linux-kernel - Re: [PATCH v5 05/24] x86/resctrl: Allow RMID allocation to be scoped by CLOSID

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Date:   Wed, 9 Aug 2023 15:33:24 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        <shameerali.kolothum.thodi@...wei.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        <carl@...amperecomputing.com>, <lcherian@...vell.com>,
        <bobo.shaobowang@...wei.com>, <tan.shaopeng@...itsu.com>,
        <xingxin.hx@...nanolis.org>, <baolin.wang@...ux.alibaba.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, <peternewman@...gle.com>,
        <dfustini@...libre.com>
Subject: Re: [PATCH v5 05/24] x86/resctrl: Allow RMID allocation to be scoped
 by CLOSID

Hi James,

On 7/28/2023 9:42 AM, James Morse wrote:
> -int alloc_rmid(void)
> +static struct rmid_entry *resctrl_find_free_rmid(u32 closid)
>  {
> -	struct rmid_entry *entry;
> -
> -	lockdep_assert_held(&rdtgroup_mutex);
> +	struct rmid_entry *itr;
> +	u32 itr_idx, cmp_idx;
>  
>  	if (list_empty(&rmid_free_lru))
> -		return rmid_limbo_count ? -EBUSY : -ENOSPC;
> +		return rmid_limbo_count ? ERR_PTR(-EBUSY) : ERR_PTR(-ENOSPC);
> +
> +	list_for_each_entry(itr, &rmid_free_lru, list) {
> +		/*
> +		 * Get the index of this free RMID, and the index it would need
> +		 * to be if it were used with this CLOSID.
> +		 * If the CLOSID is irrelevant on this architecture, these will
> +		 * always be the same meaning the compiler can reduce this loop
> +		 * to a single list_entry_first() call.
> +		 */
> +		itr_idx = resctrl_arch_rmid_idx_encode(itr->closid, itr->rmid);
> +		cmp_idx = resctrl_arch_rmid_idx_encode(closid, itr->rmid);
> +
> +		if (itr_idx == cmp_idx)
> +			return itr;
> +	}
> +
> +	return ERR_PTR(-ENOSPC);
> +}
> +
> +/*
> + * For MPAM the RMID value is not unique, and has to be considered with
> + * the CLOSID. The (CLOSID, RMID) pair is allocated on all domains, which
> + * allows all domains to be managed by a single limbo list.
> + * Each domain also has a rmid_busy_llc to reduce the work of the limbo handler.
> + */

I find the above comment to be contradicting - it talks about a single limbo list
yet there is "also" a limbo list/bitmask per domain. Should "single limbo list"
perhaps be "single free list"?

Reinette