linux-kernel - Re: [PATCH v4 19/21] x86/resctrl: Rename and change the units of resctrl_cqm

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Message-ID: <7991d014-6190-de87-9310-9b5109193519@intel.com>
Date:   Tue, 17 May 2022 14:23:38 -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>,
        <lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
        <tan.shaopeng@...itsu.com>, Jamie Iles <quic_jiles@...cinc.com>,
        Cristian Marussi <cristian.marussi@....com>,
        "Xin Hao" <xhao@...ux.alibaba.com>, <xingxin.hx@...nanolis.org>,
        <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v4 19/21] x86/resctrl: Rename and change the units of
 resctrl_cqm_threshold

Hi James,

On 4/12/2022 5:44 AM, James Morse wrote:
> resctrl_cqm_threshold is stored in a hardware specific chunk size,
> but exposed to user-space as bytes.
> 
> This means the filesystem parts of resctrl need to know how the hardware
> counts, to convert the user provided byte value to chunks. The interface
> between the architecture's resctrl code and the filesystem ought to
> treat everything as bytes.
> 
> Change the unit of resctrl_cqm_threshold to bytes. resctrl_arch_rmid_read()
> still returns its value in chunks, so this needs converting to bytes.
> As all the users have been touched, rename the variable to
> resctrl_rmid_realloc_threshold, which describes what the value is for.
> 
> Neither r->num_rmid nor hw_res->mon_scale are guaranteed to be a power
> of 2, so the existing code introduces a rounding error from resctrl's
> theoretical fraction of the cache usage. This behaviour is kept as it
> ensures the user visible value matches the value read from hardware
> when the rmid will be reallocated.

The goal mentioned above does not seem to match the changes made to
the code parsing the user provided value ...

... 


> @@ -1066,8 +1062,7 @@ static ssize_t max_threshold_occ_write(struct kernfs_open_file *of,
>  	if (bytes > (boot_cpu_data.x86_cache_size * 1024))
>  		return -EINVAL;
>  
> -	hw_res = resctrl_to_arch_res(of->kn->parent->priv);
> -	resctrl_cqm_threshold = bytes / hw_res->mon_scale;
> +	resctrl_rmid_realloc_threshold = bytes;
>  
>  	return nbytes;
>  }

... originally resctrl_cqm_threshold would take user provided value and adjust it
to the mon_scale but in this change the bytes provided by user space is kept as-is
and would thus no longer match what can be read from hardware.

Reinette