From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753526AbcEYJtL (ORCPT ); Wed, 25 May 2016 05:49:11 -0400 Received: from foss.arm.com ([217.140.101.70]:33519 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752251AbcEYJtJ (ORCPT ); Wed, 25 May 2016 05:49:09 -0400 Date: Wed, 25 May 2016 10:49:50 +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: <20160525094949.GL27946@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> 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 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? 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 question though as the second condition would still evaluate true if util_avg == capacity_orig_of(task_cpu(p)), but as said above the first condition should bail out before we get here. Morten > > + > > /* > > * select_task_rq_fair: Select target runqueue for the waking task in domains > > * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE, > > @@ -5316,7 +5341,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > > > > if (sd_flag & SD_BALANCE_WAKE) { > > record_wakee(p); > > - want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p)); > > + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) > > + && cpumask_test_cpu(cpu, tsk_cpus_allowed(p)); > > } > > > > rcu_read_lock(); > > -- > > 1.9.1 > >