Re: [PATCH 6/8] lib: bitmap: support "N" as an alias for size of bitmap

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: linux-kernel@vger.kernel.org, lizefan@huawei.com,
	mingo@kernel.org, tglx@linutronix.de, josh@joshtriplett.org,
	yury.norov@gmail.com, peterz@infradead.org, paulmck@kernel.org,
	fweisbec@gmail.com, linux@rasmusvillemoes.dk
Subject: Re: [PATCH 6/8] lib: bitmap: support "N" as an alias for size of bitmap
Date: Tue, 26 Jan 2021 23:37:30 +0200	[thread overview]
Message-ID: <YBCLmrCSwBRkTAhT@smile.fi.intel.com> (raw)
In-Reply-To: <20210126171141.122639-7-paul.gortmaker@windriver.com>

On Tue, Jan 26, 2021 at 12:11:39PM -0500, Paul Gortmaker wrote:
> While this is done for all bitmaps, the original use case in mind was
> for CPU masks and cpulist_parse() as described below.
> 
> It seems that a common configuration is to use the 1st couple cores for
> housekeeping tasks.  This tends to leave the remaining ones to form a
> pool of similarly configured cores to take on the real workload of
> interest to the user.
> 
> So on machine A - with 32 cores, it could be 0-3 for "system" and then
> 4-31 being used in boot args like nohz_full=, or rcu_nocbs= as part of
> setting up the worker pool of CPUs.
> 
> But then newer machine B is added, and it has 48 cores, and so while
> the 0-3 part remains unchanged, the pool setup cpu list becomes 4-47.
> 
> Multiple deployment becomes easier when we can just simply replace 31
> and 47 with "N" and let the system substitute in the actual number at
> boot; a number that it knows better than we do.

I would accept lower 'n' as well.

...

> -static const char *bitmap_getnum(const char *str, unsigned int *num)
> +static const char *__bitmap_getnum(const char *str, unsigned int nbits,
> +				    unsigned int *num)
>  {
>  	unsigned long long n;
>  	unsigned int len;
>  
> +	if (str[0] == 'N') {
> +		*num = nbits - 1;
> +		return str + 1;
> +	}

But locating it here makes possible to enter a priori invalid input, like N for
start of the region.

I think this should be separate helper which is called in places where it makes
sense.

>  	len = _parse_integer(str, 10, &n);
>  	if (!len)
>  		return ERR_PTR(-EINVAL);

-- 
With Best Regards,
Andy Shevchenko