From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>,
Phil Auld <pauld@redhat.com>, Michal Hocko <mhocko@kernel.org>,
Rik van Riel <riel@surriel.com>, Waiman Long <longman@redhat.com>
Subject: [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads
Date: Mon, 29 Jul 2019 17:07:28 -0400 [thread overview]
Message-ID: <20190729210728.21634-1-longman@redhat.com> (raw)
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 <longman@redhat.com>
---
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
next reply other threads:[~2019-07-29 21:08 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-29 21:07 Waiman Long [this message]
2019-07-29 21:12 ` [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads Waiman Long
2019-07-29 21:21 ` Rik van Riel
2019-07-29 21:42 ` Waiman Long
2019-07-30 0:26 ` Rik van Riel
2019-07-30 21:01 ` Waiman Long
2019-07-31 13:48 ` Rik van Riel
2019-07-31 14:15 ` Waiman Long
2019-07-31 15:07 ` Rik van Riel
2019-07-31 15:49 ` Waiman Long
2019-07-30 7:24 ` Michal Hocko
2019-07-30 21:05 ` Waiman Long
2019-07-30 8:43 ` Peter Zijlstra
2019-07-30 13:59 ` Waiman Long
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190729210728.21634-1-longman@redhat.com \
--to=longman@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@kernel.org \
--cc=mingo@redhat.com \
--cc=pauld@redhat.com \
--cc=peterz@infradead.org \
--cc=riel@surriel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).