On Thu, Sep 19 2024 at 18:50, Jeff Layton wrote:
> The fix for this is to establish a floor value for the coarse-grained
> clock. When stamping a file with a fine-grained timestamp, we update
> the floor value with the current monotonic time (using cmpxchg). Then
> later, when a coarse-grained timestamp is requested, check whether the
> floor is later than the current coarse-grained time. If it is, then the
> kernel will return the floor value (converted to realtime) instead of
> the current coarse-grained clock. That allows us to maintain the
> ordering guarantees.
>
> My original implementation of this tracked the floor value in
> fs/inode.c (also using cmpxchg), but that caused a performance
> regression, mostly due to multiple calls into the timekeeper functions
> with seqcount loops. By adding the floor to the timekeeper we can get
> that back down to 1 seqcount loop.
>
> Let me know if you have more questions about this, or suggestions about
> how to do this better. The timekeeping code is not my area of expertise
> (obviously) so I'm open to doing this a better way if there is one.
The comments I made about races and the clock_settime() inconsistency
vs. the change log aside, I don't see room for improvement there.
What worries me is the atomic_cmpxchg() under contention on large
machines, but as it is not a cmpxchg() loop it might be not completely
horrible.
Thanks,
tglx
