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().
> >
> Sorry, the was a typo. I am checking 5.10 kernel and the trace was taken
> on that kernel.
> 
OK. It was added on the latest kernel:

root@pc638:/home/urezki# cat /sys/kernel/debug/tracing/trace_pipe
  vmalloc_test/0-1296    [062] b....    18.157470: 0xffffffffc044e5dc: -> in the local_bh_disable()

root@pc638:/home/urezki# cat /sys/kernel/debug/tracing/trace
# tracer: nop
#
# entries-in-buffer/entries-written: 0/0   #P:64
#
#                                _-----=> irqs-off/BH-disabled
#                               / _----=> need-resched
#                              | / _---=> hardirq/softirq
#                              || / _--=> preempt-depth
#                              ||| / _-=> migrate-disable
#                              |||| /     delay
#           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
#              | |         |   |||||     |         |
root@pc638:/home/urezki# uname -a
Linux pc638 5.17.0-rc2-next-20220201 #63 SMP PREEMPT Tue May 10 20:39:08 CEST 2022 x86_64 GNU/Linux
root@pc638:/home/urezki#

so it shows *bh* disabled sections.

--
Uladzislau Rezki