On 8.01.21 г. 7:36 ч., Qu Wenruo wrote:
> There are several qgroup flush related bugs fixed recently, all of them
> are caused by the fact that we can trigger qgroup metadata space
> reservation holding a transaction handle.
>
> Thankfully the only situation to trigger above reservation is
> btrfs_dirty_inode().
>
> Currently btrfs_dirty_inode() will try join transactio first, then
> update the inode.
> If btrfs_update_inode() fails with -ENOSPC, then it retry to start
> transaction to reserve metadata space.
>
> This not only forces us to reserve metadata space with a transaction
> handle hold, but can't handle other errors like -EDQUOT.
>
> This patch will make btrfs_dirty_inode() to call
> btrfs_start_transaction() directly without first try joining then
> starting, so that in try_flush_qgroup() we won't hold a trans handle.
>
> This will slow down btrfs_dirty_inode() but my fstests doesn't show too
> much different for most test cases, thus it may be worthy to skip such
> performance "optimization".
>
> Signed-off-by: Qu Wenruo <w...@suse.com>
Ok I actually run 2 tests against this patch. The first one is a 10
second run of stress-ng's utime test (stress-ng --temp-path
/media/scratch --utime 4 -M -t 10 ; done) to see if I can reproduce
intel's results and here's what I found:
bogo ops/s real (Before-patch) bogo ops/s real (After Patch)
35993 32968
35712 33146
35369 32996
35544 33159
35623 33000
35939 33016
35693 32829
35562 32685
35675 32815
Std dev 182.161981912585 146.829034703967
HMean 35677.9600871036 32957.1111111111
Diff%: -7.626
So there's a 7.6% decrease in the rate of utime() calls we can make,
given that we now start a transaction I'd say that's expected.
The other test was a randwrite with fio as I was mostly worried that
making btrfs_dirty_inode more expensive would hit write performance
since file_update_times is called from the generic iter. But inspecting
the code btrfs uses update_time_for_write which doesn't dirty the inode
per-se as this is deferred to endio completion time. I also measured
the impact during buffered read time as file_accessed is called a lot of
times but the following bpftrace script:
BEGIN {@execs = 0; }
kprobe:btrfs_dirty_inode
{
@test[kstack] = count();
@execs++;
}
kprobe:touch_atime
{
@test[kstack] = count();
}
END{
printf("Total btrfs_dirty_inode calls: %llu\n", @execs);
}
confirmed we only ever execute around 8 btrfs_dirty_inode out of 1048773
execution of touch_atimes from generic_file_buffered_read with the
following fio workload:
fio --name=random-readers --thread --ioengine=sync --iodepth=4
--rw=randread --bs=4k --direct=0 --size=1g --numjobs=4
--directory=/media/scratch --filename_format=FioWorkloads.\$jobnum
--new_group --group_reporting=1
So performance-wise I'm inclined to give it a "pass".
