linux-kernel - Re: [PATCH v5 8/8] selftests/resctrl: Adjust effective L3 cache size when SNC enabled

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Date:   Thu, 31 Aug 2023 08:51:15 +0200
From:   Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
CC:     "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        "Jonathan Corbet" <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        "Babu Moger" <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH v5 8/8] selftests/resctrl: Adjust effective L3 cache size
 when SNC enabled

On 2023-08-30 at 15:43:05 +0000, Luck, Tony wrote:
>> >+static int count_sys_bitmap_bits(char *name)
>> >+{
>> >+    FILE *fp = fopen(name, "r");
>> >+    int count = 0, c;
>> >+
>> >+    if (!fp)
>> >+            return 0;
>> >+
>> >+    while ((c = fgetc(fp)) != EOF) {
>> >+            if (!isxdigit(c))
>> >+                    continue;
>> >+            switch (c) {
>> >+            case 'f':
>> >+                    count++;
>> >+            case '7': case 'b': case 'd': case 'e':
>> >+                    count++;
>> >+            case '3': case '5': case '6': case '9': case 'a': case 'c':
>> >+                    count++;
>> >+            case '1': case '2': case '4': case '8':
>> >+                    count++;
>> >+            }
>> >+    }
>> >+    fclose(fp);
>> >+
>> >+    return count;
>> >+}
>> >+
>>
>> The resctrl selftest has a function for counting bits, could it be used
>> here instead of the switch statement like this for example?
>>
>> count = count_bits(c);
>>
>> Or is there some reason this wouldn't be a good fit here?
>
>Thanks for looking at my patch.
>
>That count_bits() function is doing so with input from an "unsigned long"
>argument.  My function is parsing the string result from a sysfs file which
>might look like this:
>
>$ cat shared_cpu_map
>0000,00000fff,ffffff00,0000000f,ffffffff
>
>To use count_bits() I'd have to use something like strtol() on each of the
>comma separated fields first to convert from ascii strings to binary
>values to feed into count_bits().

I missed they are being read as characters and not bytes, sorry.

Out of curiosity, what about using fscanf instead of fgetc? With the
format being %x and reading one byte at the time. Then instead of
isxdigit just checking if the read number was bigger than 0xF.

I also remembered there is a gcc (and I think clang has it as well)
builtin function that returns the number of set bits in a number.

So it would look like this:

	while ((fscanf(fp, "%x", c)) != EOF ) {
	    if (c > 0xF)
		    continue;
	    count = __builtin_popcount(c);
	}

Are there some problems with an approach like that?

-- 
Kind regards
Maciej Wieczór-Retman