linux-kernel - Re: [PATCH v4 04/24] x86/resctrl: Move rmid allocation out of mkdir_rdt

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Date:   Thu, 15 Jun 2023 15:05:03 -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 v4 04/24] x86/resctrl: Move rmid allocation out of
 mkdir_rdt_prepare()

Hi James,

On 5/25/2023 11:01 AM, James Morse wrote:
> RMID are allocated for each monitor or control group directory, because
> each of these needs its own RMID. For control groups,
> rdtgroup_mkdir_ctrl_mon() later goes on to allocate the CLOSID.
> 
> MPAM's equivalent of RMID is not an independent number, so can't be
> allocated until the CLOSID is known. An RMID allocation for one CLOSID
> may fail, whereas another may succeed depending on how many monitor
> groups a control group has.
> 
> The RMID allocation needs to move to be after the CLOSID has been
> allocated.

This third paragraph seems redundant.

> 
> Move the RMID allocation out of mkdir_rdt_prepare() to occur in its caller,
> after the mkdir_rdt_prepare() call. This allows the RMID allocator to
> know the CLOSID.
> 
> Tested-by: Shaopeng Tan <tan.shaopeng@...itsu.com>
> Signed-off-by: James Morse <james.morse@....com>
> ---
> Changes since v2:
>  * Moved kernfs_activate() later to preserve atomicity of files being visible
> ---
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 35 +++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 05774b185eec..8346a8f2ff9f 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -3176,6 +3176,12 @@ static int mkdir_rdt_prepare_rmid_alloc(struct rdtgroup *rdtgrp)
>  	return 0;
>  }
>  
> +static void mkdir_rdt_prepare_rmid_free(struct rdtgroup *rgrp)
> +{
> +	if (rdt_mon_capable)
> +		free_rmid(rgrp->closid, rgrp->mon.rmid);
> +}
> +
>  static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
>  			     const char *name, umode_t mode,
>  			     enum rdt_group_type rtype, struct rdtgroup **r)
> @@ -3241,12 +3247,6 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
>  		goto out_destroy;
>  	}
>  
> -	ret = mkdir_rdt_prepare_rmid_alloc(rdtgrp);
> -	if (ret)
> -		goto out_destroy;
> -
> -	kernfs_activate(kn);
> -
>  	/*
>  	 * The caller unlocks the parent_kn upon success.
>  	 */
> @@ -3265,7 +3265,6 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
>  static void mkdir_rdt_prepare_clean(struct rdtgroup *rgrp)
>  {
>  	kernfs_remove(rgrp->kn);
> -	free_rmid(rgrp->closid, rgrp->mon.rmid);
>  	rdtgroup_remove(rgrp);
>  }
>  
> @@ -3287,12 +3286,21 @@ static int rdtgroup_mkdir_mon(struct kernfs_node *parent_kn,
>  	prgrp = rdtgrp->mon.parent;
>  	rdtgrp->closid = prgrp->closid;
>  
> +	ret = mkdir_rdt_prepare_rmid_alloc(rdtgrp);
> +	if (ret) {
> +		mkdir_rdt_prepare_clean(rdtgrp);
> +		goto out_unlock;
> +	}
> +
> +	kernfs_activate(rdtgrp->kn);
> +
>  	/*
>  	 * Add the rdtgrp to the list of rdtgrps the parent
>  	 * ctrl_mon group has to track.
>  	 */
>  	list_add_tail(&rdtgrp->mon.crdtgrp_list, &prgrp->mon.crdtgrp_list);
>  
> +out_unlock:
>  	rdtgroup_kn_unlock(parent_kn);
>  	return ret;
>  }
> @@ -3323,10 +3331,17 @@ static int rdtgroup_mkdir_ctrl_mon(struct kernfs_node *parent_kn,
>  	ret = 0;
>  
>  	rdtgrp->closid = closid;
> -	ret = rdtgroup_init_alloc(rdtgrp);
> -	if (ret < 0)
> +
> +	ret = mkdir_rdt_prepare_rmid_alloc(rdtgrp);
> +	if (ret)
>  		goto out_id_free;
>  
> +	kernfs_activate(rdtgrp->kn);
> +

I can see why you added this kernfs_activate() here and it certainly matches
the resctrl pattern of its use. It does seem as though resctrl often calls
kernfs_activate() as it adds files instead of a single call when all files
are ready. This is not an issue with this patch but I do think this is an
area that deserves a closer look before any more additions.

Reinette