From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755411Ab1BHSOX (ORCPT ); Tue, 8 Feb 2011 13:14:23 -0500 Received: from smtp-out.google.com ([216.239.44.51]:12315 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755393Ab1BHSOU (ORCPT ); Tue, 8 Feb 2011 13:14:20 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=from:to:cc:subject:date:message-id:x-mailer:in-reply-to:references; b=tazHd1ECgfnRgpoHq0yVFzmCXBLI9jvviAlrCglGYzQumXUPmULETIqInbRLko8X7C zIgTSegAUWNVzjY4GXJg== From: Venkatesh Pallipadi To: Suresh Siddha Cc: Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, Paul Turner , Mike Galbraith , Nick Piggin , Venkatesh Pallipadi Subject: [PATCH 1/3] sched: Resolve sd_idle and first_idle_cpu Catch-22 Date: Tue, 8 Feb 2011 10:13:37 -0800 Message-Id: <1297188819-19999-2-git-send-email-venki@google.com> X-Mailer: git-send-email 1.7.3.1 In-Reply-To: References: X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Consider a system with { [ (A B) (C D) ] [ (E F) (G H) ] }, () denoting SMT siblings, [] cores on same socket and {} system wide Further, A, C and D are idle, B is busy and one of EFGH has excess load. With sd_idle logic, a check in rebalance_domains() converts tick based load balance requests from CPU A to busy load balance for core and above domains (lower rate of balance and higher load_idx). With first_idle_cpu logic, when CPU C or D tries to balance across domains the logic finds CPU A as first idle CPU in the group and nominates CPU A to idle balance across sockets. But, sd_idle above would not allow CPU A to do cross socket idle balance as CPU A switches its higher level balancing to busy balance. So, this can result is no cross socket balancing for extended periods. The fix here adds additional check to detect sd_idle logic in first_idle_cpu code path. We will now nominate (in order or preference): * First fully idle CPU * First semi-idle CPU * First CPU Note that this solution works fine for 2 SMT siblings case and won't be perfect in picking proper semi-idle in case of more than 2 SMT threads. The problem was found by looking at the code and schedstat output. I don't yet have any data to show impact of this on any workload. Signed-off-by: Venkatesh Pallipadi --- kernel/sched_fair.c | 38 ++++++++++++++++++++++++++++++++++++-- 1 files changed, 36 insertions(+), 2 deletions(-) diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 0c26e2d..d7e6da3 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -2603,6 +2603,34 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group) return 0; } +/* + * Find if there is any busy CPUs in SD_SHARE_CPUPOWER domain of + * requested CPU. + * Bypass the check in case of SD_POWERSAVINGS_BALANCE on + * parent domain. In that case requested CPU can still be nominated as + * balancer for higher domains. + */ +static int is_cpupower_sharing_domain_idle(int cpu) +{ + struct sched_domain *sd; + int i; + + for_each_domain(cpu, sd) { + if (!(sd->flags & SD_SHARE_CPUPOWER)) + break; + + if (test_sd_parent(sd, SD_POWERSAVINGS_BALANCE)) + return 1; + + for_each_cpu(i, sched_domain_span(sd)) { + if (!idle_cpu(i)) + return 0; + } + } + + return 1; +} + /** * update_sg_lb_stats - Update sched_group's statistics for load balancing. * @sd: The sched_domain whose statistics are to be updated. @@ -2625,6 +2653,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd, unsigned long load, max_cpu_load, min_cpu_load, max_nr_running; int i; unsigned int balance_cpu = -1, first_idle_cpu = 0; + unsigned int first_semiidle_cpu = 0; unsigned long avg_load_per_task = 0; if (local_group) @@ -2644,8 +2673,13 @@ static inline void update_sg_lb_stats(struct sched_domain *sd, /* Bias balancing toward cpus of our domain */ if (local_group) { if (idle_cpu(i) && !first_idle_cpu) { - first_idle_cpu = 1; - balance_cpu = i; + if (is_cpupower_sharing_domain_idle(i)) { + first_idle_cpu = 1; + balance_cpu = i; + } else if (!first_semiidle_cpu) { + first_semiidle_cpu = 1; + balance_cpu = i; + } } load = target_load(i, load_idx); -- 1.7.3.1