Re: Proposal: Use hi-res clock for file timestamps

J. Bruce Fields
2010-08-19T02:10:19Z

On Thu, Aug 19, 2010 at 10:52:18AM +1000, Neil Brown wrote:
> On Thu, 19 Aug 2010 09:41:36 +1000
> Neil Brown <neilb@suse.de> wrote:
>
> > So I agree that this is probably more of an issue for directories than for
> > files, and that implementing it just for directories would be a sensible
> > first step with lower expected overhead - just my reasoning seems to be a bit
> > different.
>
> Just to be sure we are on the same page:
> file_update_time would always refer to current_nfsd_time, but nfsd would
> only update current_nfsd_time when a directory was examined (and the other
> conditions were met).
>
>
> So my current thinking on how this would look - names have been changed:
>
> - global timespec 'current_fs_precise_time' is zeroed when
> current_kernel_time moves backwards and is protected by a seqlock
>
> - current_fs_time would be
> now = max(current_kernel_time(), current_fs_precise_time)
> return timespec_trunc(now, sb->s_time_gran)
> (with appropriate seqlock protection)
>
> - new function in fs/inode.c
> get_precise_time(timestamp)

Odd name for something that returns nothing of interest;
bump_precise_time() might be closer?

And unique_time might be better than precise_time, since the property
we're asking for is that mtime on a changed file by new? (Or
versioned_time?)

> cft = current_fs_time()
> if (timestamp == cft)
/*
* Make sure the next mtime stored will be
* something different from timestamp:
*/
> write_seqlock()
> if cft == current_fs_precise_time
> current_fs_precise_time.tv_nsec++
> else if cft > current_fs_precise_time

What's the cft < current_fs_precise_time case?

--b.

> current_fs_precise_time = cft
> write_sequnlock()
> return timestamp
>
> - nfsd xdr response routine does
> ts = inode->i_mtime
> if (S_ISDIR(inode->i_mode))
> ts = get_precise_time(ts)
> xdr_encode_timespec(ts)
>
>
> get_precise_time() probably needs a bit more subtlety to handle different
> s_time_gran values and possible races, but I think it is fairly close.
>
> Then if we ever had an xstat or similar that could ask for precise
> timestamps, it just makes a similar call to get_precise_time.
> Also if we added code later to use a hires timer on hardware where it was
> efficient, get_precise_time could test for that and become a no-op
>
> Yes, I should probably turn this into a patch ... maybe another day.
>
> NeilBrown