Re: [PATCH] rcu/nocb: Add an option to ON/OFF an offloading from RT context

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, May 11, 2022 at 10:29:57AM -0400, Steven Rostedt wrote:
> On Wed, 11 May 2022 15:39:56 +0200
> Uladzislau Rezki <urezki@gmail.com> wrote:
> 
> > <snip>
> >  rcuop/6-54  [000] .N..  183.753018: rcu_invoke_callback:  rcu_preempt rhp=0xffffff88ffd440b0 func=__d_free.cfi_jt
> >  rcuop/6-54  [000] .N..  183.753020: rcu_invoke_callback:  rcu_preempt rhp=0xffffff892ffd8400 func=inode_free_by_rcu.cfi_jt
> >  rcuop/6-54  [000] .N..  183.753021: rcu_invoke_callback:  rcu_preempt rhp=0xffffff89327cd708 func=i_callback.cfi_jt
> >  ... 
> >  rcuop/6-54  [000] .N..  183.755941: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c5a968 func=i_callback.cfi_jt
> >  rcuop/6-54  [000] .N..  183.755942: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c4bd20 func=__d_free.cfi_jt
> >  rcuop/6-54  [000] dN..  183.755944: rcu_batch_end:        rcu_preempt CBs-invoked=2112 idle=>c<>c<>c<>c<
> >  rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      Start context switch
> >  rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      End context switch
> > <snip>
> > 
> > i spent some time in order to understand why the context was not switched,
> > even though the "rcuop" kthread was marked as TIF_NEED_RESCHED and an IPI
> > was sent to the CPU_0 to reschedule. The last "." in latency field shows
> > that a context has not disabled any preemption. So everything should be fine.
> > 
> > An explanation is that a local_bh_disable() modifies the current_thread_info()->preempt.count
> > so a task becomes non preemtable but the ftrace does not provide any signal about
> > it. So i was fooled for some time by my tracer logs.
> > 
> > Do you have any thoughts about it? Should it be solved or signaled
> > somehow that a task in fact is not preemtable if a counter > 0?
> 
> Hmm, it should show it in the first part (where the 'd' is). Is this a
> snapshot from the kernel or from trace-cmd?
> 
I do both and the behavior is the same. But the above one looks like a
kernel trace output, the trace-cmd snapshot looks differently. So you
mean "s" has to be there then?

<snip>
	entry->preempt_count		= pc & 0xff;
	entry->pid			= (tsk) ? tsk->pid : 0;
	entry->type			= type;
	entry->flags =
#ifdef CONFIG_TRACE_IRQFLAGS_SUPPORT
		(irqs_disabled_flags(flags) ? TRACE_FLAG_IRQS_OFF : 0) |
#else
		TRACE_FLAG_IRQS_NOSUPPORT |
#endif
		((pc & NMI_MASK    ) ? TRACE_FLAG_NMI     : 0) |
		((pc & HARDIRQ_MASK) ? TRACE_FLAG_HARDIRQ : 0) |
		((pc & SOFTIRQ_OFFSET) ? TRACE_FLAG_SOFTIRQ : 0) |
		(tif_need_resched() ? TRACE_FLAG_NEED_RESCHED : 0) |
		(test_preempt_need_resched() ? TRACE_FLAG_PREEMPT_RESCHED : 0);
<snip>

BTW, i am not the 5.10 kernel. I have not checked the latest kernel
and what ftrace reports under holding local_bh_disable().

--
Uladzislau Rezki