Re: [PATCH] rcu: Fix missing TICK_DEP_MASK_RCU_EXP dependency check

[Joel Fernandes]

> On Jan 7, 2023, at 9:48 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> 
> 
>>> On Jan 7, 2023, at 5:11 PM, Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>>> 
>>> On Fri, Jan 06, 2023 at 07:01:28PM -0500, Joel Fernandes wrote:
>>> (lost html content)
> 
> My problem is the iPhone wises up when I put a web link in an email. I want to look into smtp relays but then if I spent time on fixing that, I might not get time to learn from emails like these... 
> 
>> I can't find a place where the exp grace period sends an IPI to
>> CPUs slow to report a QS. But anyway you really need the tick to poll
>> periodically on the CPU to chase a quiescent state.
> 
> Ok.
> 
>> Now arguably it's probably only useful when CONFIG_PREEMPT_COUNT=y
>> and rcu_exp_handler() has interrupted a preempt-disabled or bh-disabled
>> section. Although rcu_exp_handler() sets TIF_RESCHED, which is handled
>> by preempt_enable() and local_bh_enable() when CONFIG_PREEMPT=y.
>> So probably it's only useful when CONFIG_PREEMPT_COUNT=y and CONFIG_PREEMPT=n
>> (and there is also PREEMPT_DYNAMIC to consider).
> 
> Makes sense. I think I was missing this use case and was going by the general design of exp grace periods.  I was incorrectly assuming the IPIs were being sent repeatedly for hold out CPUs, which is not the case I think. But that would another way to fix it?
> 
> But yeah I get your point, the first set of IPIs missed it, so we need the rescue-tick for long non-rcu_read_lock() implicit critical sections.. 
> 
>> If CONFIG_PREEMPT_COUNT=n, the tick can only report idle and user
>> as QS, but those are already reported explicitly on ct_kernel_exit() ->
>> rcu_preempt_deferred_qs().
> 
> Oh hmm, because that function is a NOOP for PREEMPT_COUNT=y and PREEMPT=n and will not report the deferred QS?  Maybe it should then. However I think the tick is still useful if after the preempt disabled section, will still did not exit the kernel.

I think meant I here, an atomic section (like bh or Irq disabled). There is no such thing as disabling preemption for CONFIG_PREEMPT=n. Or maybe I am confused again.  This RCU thing…

Thanks.


> 
> We ought to start another Google doc on all of this if we have not yet…
> 
> Thanks!
> 
> - Joel
> 
>> 
>> Thanks.
>> 
>>