linux-ext4 - Unnecessarily bad cache behavior for ext4

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Message-ID: <CAHk-=wivmk_j6KbTX+Er64mLrG8abXZo0M10PNdAnHc8fWXfsQ@mail.gmail.com>
Date:   Sun, 24 Nov 2019 16:19:16 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Unnecessarily bad cache behavior for ext4_getattr()

It looks from profiles like ext4_getattr() is fairly expensive,
because it unnecessarily accesses the extended inode information and
causes extra cache misses.

On an empty kernel allmodconfig build (which is a lot of "stat()"
calls by Make, and a lot of silly string stuff in user space due to
all the make variable games we play), ext4_getattr() was something
like 1% of the time according to the profile I gathered. It might be
bogus - maybe the cacheline ends up being accessed later anyway, but
it _looked_ like it was the whole "i_extra_isize" access that missed
in the cache.

That's all for gathering the STATX_BTIME information, that the caller
doesn't even *want*.

How about a patch like the attached?

                 Linus

View attachment "patch.diff" of type "text/x-patch" (623 bytes)