Re: Major btrfs fiemap slowdown on file with many extents once in cache (RCU stalls?) (Was: [PATCH 1/3] filemap: Correct the conditions for marking a folio as accessed)

[Yu Kuai]

在 2022/06/30 15:29, Dominique MARTINET 写道:
> Hi Willy, linux-btrfs@vger,
>
> Matthew Wilcox (Oracle) wrote on Sun, Jun 19, 2022 at 04:11:41PM +0100:
> > We had an off-by-one error which meant that we never marked the first page
> > in a read as accessed.  This was visible as a slowdown when re-reading
> > a file as pages were being evicted from cache too soon.  In reviewing
> > this code, we noticed a second bug where a multi-page folio would be
> > marked as accessed multiple times when doing reads that were less than
> > the size of the folio.
>
> when debugging an unrelated issue (short reads on btrfs with io_uring
> and O_DIRECT[1]), I noticed that my horrible big file copy speeds fell
> down from ~2GB/s (there's compression and lots of zeroes) to ~100MB/s
> the second time I was copying it with cp.

Hi,

With this patch ctive_page() will be called the second time that page is
mark accessed, which has some extra overhead, however, 2GB/s -> 100MB/s
is insane, I'm not sure how this is possible, but it seems like it has
something to do with this change.(Noted that it's problematic that page
will not mark accessed before this patch).

BTW, during my test, the speed of buffer read in ext4 only fell down a
little.

Thanks,
Kuai
> I've taken a moment to bisect this and came down to this patch.
>
> [1] https://lore.kernel.org/all/YrrFGO4A1jS0GI0G@xxxxxxxxxxxxxxxxx/T/#u
>
>
>
> Dropping caches (echo 3 > /proc/sys/vm/drop_caches) restore the speed,
> so there appears to be some bad effect to having the file in cache for
> fiemap?
> To be fair that file is pretty horrible:
> ---
> # compsize bigfile
> Processed 1 file, 194955 regular extents (199583 refs), 0 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL       15%      3.7G          23G          23G
> none       100%      477M         477M         514M
> zstd        14%      3.2G          23G          23G
> ---
>
> Here's what perf has to say about it on top of this patch when running
> `cp bigfile /dev/null` the first time:
>
> 98.97%     0.00%  cp       [kernel.kallsyms]    [k]
> entry_SYSCALL_64_after_hwframe
>   entry_SYSCALL_64_after_hwframe
>   do_syscall_64
>    - 93.40% ksys_read
>       - 93.36% vfs_read
>          - 93.25% new_sync_read
>             - 93.20% filemap_read
>                - 83.38% filemap_get_pages
>                   - 82.76% page_cache_ra_unbounded
>                      + 59.72% folio_alloc
>                      + 13.43% read_pages
>                      + 8.75% filemap_add_folio
>                        0.64% xa_load
>                     0.52% filemap_get_read_batch
>                + 8.75% copy_page_to_iter
>    - 4.73% __x64_sys_ioctl
>       - 4.72% do_vfs_ioctl
>          - btrfs_fiemap
>             - 4.70% extent_fiemap
>                + 3.95% btrfs_check_shared
>                + 0.70% get_extent_skip_holes
>
> and second time:
> 99.90%     0.00%  cp       [kernel.kallsyms]    [k]
> entry_SYSCALL_64_after_hwfram
>   entry_SYSCALL_64_after_hwframe
>   do_syscall_64
>    - 94.62% __x64_sys_ioctl
>         do_vfs_ioctl
>         btrfs_fiemap
>       - extent_fiemap
>          - 50.01% get_extent_skip_holes
>             - 50.00% btrfs_get_extent_fiemap
>                - 49.97% count_range_bits
>                     rb_next
>          + 28.72% lock_extent_bits
>          + 15.55% __clear_extent_bit
>    - 5.21% ksys_read
>       + 5.21% vfs_read
>
> (if this isn't readable, 95% of the time is spent on fiemap the second
> time around)
>
>
>
>
> I've also been observing RCU stalls on my laptop with the same workload
> (cp to /dev/null), but unfortunately I could not reproduce in qemu so I
> could not take traces to confirm they are caused by the same commit but
> given the workload I'd say that is it?
> I can rebuild a kernel for my laptop and confirm if you think it should
> be something else.
>
>
> I didn't look at the patch itself (yet) so have no suggestion at this
> point - it's plausible the patch fixed something and just exposed slow
> code that had been there all along so it might be better to look at the
> btrfs side first, I don't know.
> If you don't manage to reproduce I'll be happy to test anything thrown
> at me at the very least.
>
>
> Thanks,