On 9/7/2023 2:46 AM, Peter Zijlstra wrote: > On Mon, Sep 04, 2023 at 08:59:03PM -0700, Elliot Berman wrote: >> >> >> On 9/4/2023 2:23 PM, Peter Zijlstra wrote: >>> On Wed, Aug 30, 2023 at 10:42:39AM -0700, Elliot Berman wrote: >>> >>>> Avoid the spurious wakeups by saving the state of TASK_FREEZABLE tasks. >>>> If the task was running before entering TASK_FROZEN state >>>> (__refrigerator()) or if the task received a wake up for the saved >>>> state, then the task is woken on thaw. saved_state from PREEMPT_RT locks >>>> can be re-used because freezer would not stomp on the rtlock wait flow: >>>> TASK_RTLOCK_WAIT isn't considered freezable. >>> >>> You don't actually assert that anywhere I think, so the moment someone >>> makes that happen you crash and burn. >>> >> >> I can certainly add an assertion on the freezer side. > > I think the assertion we have in ttwu_state_match() might be sufficient. > That assertion checks that you only try to wake up with only TASK_RTLOCK_WAIT and no other bits. I think it is probably good to also have assertions that check that TASK_RTLOCK_WAIT and TASK_FROZEN are exclusive bits and. I can add these assertions (a separate patch?), but I think those checks would impact the hot path to do the extra tests. >>> Also: >>> >>>> -#ifdef CONFIG_PREEMPT_RT >>>> +#if IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_FREEZER) >>> >>> That makes wakeup more horrible for everyone :/ >> >> I don't think the hot wakeup path is significantly impacted because the >> added checks come after the hot path is already not taken. > > Perhaps we should start off by doing the below, instead of making it > more complicated instead. I suppose you're right about the overhead, but > run a hackbench just to make sure or something. > I ran perf bench sched message -g 40 -l 40 with the v3 patch [1]. After 60 iterations each, I don't see a significant difference on my arm64 platform: both samples ~normal and ~eq variance w/t-test p-value: 0.79. We also ran typical high level benchmarks for our SoCs (antutu, geekbench, et. al) and didn't see any regressions there. [1]: https://lore.kernel.org/all/20230908-avoid-spurious-freezer-wakeups-v3-1-d49821fda04d@xxxxxxxxxxx/ Thanks, Elliot
