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[release-branch.go1.18] runtime: clear timerModifiedEarliest when las…
…t timer is deleted timerModifiedEarliest contains the lowest possible expiration for a modified earlier timer, which may be earlier than timer0When because we haven't yet updated the heap. Note "may", as the modified earlier timer that set timerModifiedEarliest may have since been modified later or deleted. We can clear timerModifiedEarliest when the last timer is deleted because by definition there must not be any modified earlier timers. Why does this matter? checkTimersNoP claims that there is work to do if timerModifiedEarliest has passed, causing findRunnable to loop back around to checkTimers. But the code to clean up timerModifiedEarliest in checkTimers (i.e., the call to adjusttimers) is conditional behind a check that len(pp.timers) > 0. Without clearing timerModifiedEarliest, a spinning M that would otherwise go to sleep will busy loop in findRunnable until some other work is available. Note that changing the condition on the call to adjusttimers would also be a valid fix. I took this approach because it feels a bit cleaner to clean up timerModifiedEarliest as soon as it is known to be irrelevant. For golang#51654. Fixes golang#53847. Change-Id: I3f3787c67781cac7ce87939c5706cef8db927dd5 Reviewed-on: https://go-review.googlesource.com/c/go/+/417434 Auto-Submit: Michael Pratt <mpratt@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Run-TryBot: Michael Pratt <mpratt@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> (cherry picked from commit c006b7a)
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