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

[Jeff Layton]

On Wed, 2023-10-18 at 14:31 -0700, Linus Torvalds wrote:
> On Wed, 18 Oct 2023 at 13:47, Jeff Layton <jlayton@xxxxxxxxxx> 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.
> 

Yep, and that's basically what inode_set_ctime_current does (though it
does a timespec64 comparison).

> > 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.

That's a very good point. I'll see if I can get rid of that (and maybe
some other mgtime flag checks) before I send the next version. 

Back to your earlier point though:

Is a global offset really a non-starter? I can see about doing something
per-superblock, but ktime_get_mg_coarse_ts64 should be roughly as cheap
as ktime_get_coarse_ts64. I don't see the downside there for the non-
multigrain filesystems to call that.

On another note: maybe I need to put this behind a Kconfig option
initially too?
-- 
Jeff Layton <jlayton@xxxxxxxxxx>