Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751452AbdFFPv0 (ORCPT ); Tue, 6 Jun 2017 11:51:26 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:42542 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751388AbdFFPvZ (ORCPT ); Tue, 6 Jun 2017 11:51:25 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 13ECA60798 Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=jhugo@codeaurora.org Subject: Re: [PATCH V4 1/2] sched/fair: Fix load_balance() affinity redo path To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Dietmar Eggemann , Austin Christ , Tyler Baicar , Timur Tabi References: <1496442432-330-1-git-send-email-jhugo@codeaurora.org> <1496442432-330-2-git-send-email-jhugo@codeaurora.org> <20170606123011.aehmelhnakvzjybb@hirez.programming.kicks-ass.net> From: Jeffrey Hugo Message-ID: Date: Tue, 6 Jun 2017 09:51:22 -0600 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0 MIME-Version: 1.0 In-Reply-To: <20170606123011.aehmelhnakvzjybb@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5451 Lines: 141 On 6/6/2017 6:30 AM, Peter Zijlstra wrote: > On Fri, Jun 02, 2017 at 04:27:11PM -0600, Jeffrey Hugo wrote: >> If load_balance() fails to migrate any tasks because all tasks were >> affined, load_balance() removes the source cpu from consideration and >> attempts to redo and balance among the new subset of cpus. >> >> There is a bug in this code path where the algorithm considers all active >> cpus in the system (minus the source that was just masked out). This is >> not valid for two reasons: some active cpus may not be in the current >> scheduling domain and one of the active cpus is dst_cpu. These cpus should >> not be considered, as we cannot pull load from them. >> >> Instead of failing out of load_balance(), we may end up redoing the search >> with no valid cpus and incorrectly concluding the domain is balanced. >> Additionally, if the group_imbalance flag was just set, it may also be >> incorrectly unset, thus the flag will not be seen by other cpus in future >> load_balance() runs as that algorithm intends. >> >> Fix the check by removing cpus not in the current domain and the dst_cpu >> from considertation, thus limiting the evaluation to valid remaining cpus >> from which load might be migrated. >> >> Co-authored-by: Austin Christ >> Co-authored-by: Dietmar Eggemann >> Signed-off-by: Jeffrey Hugo >> Tested-by: Tyler Baicar >> --- >> kernel/sched/fair.c | 22 ++++++++++------------ >> 1 file changed, 10 insertions(+), 12 deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index d711093..84255ab 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -6737,10 +6737,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) >> * our sched_group. We may want to revisit it if we couldn't >> * meet load balance goals by pulling other tasks on src_cpu. >> * >> - * Also avoid computing new_dst_cpu if we have already computed >> - * one in current iteration. >> + * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have >> + * already computed one in current iteration. >> */ >> - if (!env->dst_grpmask || (env->flags & LBF_DST_PINNED)) >> + if (env->idle == CPU_NEWLY_IDLE || (env->flags & LBF_DST_PINNED)) >> return 0; >> >> /* Prevent to re-select dst_cpu via env's cpus */ >> @@ -8091,14 +8091,7 @@ static int load_balance(int this_cpu, struct rq *this_rq, >> .tasks = LIST_HEAD_INIT(env.tasks), >> }; >> >> - /* >> - * For NEWLY_IDLE load_balancing, we don't need to consider >> - * other cpus in our group >> - */ >> - if (idle == CPU_NEWLY_IDLE) >> - env.dst_grpmask = NULL; >> - >> - cpumask_copy(cpus, cpu_active_mask); >> + cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask); >> >> schedstat_inc(sd->lb_count[idle]); >> >> @@ -8220,7 +8213,12 @@ static int load_balance(int this_cpu, struct rq *this_rq, >> /* All tasks on this runqueue were pinned by CPU affinity */ >> if (unlikely(env.flags & LBF_ALL_PINNED)) { >> cpumask_clear_cpu(cpu_of(busiest), cpus); >> - if (!cpumask_empty(cpus)) { >> + /* >> + * Go back to "redo" iff the load-balance cpumask >> + * contains other potential busiest cpus for the >> + * current sched domain. >> + */ >> + if (!cpumask_subset(cpus, env.dst_grpmask)) { >> env.loop = 0; >> env.loop_break = sched_nr_migrate_break; >> goto redo; > > So I was struggling with that subset condition. You want to ensure there > are CPUs outside of dst_grpmask left, otherwise balancing at this SD > level doesn't make sense anymore, right? Correct. > > I think you might want to spell that out a little in that comment. > Currently the comment only explains what it does, which is something we > can read from the code. Comments should explain _why_ we do things and > its failing there. Agreed. We will attempt to improve the comment in the next version. > > > So with that the problem is that active_load_balance_cpu_stop() calls > into can_migrate_task() with ->idle = CPU_IDLE and !dst_grpmask, which > then goes *bang*. Now active_load_balance_cpu_stop() doesn't need to > re-evaluate anything, so ideally it would just skip this entirely, > right? Correct. Your #3 option below would also work based on our understanding, and seems cleaner. We will validate it as a solution, and incorporate into the next version. Thanks > > So why not do #3: > > --- > kernel/sched/fair.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 47a0c552c77b..fd639d32fa4c 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8523,6 +8523,13 @@ static int active_load_balance_cpu_stop(void *data) > .src_cpu = busiest_rq->cpu, > .src_rq = busiest_rq, > .idle = CPU_IDLE, > + /* > + * can_migrate_task() doesn't need to compute new_dst_cpu > + * for active balancing. Since we have CPU_IDLE, but no > + * @dst_grpmask we need to make that test go away with lying > + * about DST_PINNED. > + */ > + .flags = LBF_DST_PINNED, > }; > > schedstat_inc(sd->alb_count); > -- Jeffrey Hugo Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.