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From: Thomas Gleixner <tglx@linutronix.de>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Paul McKenney <paulmck@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [patch V2 2/9] irqtime: Make accounting correct on RT
Date: Mon, 07 Dec 2020 15:44:28 +0100	[thread overview]
Message-ID: <874kkx7k1f.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20201207132702.GC122233@lothringen>

On Mon, Dec 07 2020 at 14:27, Frederic Weisbecker wrote:
> 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@linutronix.de>
>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>
> Reviewed-by: Frederic Weisbecker <frederic@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?

I ignored sh so far and RCU has some quirks for RT vs. softirqs.

> 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)

Bah, I somehow lost the cure for that while rebasing.

Thanks,

        tglx

  reply	other threads:[~2020-12-07 14:45 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-04 17:01 [patch V2 0/9] softirq: Make it RT aware Thomas Gleixner
2020-12-04 17:01 ` [patch V2 1/9] softirq: Add RT specific softirq accounting Thomas Gleixner
2020-12-07 13:06   ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 2/9] irqtime: Make accounting correct on RT Thomas Gleixner
2020-12-07  0:23   ` Frederic Weisbecker
2020-12-07  0:57     ` Thomas Gleixner
2020-12-07  1:14       ` Frederic Weisbecker
2020-12-07 13:27   ` Frederic Weisbecker
2020-12-07 14:44     ` Thomas Gleixner [this message]
2020-12-04 17:01 ` [patch V2 3/9] softirq: Move various protections into inline helpers Thomas Gleixner
2020-12-07 13:37   ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 4/9] softirq: Make softirq control and processing RT aware Thomas Gleixner
2020-12-07 14:16   ` Frederic Weisbecker
2020-12-07 15:08     ` Thomas Gleixner
2020-12-08  0:08   ` Frederic Weisbecker
2020-12-09 10:11   ` Peter Zijlstra
2020-12-09 12:36     ` Thomas Gleixner
2020-12-09 12:42       ` Peter Zijlstra
2020-12-09 13:30         ` Thomas Gleixner
2020-12-09 10:34   ` Peter Zijlstra
2020-12-04 17:01 ` [patch V2 5/9] tick/sched: Prevent false positive softirq pending warnings on RT Thomas Gleixner
2020-12-08 12:23   ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 6/9] rcu: Prevent false positive softirq warning " Thomas Gleixner
2020-12-04 17:59   ` Paul E. McKenney
2020-12-04 17:01 ` [patch V2 7/9] softirq: Replace barrier() with cpu_relax() in tasklet_unlock_wait() Thomas Gleixner
2020-12-07 11:39   ` Peter Zijlstra
2020-12-07 15:21     ` Thomas Gleixner
2020-12-04 17:01 ` [patch V2 8/9] tasklets: Use static inlines for stub implementations Thomas Gleixner
2020-12-04 17:02 ` [patch V2 9/9] tasklets: Prevent kill/unlock_wait deadlock on RT Thomas Gleixner
2020-12-07 11:47   ` Peter Zijlstra
2020-12-07 14:00     ` Sebastian Andrzej Siewior
2020-12-07 14:27       ` Peter Zijlstra
2020-12-07 17:55         ` Thomas Gleixner
2020-12-07 15:22       ` Thomas Gleixner
2020-12-07 15:39         ` Sebastian Andrzej Siewior
2020-12-07 17:49           ` Thomas Gleixner
2020-12-07 17:50             ` Thomas Gleixner
2020-12-06 10:05 ` [patch V2 0/9] softirq: Make it RT aware Sebastian Andrzej Siewior

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