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

[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@redhat.com>
CC: Ingo Molnar <mingo@redhat.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Juri Lelli <juri.lelli@redhat.com>
CC: Luiz Capitulino <lcapitulino@redhat.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
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

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

[Peter Xu]

On Thu, Apr 02, 2020 at 10:59:29AM -0400, Peter Xu wrote:
> 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@redhat.com>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Juri Lelli <juri.lelli@redhat.com>
> CC: Luiz Capitulino <lcapitulino@redhat.com>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> 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++;
>  	}

I just noticed this is still problematic if we want to mark this as
stable, because "managed_irq" violate the "isalpha()" rule already...
It means even if we apply this patch to the stable trees it'll still
think managed_irq as illegal and ignore the whole isolcpus=.

Thomas, do you want me to repost a v3 as v1 plus some pr_warn()s?

Thanks,

-- 
Peter Xu

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

[Thomas Gleixner]

Peter Xu <peterx@redhat.com> writes:
> On Thu, Apr 02, 2020 at 10:59:29AM -0400, Peter Xu wrote:
>> -		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++)

lacks {


>> +			if (!isalpha(*str))
>> +				illegal = true;

lacks }

>> +
>> +		if (illegal) {
>> +			pr_warn("isolcpus: Invalid flag %.*s\n", len, par);
>> +			return 0;
>> +		}
>> +
>> +		pr_info("isolcpus: Skipped unknown flag %.*s\n", len, par);
>> +		str++;
>>  	}
>
> I just noticed this is still problematic if we want to mark this as
> stable, because "managed_irq" violate the "isalpha()" rule already...
> It means even if we apply this patch to the stable trees it'll still
> think managed_irq as illegal and ignore the whole isolcpus=.

		if (!isalpha(*str) && *str != '_')

which is what I told you a couple of days ago already.

Thanks,

        tglx