sched/debug: Output SD flag names rather than their values

Decoding the output of /proc/sys/kernel/sched_domain/cpu*/domain*/flags has
always been somewhat annoying, as one needs to go fetch the bit -> name
mapping from the source code itself. This encoding can be saved in a script
somewhere, but that isn't safe from flags being added, removed or even
shuffled around.

What matters for debugging purposes is to get *which* flags are set in a
given domain, their associated value is pretty much meaningless.

Make the sd flags debug file output flag names.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20200817113003.20802-7-valentin.schneider@arm.com

kernel/sched/debug.c

@@ -245,6 +245,60 @@ set_table_entry(struct ctl_table *entry,
 	entry->proc_handler = proc_handler;
 }
 
+static int sd_ctl_doflags(struct ctl_table *table, int write,
+			  void *buffer, size_t *lenp, loff_t *ppos)
+{
+	unsigned long flags = *(unsigned long *)table->data;
+	size_t data_size = 0;
+	size_t len = 0;
+	char *tmp;
+	int idx;
+
+	if (write)
+		return 0;
+
+	for_each_set_bit(idx, &flags, __SD_FLAG_CNT) {
+		char *name = sd_flag_debug[idx].name;
+
+		/* Name plus whitespace */
+		data_size += strlen(name) + 1;
+	}
+
+	if (*ppos > data_size) {
+		*lenp = 0;
+		return 0;
+	}
+
+	tmp = kcalloc(data_size + 1, sizeof(*tmp), GFP_KERNEL);
+	if (!tmp)
+		return -ENOMEM;
+
+	for_each_set_bit(idx, &flags, __SD_FLAG_CNT) {
+		char *name = sd_flag_debug[idx].name;
+
+		len += snprintf(tmp + len, strlen(name) + 2, "%s ", name);
+	}
+
+	tmp += *ppos;
+	len -= *ppos;
+
+	if (len > *lenp)
+		len = *lenp;
+	if (len)
+		memcpy(buffer, tmp, len);
+	if (len < *lenp) {
+		((char *)buffer)[len] = '\n';
+		len++;
+	}
+
+	*lenp = len;
+	*ppos += len;
+
+	kfree(tmp);
+
+	return 0;
+}
+
 static struct ctl_table *
 sd_alloc_ctl_domain_table(struct sched_domain *sd)
 {
@@ -258,7 +312,7 @@ sd_alloc_ctl_domain_table(struct sched_domain *sd)
 	set_table_entry(&table[2], "busy_factor",	  &sd->busy_factor,	    sizeof(int),  0644, proc_dointvec_minmax);
 	set_table_entry(&table[3], "imbalance_pct",	  &sd->imbalance_pct,	    sizeof(int),  0644, proc_dointvec_minmax);
 	set_table_entry(&table[4], "cache_nice_tries",	  &sd->cache_nice_tries,    sizeof(int),  0644, proc_dointvec_minmax);
-	set_table_entry(&table[5], "flags",		  &sd->flags,		    sizeof(int),  0444, proc_dointvec_minmax);
+	set_table_entry(&table[5], "flags",		  &sd->flags,		    sizeof(int),  0444, sd_ctl_doflags);
 	set_table_entry(&table[6], "max_newidle_lb_cost", &sd->max_newidle_lb_cost, sizeof(long), 0644, proc_doulongvec_minmax);
 	set_table_entry(&table[7], "name",		  sd->name,	       CORENAME_MAX_SIZE, 0444, proc_dostring);
 	/* &table[8] is terminator */