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

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Message-Id: <20200204161639.267026-1-peterx@redhat.com>
Date:   Tue,  4 Feb 2020 11:16:39 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterx@...hat.com, Ming Lei <ming.lei@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters

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@...hat.com>
CC: Ingo Molnar <mingo@...hat.com>
CC: Peter Zijlstra <peterz@...radead.org>
CC: Juri Lelli <juri.lelli@...hat.com>
CC: Luiz Capitulino <lcapitulino@...hat.com>
Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 kernel/sched/isolation.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 008d6ac2342b..d5defb667bbc 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -169,8 +169,12 @@ static int __init housekeeping_isolcpus_setup(char *str)
 			continue;
 		}

-		pr_warn("isolcpus: Error, unknown flag\n");
-		return 0;
+		str = strchr(str, ',');
+		if (str)
+			/* Skip unknown sub-parameter */
+			str++;
+		else
+			return 0;
 	}

 	/* Default behaviour for isolcpus without flags */
-- 
2.24.1