Messages in this thread | | | Date | Wed, 20 Mar 2024 17:15:48 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 2/2] timers: Fix removed self-IPI on global timer's enqueue in nohz_full |
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Le Wed, Mar 20, 2024 at 04:14:24AM -0700, Paul E. McKenney a écrit : > On Tue, Mar 19, 2024 at 02:18:00AM -0700, Paul E. McKenney wrote: > > On Tue, Mar 19, 2024 at 12:07:29AM +0100, Frederic Weisbecker wrote: > > > While running in nohz_full mode, a task may enqueue a timer while the > > > tick is stopped. However the only places where the timer wheel, > > > alongside the timer migration machinery's decision, may reprogram the > > > next event accordingly with that new timer's expiry are the idle loop or > > > any IRQ tail. > > > > > > However neither the idle task nor an interrupt may run on the CPU if it > > > resumes busy work in userspace for a long while in full dynticks mode. > > > > > > To solve this, the timer enqueue path raises a self-IPI that will > > > re-evaluate the timer wheel on its IRQ tail. This asynchronous solution > > > avoids potential locking inversion. > > > > > > This is supposed to happen both for local and global timers but commit: > > > > > > b2cf7507e186 ("timers: Always queue timers on the local CPU") > > > > > > broke the global timers case with removing the ->is_idle field handling > > > for the global base. As a result, global timers enqueue may go unnoticed > > > in nohz_full. > > > > > > Fix this with restoring the idle tracking of the global timer's base, > > > allowing self-IPIs again on enqueue time. > > > > Testing with the previous patch (1/2 in this series) reduced the number of > > problems by about an order of magnitude, down to two sched_tick_remote() > > instances and one enqueue_hrtimer() instance, very good! > > > > I have kicked off a test including this patch. Here is hoping! ;-) > > And 22*100 hours of TREE07 got me one run with a sched_tick_remote() > complaint and another run with a starved RCU grace-period kthread. > So this is definitely getting more reliable, but still a little ways > to go.
Right, there is clearly something else. Investigation continues...
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