Re: [PATCH 1/3] make migrate disable/enable conditioned on softirq_nestcnt transition

Re: [PATCH 1/3] make migrate disable/enable conditioned on softirq_nestcnt transition

[Steven Rostedt]

BTW, Please Cc LKML when sending patches. It is the kernel you are
modifying, not userspace.

On Fri, 6 Dec 2013 00:42:22 +0100
Nicholas Mc Guire <der.herr@hofr.at> wrote:

> 
>  This patch removes the recursive calls to migrate_disable/enable in 
>  local_bh_disable/enable
> 
>  the softirq-local-lock.patch introduces local_bh_disable/enable wich 
>  decrements/increments the current->softirq_nestcnt and disable/enables
>  migration as well. as softirq_nestcnt (include/linux/sched.h conditioned
>  on CONFIG_PREEMPT_RT_BASE) already is tracking the nesting level of the
>  recursive calls to local_bh_disable/enable (all in kernel/softirq.c) - no 
>  need to do it twice.
> 
>  migrate_disable/enable thus can be conditionsed on softirq_nestcnt making
>  a transition from 0-1 to disable migration and 1-0 to re-enable it.
> 
>  patch on top of 3.12.1-rt4
> 
>  No change of functional behavior, this does noticably reduce the observed 
>  nesting level of migrate_disable/enable 
> 
> Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
> ---
>  kernel/softirq.c |   14 ++++++++------
>  1 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 9a7268f..16ebbd9 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -569,8 +569,8 @@ static void do_current_softirqs(int need_rcu_bh_qs)
>  
>  void local_bh_disable(void)
>  {
> -	migrate_disable();
> -	current->softirq_nestcnt++;
> +	if (++current->softirq_nestcnt == 1)
> +		migrate_disable();
>  }
>  EXPORT_SYMBOL(local_bh_disable);
>  
> @@ -584,8 +584,8 @@ void local_bh_enable(void)
>  		do_current_softirqs(1);
>  	local_irq_enable();
>  
> -	current->softirq_nestcnt--;
> -	migrate_enable();
> +	if (--current->softirq_nestcnt == 0)
> +		migrate_enable();

I wonder if we should add a:

	BUG_ON(current->softirq_nestcnt < 0);

As for the patch, I haven't found anything wrong with it.

Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

>  }
>  EXPORT_SYMBOL(local_bh_enable);
>  
> @@ -597,8 +597,10 @@ EXPORT_SYMBOL(local_bh_enable_ip);
>  
>  void _local_bh_enable(void)
>  {
> -	current->softirq_nestcnt--;
> -	migrate_enable();
> +	if (WARN_ON(current->softirq_nestcnt == 0))
> +		return;
> +	if (--current->softirq_nestcnt == 0)
> +		migrate_enable();
>  }
>  EXPORT_SYMBOL(_local_bh_enable);
>  

Re: [PATCH 1/3] make migrate disable/enable conditioned on softirq_nestcnt transition

[Sebastian Andrzej Siewior]

* Steven Rostedt | 2013-12-05 19:45:30 [-0500]:

>On Fri, 6 Dec 2013 00:42:22 +0100
>Nicholas Mc Guire <der.herr@hofr.at> wrote:
>> --- a/kernel/softirq.c
>> +++ b/kernel/softirq.c
>> @@ -569,8 +569,8 @@ static void do_current_softirqs(int need_rcu_bh_qs)
>>  
>>  void local_bh_disable(void)
>>  {
>> -	migrate_disable();
>> -	current->softirq_nestcnt++;
>> +	if (++current->softirq_nestcnt == 1)
>> +		migrate_disable();
>>  }
>>  EXPORT_SYMBOL(local_bh_disable);
>>  
>> @@ -584,8 +584,8 @@ void local_bh_enable(void)
>>  		do_current_softirqs(1);
>>  	local_irq_enable();
>>  
>> -	current->softirq_nestcnt--;
>> -	migrate_enable();
>> +	if (--current->softirq_nestcnt == 0)
>> +		migrate_enable();
>
>I wonder if we should add a:
>
>	BUG_ON(current->softirq_nestcnt < 0);

We have a WARN_ON() in each enable path. That one in local_bh_enable()
isn't part of the context here. If you want to s/WARN_/BUG_/ then I
would prefer not to since there are a few people with no UART and this
could break the system while the current sollution keeps the system
running.

>As for the patch, I haven't found anything wrong with it.
>
>Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Thanks.

Sebastian