linux-kernel - [PATCH v2] sched/fair: reduce long-tail newly idle balance cost

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <1614154549-116078-1-git-send-email-aubrey.li@intel.com>
Date:   Wed, 24 Feb 2021 16:15:49 +0800
From:   Aubrey Li <aubrey.li@...el.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...hsingularity.net, bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>
Subject: [PATCH v2] sched/fair: reduce long-tail newly idle balance cost

A long-tail load balance cost is observed on the newly idle path,
this is caused by a race window between the first nr_running check
of the busiest runqueue and its nr_running recheck in detach_tasks.

Before the busiest runqueue is locked, the tasks on the busiest
runqueue could be pulled by other CPUs and nr_running of the busiest
runqueu becomes 1 or even 0 if the running task becomes idle, this
causes detach_tasks breaks with LBF_ALL_PINNED flag set, and triggers
load_balance redo at the same sched_domain level.

In order to find the new busiest sched_group and CPU, load balance will
recompute and update the various load statistics, which eventually leads
to the long-tail load balance cost.

This patch clears LBF_ALL_PINNED flag for this race condition, and hence
reduces the long-tail cost of newly idle balance.

Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>
---
 kernel/sched/fair.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04a3ce2..5c67804 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7675,6 +7675,15 @@ static int detach_tasks(struct lb_env *env)

 	lockdep_assert_held(&env->src_rq->lock);

+	/*
+	 * Source run queue has been emptied by another CPU, clear
+	 * LBF_ALL_PINNED flag as we will not test any task.
+	 */
+	if (env->src_rq->nr_running <= 1) {
+		env->flags &= ~LBF_ALL_PINNED;
+		return 0;
+	}
+
 	if (env->imbalance <= 0)
 		return 0;

-- 
2.7.4