lkml.org 
[lkml]   [2010]   [Mar]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
SubjectRe: [PATCH] sched: prevent compiler from optimising sched_avg_update loop
From
Date
On Tue, 2010-03-23 at 17:36 +0000, Will Deacon wrote:
> 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@arm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Will Deacon <will.deacon@arm.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;
> }

Hmm, and that does an unconditional division, thing is, I don't expect
(under normal circumstances) for that loop to go round more than once so
that division will hurt for no reason.

Should we maybe write it like so:

if ((s64)(rq->clock - rq->age_stamp) > period) {
rq->age_stamp += period;
rq->rt_avg >>= 1;
}
if (unlikely((s64)(rq->clock - rq->age_stamp) > period)) {
s64 overflows = div_s64(rq->clocks - rq->age_stamp, period);
int width = sizeof(rq->rt_avg) * 8;

rq->age_stamp += overflows * period;
if (unlikely(overflows >= width))
rq->rt_avg = 0;
else
rq->rt_avg >>= overflows;
}
?



\
 
 \ /
  Last update: 2010-03-23 19:11    [W:0.048 / U:0.600 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site