linux-kernel - Re: [PATCH 7/8] x86/intel_rdt/mba: Add schemata file support for MBA

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Date:   Mon, 16 Jan 2017 17:07:17 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>
cc:     vikas.shivappa@...el.com, linux-kernel@...r.kernel.org,
        x86@...nel.org, hpa@...or.com, mingo@...nel.org,
        peterz@...radead.org, ravi.v.shankar@...el.com,
        tony.luck@...el.com, fenghua.yu@...el.com, h.peter.anvin@...el.com
Subject: Re: [PATCH 7/8] x86/intel_rdt/mba: Add schemata file support for
 MBA

On Tue, 10 Jan 2017, Vikas Shivappa wrote:
> + * @display_str:		Format string to show schemata
> + * @validate:			API to validate the ctrl values.
>   * @info_files:		resctrl info files for the resource
>   * @infofiles_len:		Number of info files
>   * @max_delay:		Max throttle delay
> @@ -99,6 +101,9 @@ struct rdt_resource {
>  	int			cbm_len;
>  	int			min_cbm_bits;
>  	u32			no_ctrl;
> +	char			*display_str;
> +	int (*validate)		(char *buf, unsigned long *data,
> +				  struct rdt_resource *r);

Again this display and validation change wants to be seperate from the
bandwidth stuff.

It's not rocket science to split patches into preparatory and
implementation parts.

> +	r->display_str = kstrdup("%d=%d", GFP_KERNEL);
> +	if (!r->display_str)
> +		return -ENOMEM;

And the point of this allocation is? To consume extra memory for a constant
string which is in const data anyway.

       r->display_str = "%d=%d";

does not need allcotion and consumes exactly the same amount of const data
as the above. Oh well...

> -static inline bool get_rdt_resources(void)
> +static inline int get_rdt_resources(void)
>  {

And the point of this change is? Lots of churn to return the same -ENODEV
value at the call site. So why are you trying to return other values
instead of the simple boolean success/fail decision?

>  /*
> + * Check whether MBE 'throttle by' value is correct.
> + *	As per the SDM, when the scale is linear the
> + *	throttle_by granularity is '100 - max_thrtl_by'
> + *	and when its non-linear it is 'power of 2'.

That's wrong. We really want to let the user set a bandwidth percentage
value from 0 - 100 %. And then adjust it to the proper value which the
hardware can provide. So the user value is independent from granularity,
linear and the max throttling allowed.

>  /*
> - * Read one cache bit mask (hex). Check that it is valid for the current
> - * resource type.
> + * Read the user RDT control value into tempory buffer:
> + * Cache bit mask (hex) or Memory b/w throttle (decimal).
> + * Check that it is valid for the current resource type.
>   */
> -static int parse_cbm(char *buf, struct rdt_resource *r)
> +static int parse_ctrls(char *buf, struct rdt_resource *r)
>  {
>  	unsigned long data;
> -	int ret;
> +	int ret = 0;

What's the purpose of initializing ret to 0 if the next action is assigning
ret the return value of the validate function?

> -	ret = kstrtoul(buf, 16, &data);
> -	if (ret)
> -		return ret;
> -	if (!cbm_validate(data, r))
> -		return -EINVAL;
> +	ret = r->validate(buf, &data, r);
>  	r->tmp_ctrl[r->num_tmp_ctrl++] = data;
>  
> -	return 0;
> +	return ret;
>  }

Thanks,

	tglx