On Thu, Jul 10, 2025 at 2:59 PM John Stultz <jstu...@google.com> wrote: > On Thu, Jul 10, 2025 at 12:52 PM Ben Hutchings <b...@decadent.org.uk> wrote: > > There seems to be a longstanding issue with the combination of user- > > space watchdog timers (using CLOCK_MONOTONIC) and suspend-to-idle. This > > was reported at <https://bugzilla.kernel.org/show_bug.cgi?id=200595> and > > more recently at <https://bugs.debian.org/1107785>. > > > > During suspend-to-idle the system may be woken by interrupts and the > > CLOCK_MONOTONIC clock may tick while that happens, but no user-space > > tasks are allowed to run. So when the system finally exits suspend, a > > watchdog timer based on CLOCK_MONOTONIC may expire immediately without > > the task being supervised ever having an opportunity to pet the > > watchdog. > > > > This seems like a hard problem to solve! > > So I don't know much about suspend-to-idle, but I'm surprised it's not > suspending timekeeping! That definitely seems problematic.
Hrm. The docs here seem to call out that timekeeping is supposed to be suspended in s2idle: https://docs.kernel.org/admin-guide/pm/sleep-states.html#suspend-to-idle Looking at enter_s2idle_proper(): https://elixir.bootlin.com/linux/v6.16-rc5/source/drivers/cpuidle/cpuidle.c#L154 We call tick_freeze(): https://elixir.bootlin.com/linux/v6.16-rc5/source/kernel/time/tick-common.c#L524 Which calls timekeeping_suspend() when the last cpu's tick has been frozen. So it seems like the problem might be somehow all the cpus maybe aren't entering s2idle, causing time to keep running? thanks -john
