   - 5.10: 1m23.124s
> > > >   - with c1f6925e1091 reverted: 0m3.323s
> > > >   - other LTS kernel (eg. 5.4): 0m3.066s
> > > >
> > > > The slowest part is aops->readpage() in read_pages() called in
> > > > read_pages(ractl, &page_pool, false); (the 3rd in
> > > > page_cache_ra_unbounded())
> > >
> > > What filesystem are you using?
> > >
> > ext4, block size 4096
>
> That's confusing.  ext4 shouldn't hit that path; it has a ->readahead
> address space operation.

Sorry for the confusion, both readahead and readpage are called.
The ->readpage is called by vfs: vfs_fadvise.
(Full path)
read_pages
page_cache_ra_unbounded
do_page_cache_ra
force_page_cache_ra
generic_fadvise
vfs_fadvise
ksys_readahead
__arm64_compat_sys_aarch32_readahead
el0_svc_common
do_el0_svc_compat
el0_svc_compat
el0_sync_compat_handler
el0_sync_compat

The ->readahead is called by ext4: ext4_file_read_iter. But this part is fast.
(Full path)
read_pages
page_cache_ra_unbounded
do_page_cache_ra
ondemand_readahead
page_cache_sync_ra
generic_file_buffered_read
generic_file_read_iter
ext4_file_read_iter
do_iter_readv_writev
do_iter_read
vfs_iter_read
loop_queue_work
kthread_worker_fn
loop_kthread_worker_fn
kthread
ret_from_fork