[PATCH v2] sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters

[Peter Xu]

The "isolcpus=" parameter allows sub-parameters to exist before the
cpulist is specified, and if it sees unknown sub-parameters the whole
parameter will be ignored.  This design is incompatible with itself
when we add more sub-parameters to "isolcpus=", because the old
kernels will not recognize the new "isolcpus=" sub-parameters, then it
will invalidate the whole parameter so the CPU isolation will not
really take effect if we start to use the new sub-parameters while
later we reboot into an old kernel. Instead we will see this when
booting the old kernel:

    isolcpus: Error, unknown flag

The better and compatible way is to allow "isolcpus=" to skip unknown
sub-parameters, so that even if we add new sub-parameters to it the
old kernel will still be able to behave as usual even if with the new
sub-parameter is specified.

Ideally this patch should be there when we introduce the first
sub-parameter for "isolcpus=", so it's already a bit late.  However
late is better than nothing.

CC: Ming Lei <ming.lei@xxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Juri Lelli <juri.lelli@xxxxxxxxxx>
CC: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
v2:
- only allow isalpha() for sub-parameters [tglx]
---
 kernel/sched/isolation.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 008d6ac2342b..c2e8b4a778d6 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -149,6 +149,9 @@ __setup("nohz_full=", housekeeping_nohz_full_setup);
 static int __init housekeeping_isolcpus_setup(char *str)
 {
 	unsigned int flags = 0;
+	char *par;
+	int len;
+	bool illegal = false;
 
 	while (isalpha(*str)) {
 		if (!strncmp(str, "nohz,", 5)) {
@@ -169,8 +172,21 @@ static int __init housekeeping_isolcpus_setup(char *str)
 			continue;
 		}
 
-		pr_warn("isolcpus: Error, unknown flag\n");
-		return 0;
+		/*
+		 * Skip unknown sub-parameter and validate that it is not
+		 * containing an invalid character.
+		 */
+		for (par = str, len = 0; *str && *str != ','; str++, len++)
+			if (!isalpha(*str))
+				illegal = true;
+
+		if (illegal) {
+			pr_warn("isolcpus: Invalid flag %.*s\n", len, par);
+			return 0;
+		}
+
+		pr_info("isolcpus: Skipped unknown flag %.*s\n", len, par);
+		str++;
 	}
 
 	/* Default behaviour for isolcpus without flags */
-- 
2.24.1