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

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Date:	Thu, 19 Aug 2010 10:52:18 +1000
From:	Neil Brown <neilb@...e.de>
To:	Chuck Lever <chuck.lever@...cle.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Patrick J. LoPresti" <lopresti@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Proposal: Use hi-res clock for file timestamps

On Thu, 19 Aug 2010 09:41:36 +1000
Neil Brown <neilb@...e.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)
                cft = current_fs_time()
                if (timestamp == cft)
                   write_seqlock()
                   if cft == current_fs_precise_time
                        current_fs_precise_time.tv_nsec++
                   else if cft > current_fs_precise_time
                        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
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