Re: [Intel-gfx] [PATCH v5 2/3] drm/i915: Move on the new pm runtime interface

From: Tvrtko Ursulin
Date: Mon Dec 31 2018 - 07:32:39 EST

On 21/12/2018 13:26, Vincent Guittot wrote:
On Fri, 21 Dec 2018 at 12:33, Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:


On 21/12/2018 10:33, Vincent Guittot wrote:
Use the new pm runtime interface to get the accounted suspended time:
This new interface helps to simplify and cleanup the code that computes
__I915_SAMPLE_RC6_ESTIMATED and to remove direct access to internals of
PM runtime.

Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
drivers/gpu/drm/i915/i915_pmu.c | 16 ++++++----------
drivers/gpu/drm/i915/i915_pmu.h | 4 ++--
2 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index d6c8f8f..3f76f60 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -5,6 +5,7 @@

#include <linux/irq.h>
+#include <linux/pm_runtime.h>
#include "i915_pmu.h"
#include "intel_ringbuffer.h"
#include "i915_drv.h"
@@ -478,7 +479,6 @@ static u64 get_rc6(struct drm_i915_private *i915)
* counter value.
spin_lock_irqsave(&i915->pmu.lock, flags);
- spin_lock(&kdev->power.lock);

* After the above branch intel_runtime_pm_get_if_in_use failed
@@ -491,16 +491,13 @@ static u64 get_rc6(struct drm_i915_private *i915)
* suspended and if not we cannot do better than report the last
* known RC6 value.
- if (kdev->power.runtime_status == RPM_SUSPENDED) {
- if (!i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur)
- i915->pmu.suspended_jiffies_last =
- kdev->power.suspended_jiffies;
+ if (pm_runtime_status_suspended(kdev)) {
+ val = pm_runtime_suspended_time(kdev);

There is a race condition between the status check and timestamp access
which the existing code solves by holding the power.lock over it. But I
don't exactly remember how this issue was manifesting. Is
kdev->power.suspended_jiffies perhaps reset on exit from runtime
suspend, which was then underflowing the val, not sure.

Anyways, is the new way of doing this safe with regards to this race? In

AFAICT it is safe.
The current version does:
1-take lock,
2-test if dev is suspended
3-read some internals field to computed an up-to-date suspended time
4-update __I915_SAMPLE_RC6_ESTIMATED
5-release lock

The new version does:
1-test if dev is suspended
2-get an up-to-date suspended time with pm_runtime_suspended_time.
This is atomic and monotonic
3-update __I915_SAMPLE_RC6_ESTIMATED

A change from suspended to another states that happens just before
step 1 is ok for both as we will run the else if
No change of the state can happen after step 1 in current code and the
estimated suspended time will be the time up to step2. In parallel,
Any state change will have to wait step5 to continue
If a change from suspended to another state happens after step 1 in
new code, the suspended time return by PM core will be the time up to
this change. So I would say you don't delay state transition and you
get a more accurate estimated suspended time (even if the difference
should be small).
If a change from suspended to another state happens after step 2 in
new code, the suspended time return by PM core will be the time up to
step 2 so there is no changes

other words is the value pm_runtime_suspended_time always monotonic,
even when not suspended? If not we have to handle the race somehow.

Yes pm_runtime_suspended_time is monotonic and stays unchanged when
not suspended

If it is always monotonic, then worst case we report one wrong sample,
which I guess is still not ideal since someone could be querying the PMU
with quite low frequency.

There are tests which probably can hit this, but to run them
automatically your patches would need to be rebased on drm-tip and maybe
sent to our trybot. I can do that after the holiday break if you are
okay with having the series waiting until then.

yes looks good to me

Looks good to me as well. And our CI agrees with it. So:

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

I assume you will take the patch through some power related tree and we will eventually pull it back to drm-tip.