linux-kernel - Re: [PATCH v2 27/29] arm_mpam: Add helper to reset saved mbwu state

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Message-ID: <20250912143302.00004d0e@huawei.com>
Date: Fri, 12 Sep 2025 14:33:02 +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>,
	<linux-acpi@...r.kernel.org>, D Scott Phillips OS
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 Deacon" <will@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 27/29] arm_mpam: Add helper to reset saved mbwu state

On Wed, 10 Sep 2025 20:43:07 +0000
James Morse <james.morse@....com> wrote:

> resctrl expects to reset the bandwidth counters when the filesystem
> is mounted.
> 
> To allow this, add a helper that clears the saved mbwu state. Instead
> of cross calling to each CPU that can access the component MSC to
> write to the counter, set a flag that causes it to be zero'd on the
> the next read. This is easily done by forcing a configuration update.
> 
> Signed-off-by: James Morse <james.morse@....com>
Minor comments inline.

Jonathan

> @@ -1245,6 +1257,37 @@ int mpam_msmon_read(struct mpam_component *comp, struct mon_cfg *ctx,
>  	return err;
>  }
>  
> +void mpam_msmon_reset_mbwu(struct mpam_component *comp, struct mon_cfg *ctx)
> +{
> +	int idx;
> +	struct mpam_msc *msc;
> +	struct mpam_vmsc *vmsc;
> +	struct mpam_msc_ris *ris;
> +
> +	if (!mpam_is_enabled())
> +		return;
> +
> +	idx = srcu_read_lock(&mpam_srcu);

Maybe guard() though it doesn't add that much here.

> +	list_for_each_entry_rcu(vmsc, &comp->vmsc, comp_list) {

Reason not to use _srcu variants?

> +		if (!mpam_has_feature(mpam_feat_msmon_mbwu, &vmsc->props))
> +			continue;
> +
> +		msc = vmsc->msc;
> +		list_for_each_entry_rcu(ris, &vmsc->ris, vmsc_list) {
> +			if (!mpam_has_feature(mpam_feat_msmon_mbwu, &ris->props))
> +				continue;
> +
> +			if (WARN_ON_ONCE(!mpam_mon_sel_lock(msc)))
> +				continue;
> +
> +			ris->mbwu_state[ctx->mon].correction = 0;
> +			ris->mbwu_state[ctx->mon].reset_on_next_read = true;
> +			mpam_mon_sel_unlock(msc);
> +		}
> +	}
> +	srcu_read_unlock(&mpam_srcu, idx);
> +}
> +
>  static void mpam_reset_msc_bitmap(struct mpam_msc *msc, u16 reg, u16 wd)
>  {
>  	u32 num_words, msb;
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index c190826dfbda..7cbcafe8294a 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -223,10 +223,12 @@ struct mon_cfg {
>  
>  /*
>   * Changes to enabled and cfg are protected by the msc->lock.
> - * Changes to prev_val and correction are protected by the msc's mon_sel_lock.
> + * Changes to reset_on_next_read, prev_val and correction are protected by the
> + * msc's mon_sel_lock.
Getting close to the point where a list of one per line would reduce churn.
If you anticipate adding more to this in future I'd definitely consider it.
e.g.
 * msc's mon_sel_lcok protects:
 * - reset_on_next_read
 * - prev_val
 * - correction
 */
>   */
>  struct msmon_mbwu_state {
>  	bool		enabled;
> +	bool		reset_on_next_read;
>  	struct mon_cfg	cfg;
>  
>  	/* The value last read from the hardware. Used to detect overflow. */
> @@ -393,6 +395,7 @@ int mpam_apply_config(struct mpam_component *comp, u16 partid,
>  
>  int mpam_msmon_read(struct mpam_component *comp, struct mon_cfg *ctx,
>  		    enum mpam_device_features, u64 *val);
> +void mpam_msmon_reset_mbwu(struct mpam_component *comp, struct mon_cfg *ctx);
>  
>  int mpam_get_cpumask_from_cache_id(unsigned long cache_id, u32 cache_level,
>  				   cpumask_t *affinity);