From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752942AbcEYK3g (ORCPT ); Wed, 25 May 2016 06:29:36 -0400 Received: from mail-oi0-f68.google.com ([209.85.218.68]:34732 "EHLO mail-oi0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750765AbcEYK3f (ORCPT ); Wed, 25 May 2016 06:29:35 -0400 MIME-Version: 1.0 In-Reply-To: <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> <20160525094949.GL27946@e105550-lin.cambridge.arm.com> Date: Wed, 25 May 2016 18:29:33 +0800 Message-ID: Subject: Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations balance at wake-up From: Wanpeng Li To: Morten Rasmussen Cc: Peter Zijlstra , Ingo Molnar , Dietmar Eggemann , yuyang.du@intel.com, Vincent Guittot , Mike Galbraith , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Regards, Wanpeng Li