[Linux-ia64] ping(8) faster than light (do_get_fast_time) (fwd)

[Linux-ia64] ping(8) faster than light (do_get_fast_time) (fwd)

[Martin Wilck]

Hi,

I have encountered the following strange problem that is related
to the RedHat bug

https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id7485

A "ping localhost" on RedHat 7.1 will generate the warning

Warning: time of day goes back, taking countermeasures.

Tracing ping(8) with tcpdump shows that echo request and echo reply
packets have exactly the same time stamp.

The "RELNOTES" file of the RedHat iputils package says

* By Pekka Savola <pekkas@netcore.fi>
  - SIOCGSTAMP/SO_TIMESTAMP are sensitive to bug in kernel.
    When get_fast_time != gettimeofday (f.e. timestampless x86),
    returned stamp can be out of sync with gettimeofday.
    Workaround is not to use SIOCGSTAMP/SO_TIMESTAMP on such systems.

It appears that this is actually the reason for the above problem.

The deeper reason is that on IA64, *do_get_fast_time() equals
do_normal_gettime(), which simply returns xtime. On other architectures
do_get_fast_time() is set to do_gettimeofday() in time_init(), which
avoids the problem.

1) Is do_gettimeofday () too slow on IA64 to use it for
   do_get_fast_time() ?

2) If yes, would it be possible to susbtitute a do_get_fast_time()
   that is faster and less accurate than do_gettimeofday(), but still
   better than do_normal_gettime() ?

3) If no, the ping(8) program on IA64 should IMO be changed to not use
   SO_TIMESTAMP (equivalent to "ping -U" with the RH7.1 ping).

Martin

-- 
Martin Wilck     <Martin.Wilck@fujitsu-siemens.com>
FSC EP PS DS1, Paderborn      Tel. +49 5251 8 15113

Re: [Linux-ia64] ping(8) faster than light (do_get_fast_time) (fwd)

[David Mosberger]

>>>>> On Fri, 10 Aug 2001 15:15:56 +0200 (CEST), Martin Wilck <Martin.Wilck@fujitsu-siemens.com> said:

  Martin> The deeper reason is that on IA64, *do_get_fast_time()
  Martin> equals do_normal_gettime(), which simply returns xtime.

This distinction doesn't make sense on ia64 because there is no
(slow) gettimeofday() to begin with.

  Martin> 1) Is do_gettimeofday () too slow on IA64 to use it for
  Martin> do_get_fast_time() ?

I have no idea what you mean here.  On ia64 do_gettimeofday() returns
xtime plus an itc based time offset.  The kernel is careful to return
only monotonically increasing time values, even in cases where the
ITCs in different CPUs are not perfectly synchronized.

  Martin> 3) If no, the ping(8) program on IA64 should IMO be changed
  Martin> to not use SO_TIMESTAMP (equivalent to "ping -U" with the
  Martin> RH7.1 ping).

We should first find out what the real problem is.  What timestamps
are present at the time ping is complaining?  If the earlier timestamp
is bigger than the later one, there is a bug somewhere.  If they're
equal (unlikely, given that we do ITC-based interpolation), ping
shouldn't complain.

	--david

Re: [Linux-ia64] ping(8) faster than light (do_get_fast_time) (fwd)

[Martin Wilck]

On Fri, 10 Aug 2001, David Mosberger wrote:

> >>>>> On Fri, 10 Aug 2001 15:15:56 +0200 (CEST), Martin Wilck <Martin.Wilck@fujitsu-siemens.com> said:
>
>   Martin> The deeper reason is that on IA64, *do_get_fast_time()
>   Martin> equals do_normal_gettime(), which simply returns xtime.
>
> This distinction doesn't make sense on ia64 because there is no
> (slow) gettimeofday() to begin with.

Then, unless I'm completely mistaken, time_init() should change the
pointer do_get_fast_time to use do_gettimeofday() rather than
do_normal gettime(). Unless this is done, get_fast_time()
will simply return xtime. Or am I overlooking something?

>   Martin> 1) Is do_gettimeofday () too slow on IA64 to use it for
>   Martin> do_get_fast_time() ?
>
> I have no idea what you mean here.  On ia64 do_gettimeofday() returns
> xtime plus an itc based time offset.

Yes, but get_fast_time() doesn't (see above), and that's the function
that is called for getting timestamps for socket buffers.

Martin
-- 
Martin Wilck     <Martin.Wilck@fujitsu-siemens.com>
FSC EP PS DS1, Paderborn      Tel. +49 5251 8 15113

Re: [Linux-ia64] ping(8) faster than light (do_get_fast_time) (fwd)

[David Mosberger]

>>>>> On Fri, 10 Aug 2001 17:41:26 +0200 (CEST), Martin Wilck <Martin.Wilck@fujitsu-siemens.com> said:

  Martin> Yes, but get_fast_time() doesn't (see above), and that's the
  Martin> function that is called for getting timestamps for socket
  Martin> buffers.

Ah, yes, my oversight.  We should initialize do_get_fast_time.  Thanks
for tracking this down.

	--david