linux-ext4 - code questions about ext4_inode_datasync

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Date:   Tue, 12 Jan 2021 19:45:06 +0800
From:   Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     joseph qi <joseph.qi@...ux.alibaba.com>
Subject: code questions about ext4_inode_datasync_dirty()

hi,

I use io_uring to evaluate ext4 randread performance(direct io), observed
obvious overhead in jbd2_transaction_committed():
Samples: 124K of event 'cycles:ppp', Event count (approx.): 80630088951
Overhead  Command          Shared Object      Symbol
    7.02%  io_uring-sq-per  [kernel.kallsyms]  [k] jbd2_transaction_committed

The codes:
	/*
	 * Writes that span EOF might trigger an I/O size update on completion,
	 * so consider them to be dirty for the purpose of O_DSYNC, even if
	 * there is no other metadata changes being made or are pending.
	 */
	iomap->flags = 0;
	if (ext4_inode_datasync_dirty(inode) ||
	    offset + length > i_size_read(inode))
		iomap->flags |= IOMAP_F_DIRTY;

ext4_inode_datasync_dirty() calls jbd2_transaction_committed(). Sorry, I don't spend
much time to learn iomap codes yet, just ask a quick question here. Do we need to call
ext4_inode_datasync_dirty() for a read operation?

If we must call ext4_inode_datasync_dirty() for a read operation, can we improve
jbd2_transaction_committed() a bit, for example, have a quick check between
inode->i_datasync_tid and j_commit_sequence, if inode->i_datasync_tid is less than
or equal to j_commit_sequence, we also don't call jbd2_transaction_committed()?

Regards,
Xiaoguang Wang