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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Clark Williams <clark.williams@gmail.com>
Cc: bigeasy@linutronix.com, tglx@linutronix.com,
	linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PREEMPT_RT PATCH 2/3] i915: convert all irq_locks spinlocks to raw spinlocks
Date: Tue, 3 Sep 2019 10:03:35 +0200
Message-ID: <20190903080335.pe45dmgmjvdvbyd4@linutronix.de> (raw)
In-Reply-To: <20190820003319.24135-3-clark.williams@gmail.com>

On 2019-08-19 19:33:18 [-0500], Clark Williams wrote:
> From: Clark Williams <williams@redhat.com>
> 
> The following structures contain a member named 'irq_lock'.
> These three locks are of type spinlock_t and are used in
> multiple contexts including atomic:
> 
>     struct drm_i915_private
>     struct intel_breadcrumbs
>     strict intel_guc
> 
> Convert them all to be raw_spinlock_t so that lockdep and the lock
> debugging code will be happy.

What is your motivation to make the lock raw?
I did the following:

 void intel_engine_signal_breadcrumbs(struct intel_engine_cs *engine)
 {
-       local_irq_disable();
-       intel_engine_breadcrumbs_irq(engine);
-       local_irq_enable();
+       if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
+               intel_engine_breadcrumbs_irq(engine);
+       } else {
+               local_irq_disable();
+               intel_engine_breadcrumbs_irq(engine);
+               local_irq_enable();
+       }
 }

and lockdep was quiet (+ ignoring/patching the lockdep-irq-off-asserts).
The local_irq_disable() is here (my interpretation of the situation)
because that function is called from process context while the remaining
callers invoke intel_engine_breadcrumbs_irq() from the interrupt
handler and it acquires irq_lock via a plain spin_lock().  That
local_irq_disable() would be required if everyone did a _irqsave().

I tried to check how much worse the latency gets here but I didn't see
anything in a brief test. What I saw however is that switching to
fullscreen while playing a video gives me ~0.5 to ~2ms latency. This is
has nothing to do with this change, I have to dig deeper… It might be
one of the preempt_disable() section I just noticed.
I would prefer to keep the lock non-raw unless there is actual need for
it.

Sebastian

  reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-20  0:33 [PREEMPT_RT PATCH 0/3] i915 fixups for lockdep/lockdebugging Clark Williams
2019-08-20  0:33 ` [PREEMPT_RT PATCH 1/3] i915: do not call lockdep_assert_irqs_disabled() on PREEMPT_RT Clark Williams
2019-09-25 16:47   ` Sebastian Andrzej Siewior
2019-08-20  0:33 ` [PREEMPT_RT PATCH 2/3] i915: convert all irq_locks spinlocks to raw spinlocks Clark Williams
2019-09-03  8:03   ` Sebastian Andrzej Siewior [this message]
2019-09-03 13:09     ` Sebastian Andrzej Siewior
2019-09-16 21:21     ` Sean V Kelley
2019-09-27 12:29       ` Sebastian Andrzej Siewior
     [not found]         ` <CAPAFJkofsYgSOxzM4JDkOV3Ra0C-AEpU2eZJjR9F9e_qv-6=rg@mail.gmail.com>
2019-09-27 15:18           ` Sean V Kelley
2019-08-20  0:33 ` [PREEMPT_RT PATCH 3/3] i915: convert uncore lock to raw spinlock Clark Williams

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