From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sasha Levin Subject: [PATCH AUTOSEL 5.0 133/262] cgroup, rstat: Don't flush subtree root unless necessary Date: Wed, 27 Mar 2019 13:59:48 -0400 Message-ID: <20190327180158.10245-133-sashal@kernel.org> References: <20190327180158.10245-1-sashal@kernel.org> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1553709959; bh=pylpkBLNcCZBLgMSDbploWxJ69hWPZeS0H5eeVswZy0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=T+Mejj2lrNbtgd5j/iaQ/kU1D54sTpO+YpP53CSU/0Ep+sQLRxbWU/TOhcHbdegSv H8HbQlNHJFLktHktOYMzHZfQqDJiUwQsnFPfRgWZhVGML8bf1s8jj60KcgDJpjM8Fh 6GVuZbIWXtxbNsqd6ZgCz6TYGSSsIgbRFNTO5TiM= In-Reply-To: <20190327180158.10245-1-sashal@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Tejun Heo , Sasha Levin , cgroups@vger.kernel.org From: Tejun Heo [ Upstream commit b4ff1b44bcd384d22fcbac6ebaf9cc0d33debe50 ] cgroup_rstat_cpu_pop_updated() is used to traverse the updated cgroups on flush. While it was only visiting updated ones in the subtree, it was visiting @root unconditionally. We can easily check whether @root is updated or not by looking at its ->updated_next just as with the cgroups in the subtree. * Remove the unnecessary cgroup_parent() test. The system root cgroup is never updated and thus its ->updated_next is always NULL. No need to test whether cgroup_parent() exists in addition to ->updated_next. * Terminate traverse if ->updated_next is NULL. This can only happen for subtree @root and there's no reason to visit it if it's not marked updated. This reduces cpu consumption when reading a lot of rstat backed files. In a micro benchmark reading stat from ~1600 cgroups, the sys time was lowered by >40%. Signed-off-by: Tejun Heo Signed-off-by: Sasha Levin --- kernel/cgroup/rstat.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c index d503d1a9007c..bb95a35e8c2d 100644 --- a/kernel/cgroup/rstat.c +++ b/kernel/cgroup/rstat.c @@ -87,7 +87,6 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos, struct cgroup *root, int cpu) { struct cgroup_rstat_cpu *rstatc; - struct cgroup *parent; if (pos == root) return NULL; @@ -115,8 +114,8 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos, * However, due to the way we traverse, @pos will be the first * child in most cases. The only exception is @root. */ - parent = cgroup_parent(pos); - if (parent && rstatc->updated_next) { + if (rstatc->updated_next) { + struct cgroup *parent = cgroup_parent(pos); struct cgroup_rstat_cpu *prstatc = cgroup_rstat_cpu(parent, cpu); struct cgroup_rstat_cpu *nrstatc; struct cgroup **nextp; @@ -140,9 +139,12 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos, * updated stat. */ smp_mb(); + + return pos; } - return pos; + /* only happens for @root */ + return NULL; } /* see cgroup_rstat_flush() */ -- 2.19.1