Re: 2.6.17-rc2+ regression -- audio skipping

[Con Kolivas <kernel@kolivas.org>]

On Monday 22 May 2006 10:10, Rene Herman wrote:
> Lee Revell wrote:
> > On Sun, 2006-05-21 at 22:24 +0200, Rene Herman wrote:
> >> 2.6.17-rc2 (and 3 and 4) make my audio skip. Audio player is ogg123
> >> running in an xterm. Browsing heavy sites (say, eBay) with Firefox
> >> 1.5.0.3 gets me audio underruns quickly. This does not happen on
> >> 2.6.17-rc1 and earlier (I just tested extensively; quite impossible to
> >> generate underruns on -rc1, quickly on -rc2 and later).
> >>
> >> It's not ALSA; reverted */sound/* from the rc1-rc2 interdiff. It's also
> >> not cfq-iosched.c. Any likely culprits in there? (I'm not a GIT user).
> >
> > I would suspect the scheduler interactivity patches.  Please confirm
> > this by running ogg123 at nice -20 - do the underruns persist?
>
> They do persist. Thanks for the hint though -- "sched: fix interactive
> task starvation" is the culprit:
>
> http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=comm
>it;h=5ce74abe788a26698876e66b9c9ce7e7acc25413

Good investigative work! Makes sense that falling on the expired array would 
make for terrible latency for audio apps if your cpu was stretched just the 
right amount.

> Added author and acked-by's to the CC. Mike, this patch is no good for
> me. Audio underruns galore, with only ogg123 and firefox (browsing the
> GIT tree online is also a nice trigger by the way).
>
> If I back it out, everything is fine for me again. Back-out attached as
> a patch against -rc4. This also backs out your follow-up "don't awaken
> RT tasks on expired array" as it was dependant:
>
> http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=comm
>it;h=8a5bc075b8d8cf7a87b3f08fad2fba0f5d13295e
>
> While looking at the patch I noticed there was +1 difference in the
> "limit" value between the macro and the static inline version of
> expired_starving() so I experimented with adding that back but that
> wasn't it unfortunately.
>
> I can test patches (preferably versus -rc4) although possibly not quickly.

This close to 2.6.17 the safest thing we can and should do is simply back out 
the patch.

-- 
-ck