[PATCH 3/3] drm/i915/hwmon: Block waiting for GuC reset to complete

Wed Apr 19 22:13:08 UTC 2023

On Wed, 19 Apr 2023 12:40:44 -0700, Rodrigo Vivi wrote:
>

Hi Rodrigo,

> On Tue, Apr 18, 2023 at 10:23:50AM -0700, Dixit, Ashutosh wrote:
> > On Mon, 17 Apr 2023 22:35:58 -0700, Rodrigo Vivi wrote:
> > >
> >
> > Hi Rodrigo,
> >
> > > On Mon, Apr 10, 2023 at 03:35:09PM -0700, Ashutosh Dixit wrote:
> > > > Instead of erroring out when GuC reset is in progress, block waiting for
> > > > GuC reset to complete which is a more reasonable uapi behavior.
> > > >
> > > > v2: Avoid race between wake_up_all and waiting for wakeup (Rodrigo)
> > > >
> > > > Signed-off-by: Ashutosh Dixit <ashutosh.dixit at intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/i915_hwmon.c | 38 +++++++++++++++++++++++++++----
> > > >  1 file changed, 33 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> > > > index 9ab8971679fe3..8471a667dfc71 100644
> > > > --- a/drivers/gpu/drm/i915/i915_hwmon.c
> > > > +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> > > > @@ -51,6 +51,7 @@ struct hwm_drvdata {
> > > >	char name[12];
> > > >	int gt_n;
> > > >	bool reset_in_progress;
> > > > +	wait_queue_head_t waitq;
> > > >  };
> > > >
> > > >  struct i915_hwmon {
> > > > @@ -395,16 +396,41 @@ hwm_power_max_read(struct hwm_drvdata *ddat, long *val)
> > > >  static int
> > > >  hwm_power_max_write(struct hwm_drvdata *ddat, long val)
> > > >  {
> > > > +#define GUC_RESET_TIMEOUT msecs_to_jiffies(2000)
> > > > +
> > > > +	int ret = 0, timeout = GUC_RESET_TIMEOUT;
> > > >	struct i915_hwmon *hwmon = ddat->hwmon;
> > > >	intel_wakeref_t wakeref;
> > > > -	int ret = 0;
> > > > +	DEFINE_WAIT(wait);
> > > >	u32 nval;
> > > >
> > > > -	mutex_lock(&hwmon->hwmon_lock);
> > > > -	if (hwmon->ddat.reset_in_progress) {
> > > > -		ret = -EAGAIN;
> > > > -		goto unlock;
> > > > +	/* Block waiting for GuC reset to complete when needed */
> > > > +	for (;;) {
> > > > +		mutex_lock(&hwmon->hwmon_lock);
> > >
> > > I'm really afraid of how this mutex is handled with the wait queue.
> > > some initial thought it looks like it is trying to reimplement ww_mutex?
> >
> > Sorry, but I am missing the relation with ww_mutex. No such relation is
> > intended.
> >
> > > all other examples of the wait_queue usages like this or didn't use
> > > locks or had it in a total different flow that I could not correlate.
> >
> > Actually there are several examples of prepare_to_wait/finish_wait
> > sequences with both spinlock and mutex in the kernel. See
> > e.g. rpm_suspend(), wait_for_rtrs_disconnection(), softsynthx_read().
> >
> > Also, as I mentioned, except for the lock, the sequence here is identical
> > to intel_guc_wait_for_pending_msg().
> >
> > >
> > > > +
> > > > +		prepare_to_wait(&ddat->waitq, &wait, TASK_INTERRUPTIBLE);
> > > > +
> > > > +		if (!hwmon->ddat.reset_in_progress)
> > > > +			break;
> > >
> > > If this breaks we never unlock it?
> >
> > Correct, this is the original case in Patch 2 where the mutex is acquired
> > in the beginning of the function and released just before the final exit
> > from the function (so the mutex is held for the entire duration of the
> > function).
>
> I got really confused here...

Sorry, the patch is a little confusing/tricky but I thought I'd better
stick to the standard 'for (;;)' loop pattern otherwise it will also be
hard to review.

> I looked at the patch 2 again and I don't see any place where the lock
> remains outside of the function. What was what I asked to remove on the
> initial versions.

So it was in Patch 1 where we changed the code to take the lock in the
beginning of the function and release it at the end of the function (you
can see it Patch 1).

In Patch 2 the 'unlock' label and 'goto unlock' is introduced and the lock
is released at the 'unlock' label (it is visible in Patch 2).

> But now with this one I'm even more confused because I couldn't follow
> to understand who will remove the lock and when.

In Patch 3 again the lock is released at the the 'unlock' label (i.e. the
destination of 'goto unlock', not visible in Patch 3). But we execute 'goto
unlock' only when 'ret != 0' in the 'for (;;)' loop. But when 'ret == 0'
(when 'ddat.reset_in_progress' flag is clear) we hold the mutex, execute
the entire function and finally release the lock at the end of the
function.

Hopefully this helps.

Thanks.
--
Ashutosh

>
> >
> > >
> > > > +
> > > > +		if (signal_pending(current)) {
> > > > +			ret = -EINTR;
> > > > +			break;
> > > > +		}
> > > > +
> > > > +		if (!timeout) {
> > > > +			ret = -ETIME;
> > > > +			break;
> > > > +		}
> > > > +
> > > > +		mutex_unlock(&hwmon->hwmon_lock);
> > >
> > > do we need to lock the signal pending and timeout as well?
> > > or only wrapping it around the hwmon->ddat access would be
> > > enough?
> >
> > Strictly, the mutex is only needed for the hwmon->ddat.reset_in_progress
> > flag. But because this is not a performance path, implementing it as done
> > in the patch simplifies the code flow (since there are several if/else,
> > goto's, mutex lock/unlock and prepare_to_wait/finish_wait to consider).
> >
> > So if possible I *really* want to not try to over-optimize here (I did try
> > a few other things when writing the patch but it was getting ugly). The
> > only real requirement is to drop the lock before calling schedule_timeout()
> > below (and we are reacquiring the lock as soon as we are scheduled back in,
> > as you can see in the loop above).
> >
> > >
> > > > +
> > > > +		timeout = schedule_timeout(timeout);
> > > >	}
> > > > +	finish_wait(&ddat->waitq, &wait);
> > > > +	if (ret)
> > > > +		goto unlock;
> > > > +
> > > >	wakeref = intel_runtime_pm_get(ddat->uncore->rpm);
> > > >
> > > >	/* Disable PL1 limit and verify, because the limit cannot be disabled on all platforms */
> > > > @@ -508,6 +534,7 @@ void i915_hwmon_power_max_restore(struct drm_i915_private *i915, bool old)
> > > >	intel_uncore_rmw(hwmon->ddat.uncore, hwmon->rg.pkg_rapl_limit,
> > > >			 PKG_PWR_LIM_1_EN, old ? PKG_PWR_LIM_1_EN : 0);
> > > >	hwmon->ddat.reset_in_progress = false;
> > > > +	wake_up_all(&hwmon->ddat.waitq);
> > > >
> > > >	mutex_unlock(&hwmon->hwmon_lock);
> > > >  }
> > > > @@ -784,6 +811,7 @@ void i915_hwmon_register(struct drm_i915_private *i915)
> > > >	ddat->uncore = &i915->uncore;
> > > >	snprintf(ddat->name, sizeof(ddat->name), "i915");
> > > >	ddat->gt_n = -1;
> > > > +	init_waitqueue_head(&ddat->waitq);
> > > >
> > > >	for_each_gt(gt, i915, i) {
> > > >		ddat_gt = hwmon->ddat_gt + i;
> > > > --
> > > > 2.38.0
> > > >
> >
> > From what I understand is the locking above is fine and is not the
> > point. The real race is between schedule_timeout() (which suspends the
> > thread) and wake_up_all() (which schedules it back in). But this
> > prepare_to_wait/finish_wait pattern is so widespread that the kernel
> > guarantees that this works correctly as long as you do things in the
> > correct order (otherwise we'd see a lot more kernel hangs/deadlocks).
> >
> > Thanks,
> > Ashutosh