linux-kernel - Re: [RFC PATCH 24/38] arm_mpam: resctrl: Allow resctrl to allocate monitors

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Message-ID: <20251218134953.000064b1@huawei.com>
Date: Thu, 18 Dec 2025 13:49:53 +0000
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>, D
 Scott Phillips OS <scott@...amperecomputing.com>,
	<carl@...amperecomputing.com>, <lcherian@...vell.com>,
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	<david@...nel.org>, Dave Martin <dave.martin@....com>, Koba Ko
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	<gshan@...hat.com>, Ben Horgan <ben.horgan@....com>, <rohit.mathew@....com>,
	<reinette.chatre@...el.com>, Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 24/38] arm_mpam: resctrl: Allow resctrl to allocate
 monitors

On Fri, 5 Dec 2025 21:58:47 +0000
James Morse <james.morse@....com> wrote:

> When resctrl wants to read a domain's 'QOS_L3_OCCUP', it needs
> to allocate a monitor on the corresponding resource. Monitors are
> allocated by class instead of component.
> 
> MBM monitors are much more complicated, if there are enough monitors,
> they will be pre-allocated and free-running. If ABMC is in use instead
> then 'some' are pre-allocated in a different way, and need assigning.
> 
> Add helpers to allocate a CSU monitor. These helper return an out
> of range value for MBM counters.
> 
> Allocating a montitor context is expected to block until hardware
> resources become available. This only makes sense for QOS_L3_OCCUP
> as unallocated MBM counters are losing data.
> 
> Signed-off-by: James Morse <james.morse@....com>
> ---
> Since ABMC got merged it may be possible to remove the monitor alloc
> call for MBM counters from resctrl as this work is now done by ABMC's
> assign call.
> ---
>  drivers/resctrl/mpam_internal.h | 14 ++++++-
>  drivers/resctrl/mpam_resctrl.c  | 68 +++++++++++++++++++++++++++++++++
>  include/linux/arm_mpam.h        |  4 ++
>  3 files changed, 85 insertions(+), 1 deletion(-)
> 

> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 22ad5dd3c383..a2b238d47117 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c

> +
> +void resctrl_arch_mon_ctx_free(struct rdt_resource *r,
> +			       enum resctrl_event_id evtid, void *arch_mon_ctx)
> +{
> +	u32 mon_idx = *(u32 *)arch_mon_ctx;
> +
> +	kfree(arch_mon_ctx);
> +	arch_mon_ctx = NULL;

Why is this useful?  Updating the local pointer copy which is not
used again. Maybe I need more coffee.

> +
> +	resctrl_arch_mon_ctx_free_no_wait(evtid, mon_idx);
> +}