linux-kernel - Re: [PATCH v3 18/29] arm_mpam: Register and enable IRQs

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Message-ID: <20251024190323.0000328d@huawei.com>
Date: Fri, 24 Oct 2025 19:03:23 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: James Morse <james.morse@....com>
CC: <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
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Subject: Re: [PATCH v3 18/29] arm_mpam: Register and enable IRQs

On Fri, 17 Oct 2025 18:56:34 +0000
James Morse <james.morse@....com> wrote:

> Register and enable error IRQs. All the MPAM error interrupts indicate a
> software bug, e.g. out of range partid. If the error interrupt is ever
> signalled, attempt to disable MPAM.
> 
> Only the irq handler accesses the MPAMF_ESR register, so no locking is
> needed. The work to disable MPAM after an error needs to happen at process
> context as it takes mutex. It also unregisters the interrupts, meaning
> it can't be done from the threaded part of a threaded interrupt.
> Instead, mpam_disable() gets scheduled.
> 
> Enabling the IRQs in the MSC may involve cross calling to a CPU that
> can access the MSC.
> 
> Once the IRQ is requested, the mpam_disable() path can be called
> asynchronously, which will walk structures sized by max_partid. Ensure
> this size is fixed before the interrupt is requested.
> 
> CC: Rohit Mathew <rohit.mathew@....com>
> Tested-by: Rohit Mathew <rohit.mathew@....com>
> Tested-by: Fenghua Yu <fenghuay@...dia.com>
> Signed-off-by: James Morse <james.morse@....com>
One trivial thing inline to reduce the patch churn a tiny bit.
May not be worth the hassle.

Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>

> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 545482e112b7..f18a22f825a0 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -14,6 +14,9 @@



>  static void mpam_enable_once(void)
>  {
> -	mutex_lock(&mpam_list_lock);
> -	mpam_enable_merge_features(&mpam_classes);
> -	mutex_unlock(&mpam_list_lock);

Can you shift this later in the earlier patch (ordering clearly doesn't matter)
so as to reduce churn here?

> +	int err;
>  
>  	/*
>  	 * Once the cpuhp callbacks have been changed, mpam_partid_max can no
> @@ -1318,6 +1572,27 @@ static void mpam_enable_once(void)
>  	partid_max_published = true;
>  	spin_unlock(&partid_max_lock);
>  
> +	/*
> +	 * If all the MSC have been probed, enabling the IRQs happens next.
> +	 * That involves cross-calling to a CPU that can reach the MSC, and
> +	 * the locks must be taken in this order:
> +	 */
> +	cpus_read_lock();
> +	mutex_lock(&mpam_list_lock);
> +	mpam_enable_merge_features(&mpam_classes);
> +
> +	err = mpam_register_irqs();
> +
> +	mutex_unlock(&mpam_list_lock);
> +	cpus_read_unlock();
> +
> +	if (err) {
> +		pr_warn("Failed to register irqs: %d\n", err);
> +		mpam_disable_reason = "Failed to enable.";
> +		schedule_work(&mpam_broken_work);
> +		return;
> +	}
> +
>  	mpam_register_cpuhp_callbacks(mpam_cpu_online, mpam_cpu_offline,
>  				      "mpam:online");
>  
> @@ -1385,6 +1660,8 @@ void mpam_disable(struct work_struct *ignored)
>  	}
>  	mutex_unlock(&mpam_cpuhp_state_lock);
>  
> +	mpam_unregister_irqs();
> +
>  	idx = srcu_read_lock(&mpam_srcu);
>  	list_for_each_entry_srcu(class, &mpam_classes, classes_list,
>  				 srcu_read_lock_held(&mpam_srcu))