Messages in this thread Patch in this message | | | From | John Keeping <> | Subject | [PATCH v2] PM: runtime: avoid priority inversion on PREEMPT_RT | Date | Wed, 17 Nov 2021 18:37:09 +0000 |
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With PREEMPT_RT the cpu_relax() loops in rpm_suspend and rpm_resume can cause unbounded latency if they preempt an asynchronous suspend. The main scenario where this can happen is when a realtime thread resumes a device while it is asynchronously suspending on a worker thread.
I'm not convinced this can actually happen in the rpm_suspend case, or at least it's a lot less likely for a synchronous suspend to run at the same time as an asynchronous suspend, but both functions are updated here for symmetry.
For devices setting power.irq_safe, it is possible that RPM functions will be called with a spinlock held (for example in pl330_issue_pending()). This means a normal call to schedule() can't be used, but to avoid the priority inversion it is necessary to wait and schedule. schedule_rtlock() is only available when CONFIG_PREEMPT_RT is defined, so even though the logic is correct without any preprocessor guards around schedule_rtlock(), they are necessary for compilation.
Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Ingo Molnar <mingo@kernel.org> Signed-off-by: John Keeping <john@metanate.com> --- Changes since v1: - Use schedule_rtlock() instead of schedule() for PREEMPT_RT & irq_safe - Rewritten commit description
drivers/base/power/runtime.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-)
diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c index f3de7bfc7f5b..fdf461bfae8c 100644 --- a/drivers/base/power/runtime.c +++ b/drivers/base/power/runtime.c @@ -596,7 +596,7 @@ static int rpm_suspend(struct device *dev, int rpmflags) goto out; } - if (dev->power.irq_safe) { + if (dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT)) { spin_unlock(&dev->power.lock); cpu_relax(); @@ -614,7 +614,12 @@ static int rpm_suspend(struct device *dev, int rpmflags) spin_unlock_irq(&dev->power.lock); - schedule(); +#ifdef CONFIG_PREEMPT_RT + if (dev->power.irq_safe) + schedule_rtlock(); + else +#endif + schedule(); spin_lock_irq(&dev->power.lock); } @@ -779,7 +784,7 @@ static int rpm_resume(struct device *dev, int rpmflags) goto out; } - if (dev->power.irq_safe) { + if (dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT)) { spin_unlock(&dev->power.lock); cpu_relax(); @@ -798,7 +803,12 @@ static int rpm_resume(struct device *dev, int rpmflags) spin_unlock_irq(&dev->power.lock); - schedule(); +#ifdef CONFIG_PREEMPT_RT + if (dev->power.irq_safe) + schedule_rtlock(); + else +#endif + schedule(); spin_lock_irq(&dev->power.lock); } -- 2.34.0
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