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

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Date:   Mon, 7 Dec 2020 14:27:02 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch V2 2/9] irqtime: Make accounting correct on RT

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 <bigeasy@...utronix.de>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: Frederic Weisbecker <frederic@...nel.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.