Re: [PATCH] btrfs: Use the normal writeback path for flushing delalloc

[Josef Bacik <josef@toxicpanda.com>]

On 1/4/21 12:23 PM, Filipe Manana wrote:
> On Mon, Jan 4, 2021 at 5:06 PM Josef Bacik <josef@toxicpanda.com> wrote:
>>
>> This is a revert for 38d715f494f2 ("btrfs: use
>> btrfs_start_delalloc_roots in shrink_delalloc").  A user reported a
>> problem where performance was significantly worse with this patch
>> applied.  The problem needs to be fixed with proper pre-flushing, and
>> changes to how we deal with the work queues for the inodes.  However
>> that work is much more complicated than is acceptable for stable, and
>> simply reverting this patch fixes the problem.  The original patch was
>> a cleanup of the code, so it's fine to revert it.  My numbers for the
>> original reported test, which was untarring a copy of the firefox
>> sources, are as follows
>>
>> 5.9     0m54.258s
>> 5.10    1m26.212s
>> Fix     0m35.038s
>>
>> cc: stable@vger.kernel.org # 5.10
>> Reported-by: René Rebe <rene@exactcode.de>
>> Fixes: 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in shrink_delalloc")
>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>> ---
>> Dave, this is ontop of linus's branch, because we've changed the arguments for
>> btrfs_start_delalloc_roots in misc-next, and this needs to go back to 5.10 ASAP.
>> I can send a misc-next version if you want to have it there as well while we're
>> waiting for it to go into linus's tree, just let me know.
> 
> Adding this to stable releases will also make the following fix not
> work on stable releases:
> 
> https://lore.kernel.org/linux-btrfs/39c2a60aa682f69f9823f51aa119d37ef4b9f834.1606909923.git.fdmanana@suse.com/
> 
> Since now the async reclaim task can trigger writeback through
> writeback_inodes_sb_nr() and not only through
> btrfs_start_delalloc_roots().
> Other than changing that patch to make extent_write_cache_pages() do
> nothing when the inode has the bit BTRFS_INODE_NO_DELALLOC_FLUSH set,
> I'm not seeing other simple ways to do it.

Hmmm shit, ok let me see if I can make the perf regression go away while still 
using btrfs_start_delalloc_roots().  Thanks,

Josef