linux-kernel - Re: [PREEMPT_RT PATCH 2/3] i915: convert all irq

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

On 2019-08-19 19:33:18 [-0500], Clark Williams wrote:
> From: Clark Williams <williams@...hat.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