From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753846AbcEYKx3 (ORCPT ); Wed, 25 May 2016 06:53:29 -0400 Received: from foss.arm.com ([217.140.101.70]:33896 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750969AbcEYKx2 (ORCPT ); Wed, 25 May 2016 06:53:28 -0400 Date: Wed, 25 May 2016 11:54:08 +0100 From: Morten Rasmussen To: Wanpeng Li Cc: Peter Zijlstra , Ingo Molnar , Dietmar Eggemann , yuyang.du@intel.com, Vincent Guittot , Mike Galbraith , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations balance at wake-up Message-ID: <20160525105407.GM27946@e105550-lin.cambridge.arm.com> References: <1464001138-25063-1-git-send-email-morten.rasmussen@arm.com> <1464001138-25063-10-git-send-email-morten.rasmussen@arm.com> <20160525094949.GL27946@e105550-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 25, 2016 at 06:29:33PM +0800, Wanpeng Li wrote: > 2016-05-25 17:49 GMT+08:00 Morten Rasmussen : > > On Wed, May 25, 2016 at 02:57:00PM +0800, Wanpeng Li wrote: > >> 2016-05-23 18:58 GMT+08:00 Morten Rasmussen : > >> > Currently, SD_WAKE_AFFINE always takes priority over wakeup balancing if > >> > SD_BALANCE_WAKE is set on the sched_domains. For asymmetric > >> > configurations SD_WAKE_AFFINE is only desirable if the waking task's > >> > compute demand (utilization) is suitable for the cpu capacities > >> > available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup > >> > balancing take over (find_idlest_{group, cpu}()). > >> > > >> > The assumption is that SD_WAKE_AFFINE is never set for a sched_domain > >> > containing cpus with different capacities. This is enforced by a > >> > previous patch based on the SD_ASYM_CPUCAPACITY flag. > >> > > >> > Ideally, we shouldn't set 'want_affine' in the first place, but we don't > >> > know if SD_BALANCE_WAKE is enabled on the sched_domain(s) until we start > >> > traversing them. > >> > > >> > cc: Ingo Molnar > >> > cc: Peter Zijlstra > >> > > >> > Signed-off-by: Morten Rasmussen > >> > --- > >> > kernel/sched/fair.c | 28 +++++++++++++++++++++++++++- > >> > 1 file changed, 27 insertions(+), 1 deletion(-) > >> > > >> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> > index 564215d..ce44fa7 100644 > >> > --- a/kernel/sched/fair.c > >> > +++ b/kernel/sched/fair.c > >> > @@ -114,6 +114,12 @@ unsigned int __read_mostly sysctl_sched_shares_window = 10000000UL; > >> > unsigned int sysctl_sched_cfs_bandwidth_slice = 5000UL; > >> > #endif > >> > > >> > +/* > >> > + * The margin used when comparing utilization with cpu capacity: > >> > + * util * 1024 < capacity * margin > >> > + */ > >> > +unsigned int capacity_margin = 1280; /* ~20% */ > >> > + > >> > static inline void update_load_add(struct load_weight *lw, unsigned long inc) > >> > { > >> > lw->weight += inc; > >> > @@ -5293,6 +5299,25 @@ static int cpu_util(int cpu) > >> > return (util >= capacity) ? capacity : util; > >> > } > >> > > >> > +static inline int task_util(struct task_struct *p) > >> > +{ > >> > + return p->se.avg.util_avg; > >> > +} > >> > + > >> > +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu) > >> > +{ > >> > + long delta; > >> > + long prev_cap = capacity_of(prev_cpu); > >> > + > >> > + delta = cpu_rq(cpu)->rd->max_cpu_capacity - prev_cap; > >> > + > >> > + /* prev_cpu is fairly close to max, no need to abort wake_affine */ > >> > + if (delta < prev_cap >> 3) > >> > + return 0; > >> > + > >> > + return prev_cap * 1024 < task_util(p) * capacity_margin; > >> > +} > >> > >> If one task util_avg is SCHED_CAPACITY_SCALE and running on x86 box w/ > >> SMT enabled, then each HT has capacity 589, wake_cap() will result in > >> always not wake affine, right? > > > > The idea is that SMT systems would bail out already at the previous > > condition. We should have max_cpu_capacity == prev_cap == 589, delta > > should then be zero and make the first condition true and make > > wake_cap() always return 0 for any system with symmetric capacities > > regardless of their actual capacity values. > > > > Note that this isn't entirely true as I used capacity_of() for prev_cap, > > if I change that to capacity_orig_of() it should be true. > > > > By making the !wake_cap() condition always true for want_affine, we > > should preserve existing behaviour for SMT/SMP. The only overhead is the > > capacity delta computation and comparison, which should be cheap. > > > > Does that make sense? > > Fair enough, thanks for your explanation. > > > > > Btw, task util_avg == SCHED_CAPACITY_SCALE should only be possible > > temporarily, it should decay to util_avg <= > > capacity_orig_of(task_cpu(p)) over time. That doesn't affect your > > Sorry, I didn't find it will decay to capacity_orig in > __update_load_avg(), could you elaborate? I should have checked the code before writing that :-( I thought the scaling by arch_scale_cpu_capacity() in __update_load_avg() would do that, but it turns out that the default implementation of arch_scale_cpu_capacity() doesn't do that when we pass a NULL pointer for the sched_domain, it would have returned smt_gain/span_weight == capacity_orig_of(cpu) otherwise. Sorry for the confusion, though I'm not sure if it is right to return SCHED_CAPACITY_SCALE for SMT systems.