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Date: Thu, 12 Jun 2008 10:50:34 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Lai Jiangshan <laijs@...fujitsu.com> Cc: mingo@...e.hu, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 2/2] sched: 64bit: fix overflow(was: fix divide by zero) On Thu, 2008-06-12 at 16:43 +0800, Lai Jiangshan wrote: > (overflow means weight >= 2^32 here, because inv_weigh = 2^32/weight) > > A weight of a cfs_rq is the sum of weights of which entities > are queued on this cfs_rq, so it will be overflow when entities > are too many. > > Although, overflow is very hardly occurred, but it break fairness when > it is occurred. And 64bits system have more memory than 32bits system > and 64bits system can create more process usually, so overflow many be > occurred more. > > This patch guarantee fairness when overflow in 64bits system. > Thanks to the optimization of compiler, it change nothing in 32bits system. Yeah,. bit sad for that one branch extra on 64-bit, but I get the unlikely is as good as we can get it. Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl> > Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com> > --- > diff --git a/kernel/sched.c b/kernel/sched.c > index bfb8ad8..e277533 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -1337,8 +1337,13 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight, > { > u64 tmp; > > - if (!lw->inv_weight) > - lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2)/(lw->weight+1); > + if (!lw->inv_weight) { > + if (BITS_PER_LONG > 32 && unlikely(lw->weight >= WMULT_CONST)) > + lw->inv_weight = 1; > + else > + lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2) > + / (lw->weight+1); > + } > > tmp = (u64)delta_exec * weight; > /* > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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