linux-kernel - Re: [PATCH v3 2/2] selftests/resctrl: Adjust SNC support messages

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Message-ID: <a206cbfe-83a1-4c9d-8cc9-5cff1598a6d2@intel.com>
Date: Tue, 2 Jul 2024 15:01:57 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>
CC: "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>,
	"shuah@...nel.org" <shuah@...nel.org>, "Yu, Fenghua" <fenghua.yu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"ilpo.jarvinen@...ux.intel.com" <ilpo.jarvinen@...ux.intel.com>
Subject: Re: [PATCH v3 2/2] selftests/resctrl: Adjust SNC support messages

Hi Tony,

On 7/2/24 2:46 PM, Tony Luck wrote:
> On Tue, Jul 02, 2024 at 02:26:25PM -0700, Reinette Chatre wrote:
>> Hi Tony,
>>
>> On 7/1/24 9:04 AM, Luck, Tony wrote:
>>> +static bool cpus_offline_empty(void)
>>> +{
>>> +	char offline_cpus_str[64];
>>> +	FILE *fp;
>>> +
>>> +	fp = fopen("/sys/devices/system/cpu/offline", "r");
>>>
>>> Check for fp == NULL before using it.
>>>
>>> +	if (fscanf(fp, "%s", offline_cpus_str) < 0) {
>>>
>>> fscanf() seems like a heavy hammer.
>>
>> Do you perhaps have any recommendations that should be used instead of
>> fscanf()? I checked with stat() but could not see a difference between
>> file with a CPU and a file without. Other alternative is
>> open()/read()/close()? Looks like when there are no offline CPUs then
>> the file will only contain '\n' so it may be possible to read one byte
>> from the file and confirm it is '\n' as a check for "cpus_offline_empty()".
> 
> Sorry. I replied with Outlook and didn't quote things properly
> so my alternate suggestion didn't stand out. Here it is again:
> 
> 	if (fgets(offline_cpus_str, sizeof(offline_cpus_str), fp) == NULL) {
> 		fclose(fp);
> 		return true;
> 	}

Apologies, missed this.

> 
> But that was with the assumption that /sys/devices/system/cpu/offline
> would be empty. Not that it would conatain a single "\n" as you say
> above.
> 
> So fgets( ...) followed with "if (offline_cpus_str[0] == '\n') "?

How about simplifying it more to "if (fgetc(fp) == '\n')" ?

Reinette