Re: [patch V2 2/9] irqtime: Make accounting correct on RT

[Frederic Weisbecker]

On Fri, Dec 04, 2020 at 06:01:53PM +0100, Thomas Gleixner wrote:
> vtime_account_irq and irqtime_account_irq() base checks on preempt_count()
> which fails on RT because preempt_count() does not contain the softirq
> accounting which is seperate on RT.
> 
> These checks do not need the full preempt count as they only operate on the
> hard and softirq sections.
> 
> Use irq_count() instead which provides the correct value on both RT and non
> RT kernels. The compiler is clever enough to fold the masking for !RT:
> 
>        99b:   65 8b 05 00 00 00 00    mov    %gs:0x0(%rip),%eax
>  -     9a2:   25 ff ff ff 7f          and    $0x7fffffff,%eax
>  +     9a2:   25 00 ff ff 00          and    $0xffff00,%eax
> 
> Reported-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
> Signed-off-by: Thomas Gleixner <t...@linutronix.de>

Reviewed-by: Frederic Weisbecker <frede...@kernel.org>

Also I'm seeing a few other explicit SOFTIRQ_MASK checks on top
of preempt_count(), especially on RCU:

   $ git grep SOFTIRQ_MASK
   arch/sh/kernel/irq.c:                   (irqctx->tinfo.preempt_count & 
~SOFTIRQ_MASK) |
   arch/sh/kernel/irq.c:                   (curctx->tinfo.preempt_count & 
SOFTIRQ_MASK);
   kernel/rcu/rcutorture.c:                if (preempt_count() & (SOFTIRQ_MASK 
| HARDIRQ_MASK))
   kernel/rcu/tree_exp.h:          if (!(preempt_count() & (PREEMPT_MASK | 
SOFTIRQ_MASK)) ||
   kernel/rcu/tree_plugin.h:                       !!(preempt_count() & 
(PREEMPT_MASK | SOFTIRQ_MASK));
   kernel/rcu/tree_plugin.h:           (preempt_count() & (PREEMPT_MASK | 
SOFTIRQ_MASK))) {

I guess some RT RCU config take care of that?

Also tracing:

     kernel/trace/ring_buffer.c:     if (!(pc & (NMI_MASK | HARDIRQ_MASK | 
SOFTIRQ_OFFSET)))
     kernel/trace/trace.c:           ((pc & SOFTIRQ_OFFSET) ? 
TRACE_FLAG_SOFTIRQ : 0)

Thanks.