[PATCH] radeon: Fix a false positive lockup after 10s of inactivity

[Jerome Glisse]

On Wed, Jun 12, 2013 at 6:26 AM, Michel Dänzer <michel at daenzer.net> wrote:
> On Die, 2013-06-11 at 16:23 -0700, Andy Lutomirski wrote:
>> If the device is idle for over ten seconds, then the next attempt to do
>> anything can race with the lockup detector and cause a bogus lockup
>> to be detected.
>>
>> Oddly, the situation is well-described in the lockup detector's comments
>> and a fix is even described.  This patch implements that fix (and corrects
>> some typos in the description).
>>
>> My system has been stable for about a week running this code.  Without this,
>> my screen would go blank every now and then and, when it came back, everything
>> would be remarkably slow (the latter is a separate bug).
>>
>> Signed-off-by: Andy Lutomirski <luto at amacapital.net>
>
> [...]
>
>> diff --git a/drivers/gpu/drm/radeon/radeon_ring.c b/drivers/gpu/drm/radeon/radeon_ring.c
>> index 1ef5eaa..fb7b3ea 100644
>> --- a/drivers/gpu/drm/radeon/radeon_ring.c
>> +++ b/drivers/gpu/drm/radeon/radeon_ring.c
>> @@ -547,12 +547,12 @@ void radeon_ring_lockup_update(struct radeon_ring *ring)
>>   * have CP rptr to a different value of jiffies wrap around which will force
>>   * initialization of the lockup tracking informations.
>>   *
>> - * A possible false positivie is if we get call after while and last_cp_rptr ==
>> - * the current CP rptr, even if it's unlikely it might happen. To avoid this
>> - * if the elapsed time since last call is bigger than 2 second than we return
>> - * false and update the tracking information. Due to this the caller must call
>> - * radeon_ring_test_lockup several time in less than 2sec for lockup to be reported
>> - * the fencing code should be cautious about that.
>> + * A possible false positive is if we get called after a while and
>> + * last_cp_rptr == the current CP rptr, even if it's unlikely it might
>> + * happen. To avoid this if the elapsed time since the last call is bigger
>> + * than 2 second then we return false and update the tracking
>> + * information. Due to this the caller must call radeon_ring_test_lockup
>> + * more frequently than once every 2s when waiting.
>
> Is it guaranteed that radeon_ring_test_lockup will be called more often
> than every 2s when waiting? If not, this change might prevent a real
> lockup from being detected?

Yes it will if you wait for a fence, because the fence timeout wait is
way smaller than 2sec so radeon_ring_is_lockup get call several time,
which call radeon_ring_force_activity and then
radeon_ring_test_lockup.

This also means it very very very unlikely (see below for the likely
case) to have a wrap around that give last rptr same as current one.

The likely case is when you have something like a long compute, then
nothing is lockup but you keep filling ring with
radeon_ring_force_activity but the cp is still stuck on the ib of the
compute stuff so rptr does not progress.

> Either way, I wonder if there might not be a simpler solution to the
> problem, e.g. by updating last_activity when submitting commands to a
> previously empty ring.

Maybe but i still don't think it should matter.

Andy can you test (without your patch) and see if it helps with your issue :
http://people.freedesktop.org/~glisse/0001-drm-radeon-update-lockup-tracking-when-scheduling-in.patch

Cheers,
Jerome