Re: Flush scan list on resume

[Howard Chu <hyc symas com>]

Dan Williams wrote:
> On Wed, 2009-02-04 at 18:04 -0500, Tony Espy wrote:
> > Dan Williams wrote:
> > > So here's why old access points show up after resume...
> > >
> > > Executive summary: exclusively a *driver* problem
> > >
> > >
> > > Detailed explanation:
> > >
> > > Most kernel drivers use 'jiffies' as the timestamp for each scan result.
> > > jiffies is a kernel counter that counts *while the kernel is alive*, and
> > > thus does not reflect time spent in suspend and hibernate.
> > >
> > > When the supplicant requests scan results from the driver on resume, the
> > > supplicant (and NM) gets back a list of scan results with recent-looking
> > > timestamps that do not reflect the time the machine was suspended.
> > >
> > > The solutions are to move drivers to using the actual machine clock (the
> > > kernel equivalent of time(2)) for last_beacon, or to be more agressive
> > > about expiring entries on resume with something like (10 - time spent in
> > > sleep).

OK. Having looked in my wireless-testing tree, I see that most of the drivers 
are now just allowing mac80211 to handle scanning, so this is not a per-driver 
issue. At least that makes the scope of fixing easier to manage.

> > How 'bout getting drivers to add code to their resume functions that
> > just clears the scan results queue?

I was actually thinking it makes sense to clear results on suspend, no reason 
to keep kmem allocated at that point. (No point in saving an obsolete queue on 
a hibernate, after all.)

> That's not desirable on mobile platforms like OLPC that do
> suspend/resume very quickly and very often.  The best mechanism is to
> make the *actual* last seen time available to the userspace programs
> that implement the policy (like connection managers) so they can do
> what's appropriate.

OK. I suggest a compromise then, since fetching kernel time will also be 
expensive if done frequently - just fetch it at driver load and resume time, 
and store it as a <base> in the driver. Cache entries then store base+jiffy. 
This allows cache stamps to continue to be cheap, but also solves the 
sleep/resume mismatch.

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/