Messages in this thread | | | Date | Fri, 1 Oct 2021 12:32:38 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/5] irq_work: Handle some irq_work in SOFTIRQ on PREEMPT_RT |
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On Thu, Sep 30, 2021 at 06:38:58PM +0200, Sebastian Andrzej Siewior wrote: > On 2021-09-30 16:39:51 [+0200], Peter Zijlstra wrote:
> > Runing them all at the same prio still sucks (much like the single > > net-RX thing), but at least a kthread is somewhat controllable. > > I could replace the softirq processing with a per-CPU thread. This > should work. But I would have to (still) delay the wake-up of the thread > to the timer tick - or - we try the wake from the irqwork-self-IPI.
That, just wake the thread from the hardirq.
> I > just don't know how many will arrive back-to-back. The RCU callback > (rcu_preempt_deferred_qs_handler()) pops up a lot. By my naive guesswork > I would say that the irqwork is not needed since preempt-enable > somewhere should do needed scheduling. But then commit > 0864f057b050b ("rcu: Use irq_work to get scheduler's attention in clean context") > > claims it is not enough.
Oh gawd, that was something really nasty. I'm not sure that Changelog captures all (at least I'm not sure I fully understand the problem again reading it).
But basically that thing wants to reschedule, but suffers the same problem as:
preempt_disable();
<TIF_NEED_RESCHED gets set>
local_irq_disable(); preempt_enable(); // cannea schedule because IRQs are disabled local_irq_enable(); // lost a reschedule
Yes, that will _eventually_ reschedule, but violates the PREEMPT rules because there is an unspecified amount of time until it does actually do reschedule.
So what RCU does there is basically trigger a self-IPI, which guarantees that we reschedule after IRQs are finally enabled, which then triggers a resched.
I see no problem marking that particular irq_work as HARD tho, it really doesn't do anything (other than tell RCU the GP is no longer blocked) and triggering the return-from-interrupt path.
There's also a fun comment in perf_lock_task_context() that possibly predates the above RCU fix.
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