linux-kernel - Re: [PATCH] sched: prevent compiler from optimising sched_avg

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Date:	Tue, 23 Mar 2010 18:53:24 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-kernel@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] sched: prevent compiler from optimising
 sched_avg_update loop

Le mardi 23 mars 2010 à 17:36 +0000, Will Deacon a écrit :
> GCC 4.4.1 on ARM has been observed to replace the while loop
> in sched_avg_update with a call to uldivmod, resulting in the
> following build failure at link-time:
> 
> kernel/built-in.o: In function `sched_avg_update':
> /linux-2.6/kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
> /linux-2.6/kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
> make: *** [.tmp_vmlinux1] Error 1
> 
> This patch [taken against 2.6.34-rc2] replaces the loop with a call to
> div_s64 which allows the Kernel to link.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>  kernel/sched.c |    7 +++----
>  1 files changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 9ab3cd7..6b74f21 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -1238,11 +1238,10 @@ static u64 sched_avg_period(void)
>  static void sched_avg_update(struct rq *rq)
>  {
>  	s64 period = sched_avg_period();
> +	s64 elapsed_periods = div_s64(rq->clock - rq->age_stamp - 1, period);
>  
> -	while ((s64)(rq->clock - rq->age_stamp) > period) {
> -		rq->age_stamp += period;
> -		rq->rt_avg /= 2;
> -	}
> +	rq->age_stamp += (u64)(elapsed_periods * period);
> +	rq->rt_avg >>= elapsed_periods;
>  }
>  
>  static void sched_rt_avg_update(struct rq *rq, u64 rt_delta)

Please take a look at __iter_div_u64_rem() , because we had a similar
problem in the past. We want to avoid this div_s64() call.



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