linux-ext4 - Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

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Date:   Wed, 18 Oct 2023 14:31:15 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Chandan Babu R <chandan.babu@...cle.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.de>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-mm@...ck.org,
        linux-nfs@...r.kernel.org
Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
 timestamp handing

On Wed, 18 Oct 2023 at 13:47, Jeff Layton <jlayton@...nel.org> wrote:
>
> >         old_ctime_nsec &= ~I_CTIME_QUERIED;
> >         if (ts64.tv_nsec > old_ctime_nsec + inode->i_sb->s_time_gran)
> >                 return ts64;
> >
>
> Does that really do what you expect here? current_time will return a
> value that has already been through timestamp_truncate.

Yeah, you're right.

I think you can actually remove the s_time_gran addition. Both the
old_ctime_nsec and the current ts64,tv_nsec are already rounded, so
just doing

        if (ts64.tv_nsec > old_ctime_nsec)
                return ts64;

would already guarantee that it's different enough.

> current_mgtime is calling ktime_get_real_ts64, which is an existing
> interface that does not take the global spinlock and won't advance the
> global offset. That call should be quite cheap.

Ahh, I did indeed mis-read that as the new one with the lock.

I did react to the fact that is_mgtime(inode) itself is horribly
expensive if it's not cached (following three quite possibly cold
pointers), which was part of that whole "look at I_CTIME_QUERIED
instead".

I see the pointer chasing as a huge VFS timesink in all my profiles,
although usually it's the disgusting security pointer (because selinux
creates separate security nodes for each inode, even when the contents
are often identical). So I'm a bit sensitive to "follow several
pointers from 'struct inode'" patterns from looking at too many
instruction profiles.

          Linus