linux-kernel - Re: [PATCH RFC] sched: remove implicit unsigned long - int

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Date:	Tue, 26 May 2015 15:36:16 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Nicholas Mc Guire <hofrat@...dl.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] sched: remove implicit unsigned long - int -
 unsigned long conversion

On Sat, May 23, 2015 at 07:01:25PM +0100, Nicholas Mc Guire wrote:
> Type-checking coccinelle spatches are being used to locate type mismatches
> between function signatures and return values in this case this produced:
> kernel/sched/fair.c:4987 WARNING: return of wrong type 
>          int != unsigned long
> 
> get_cpu_usage() has one user update_sg_lb_stats() in which it is called:
>   sgs->group_usage += get_cpu_usage(i);
> as group_usage (from struct sg_lb_stats) is unsigned long this
> effectively is a unsigned long -> int -> unsigned long automatic type
> conversion which has no effect as the return of get_cpu_usage() never 
> can exceed SCHED_LOAD_SCALE which is < INT_MAX. (on both 64 and 32bit
> systems).
> 
> proposal: make get_cpu_usage return unsigned long to make this type clean

Agreed. I came across this inconsistency recently when messing around
with this code and came to the same conclusion:

https://lkml.org/lkml/2015/5/12/754

> 
> patch was compile tested for x86_64_defconfig 
> 
> patch is against 4.1-rc4 (localversion-next is -next-20150522)
> 
> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
> ---
> 
> A question that popped up during code review:
> "usage" in get_cpu_usage() is in the range of [0..SCHED_LOAD_SCALE] which
> is 2**10 (could be up to 2**20 (currently disabled see sched.h:55) but
> from code reading it seems that group_usage would eventually overflow as it 
> is only being incremented but never decremented or reset ?
> 
> load_balance()
>  -> find_busiest_group()
>    -> update_sd_lb_stats()
>      -> update_sg_lb_stats()
>         ...
>         sgs->group_usage += get_cpu_usage(i); 
> 
> which returns (unsigned long) utilization_load_avg or cpu_capacity_orig
> and seems to always be positive, so group_usage would eventually overflow.
> On 64bit systems this overflow would probably simply never happen but on 
> 32bit ?
> 
> what am I missing here ?

There is a:

	memset(sgs, 0, sizeof(*sgs));

just before the loop with the sgs->group_usage increments which should
reset it every time update_sg_lb_stats() is called.

> 
>  kernel/sched/fair.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1dbeea9..7c169a8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4978,7 +4978,7 @@ done:
>   * Without capping the usage, a group could be seen as overloaded (CPU0 usage
>   * at 121% + CPU1 usage at 80%) whereas CPU1 has 20% of available capacity
>   */
> -static int get_cpu_usage(int cpu)
> +static unsigned long get_cpu_usage(int cpu)
>  {
>  	unsigned long usage = cpu_rq(cpu)->cfs.utilization_load_avg;
>  	unsigned long capacity = capacity_orig_of(cpu);
> -- 
> 1.7.10.4

FWIW: Acked-by: Morten Rasmussen <morten.rasmussen@....com>
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