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[209.132.183.28]) by mx.google.com with ESMTPS id y1si9771322vsl.56.2019.07.29.14.08.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 29 Jul 2019 14:08:37 -0700 (PDT) Received-SPF: pass (google.com: domain of longman@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of longman@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=longman@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 5F3F530860C6; Mon, 29 Jul 2019 21:08:36 +0000 (UTC) Received: from llong.com (dhcp-17-160.bos.redhat.com [10.18.17.160]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5E8175C1A1; Mon, 29 Jul 2019 21:08:30 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Phil Auld , Michal Hocko , Rik van Riel , Waiman Long Subject: [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads Date: Mon, 29 Jul 2019 17:07:28 -0400 Message-Id: <20190729210728.21634-1-longman@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Mon, 29 Jul 2019 21:08:36 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: It was found that a dying mm_struct where the owning task has exited can stay on as active_mm of kernel threads as long as no other user tasks run on those CPUs that use it as active_mm. This prolongs the life time of dying mm holding up some resources that cannot be freed on a mostly idle system. Fix that by forcing the kernel threads to use init_mm as the active_mm during a kernel thread to kernel thread transition if the previous active_mm is dying (!mm_users). This will allows the freeing of resources associated with the dying mm ASAP. The presence of a kernel-to-kernel thread transition indicates that the cpu is probably idling with no higher priority user task to run. So the overhead of loading the mm_users cacheline should not really matter in this case. My testing on an x86 system showed that the mm_struct was freed within seconds after the task exited instead of staying alive for minutes or even longer on a mostly idle system before this patch. Signed-off-by: Waiman Long --- kernel/sched/core.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 795077af4f1a..41997e676251 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3214,6 +3214,8 @@ static __always_inline struct rq * context_switch(struct rq *rq, struct task_struct *prev, struct task_struct *next, struct rq_flags *rf) { + struct mm_struct *next_mm = next->mm; + prepare_task_switch(rq, prev, next); /* @@ -3229,8 +3231,22 @@ context_switch(struct rq *rq, struct task_struct *prev, * * kernel -> user switch + mmdrop() active * user -> user switch + * + * kernel -> kernel and !prev->active_mm->mm_users: + * switch to init_mm + mmgrab() + mmdrop() */ - if (!next->mm) { // to kernel + if (!next_mm) { // to kernel + /* + * Checking is only done on kernel -> kernel transition + * to avoid any performance overhead while user tasks + * are running. + */ + if (unlikely(!prev->mm && + !atomic_read(&prev->active_mm->mm_users))) { + next_mm = next->active_mm = &init_mm; + mmgrab(next_mm); + goto mm_switch; + } enter_lazy_tlb(prev->active_mm, next); next->active_mm = prev->active_mm; @@ -3239,6 +3255,7 @@ context_switch(struct rq *rq, struct task_struct *prev, else prev->active_mm = NULL; } else { // to user +mm_switch: /* * sys_membarrier() requires an smp_mb() between setting * rq->curr and returning to userspace. @@ -3248,7 +3265,7 @@ context_switch(struct rq *rq, struct task_struct *prev, * finish_task_switch()'s mmdrop(). */ - switch_mm_irqs_off(prev->active_mm, next->mm, next); + switch_mm_irqs_off(prev->active_mm, next_mm, next); if (!prev->mm) { // from kernel /* will mmdrop() in finish_task_switch(). */ -- 2.18.1