[PATCH] cgroup: Return if dst_cgrp equals to src_cgrp

[PATCH] cgroup: Return if dst_cgrp equals to src_cgrp

[shisiyuan]

In function cgroup_migrate_add_src(), if dst_cgrp
equals to src_cgroup which the tasks link, dont
go on migrating tasks to another css_set.
This can save the cost of unnecessary migration.

Signed-off-by: shisiyuan <shisiyuan@xiaomi.com>
---
 kernel/cgroup/cgroup.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6139460..97d7f68 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2634,6 +2634,9 @@ void cgroup_migrate_add_src(struct css_set *src_cset,
 
 	src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
 
+	if (src_cgrp == dst_cgrp)
+		return;
+
 	if (!list_empty(&src_cset->mg_preload_node))
 		return;
 
@@ -2780,6 +2783,9 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 	rcu_read_unlock();
 	spin_unlock_irq(&css_set_lock);
 
+	if (list_empty(&mgctx.preloaded_src_csets))
+		return ret;
+
 	/* prepare dst csets and commit */
 	ret = cgroup_migrate_prepare_dst(&mgctx);
 	if (!ret)
@@ -2927,7 +2933,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	struct cgroup_subsys_state *d_css;
 	struct cgroup *dsct;
 	struct css_set *src_cset;
-	int ret;
+	int ret = 0;
 
 	lockdep_assert_held(&cgroup_mutex);
 
@@ -2943,6 +2949,9 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	}
 	spin_unlock_irq(&css_set_lock);
 
+	if (list_empty(&mgctx.preloaded_src_csets))
+		goto out_finish;
+
 	/* NULL dst indicates self on default hierarchy */
 	ret = cgroup_migrate_prepare_dst(&mgctx);
 	if (ret)
-- 
2.7.4

[PATCH] cgroup: Return if dst_cgrp equals to src_cgrp

[shisiyuan]

In function cgroup_migrate_add_src(), if dst_cgrp
equals to src_cgroup which the tasks link, dont
go on migrating tasks to another css_set.
This can save the cost of unnecessary migration.

Signed-off-by: shisiyuan <shisiyuan@xiaomi.com>
---
 kernel/cgroup/cgroup.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6139460..97d7f68 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2634,6 +2634,9 @@ void cgroup_migrate_add_src(struct css_set *src_cset,
 
 	src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
 
+	if (src_cgrp == dst_cgrp)
+		return;
+
 	if (!list_empty(&src_cset->mg_preload_node))
 		return;
 
@@ -2780,6 +2783,9 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 	rcu_read_unlock();
 	spin_unlock_irq(&css_set_lock);
 
+	if (list_empty(&mgctx.preloaded_src_csets))
+		return ret;
+
 	/* prepare dst csets and commit */
 	ret = cgroup_migrate_prepare_dst(&mgctx);
 	if (!ret)
@@ -2927,7 +2933,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	struct cgroup_subsys_state *d_css;
 	struct cgroup *dsct;
 	struct css_set *src_cset;
-	int ret;
+	int ret = 0;
 
 	lockdep_assert_held(&cgroup_mutex);
 
@@ -2943,6 +2949,9 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	}
 	spin_unlock_irq(&css_set_lock);
 
+	if (list_empty(&mgctx.preloaded_src_csets))
+		goto out_finish;
+
 	/* NULL dst indicates self on default hierarchy */
 	ret = cgroup_migrate_prepare_dst(&mgctx);
 	if (ret)
-- 
2.7.4

Re: [PATCH] cgroup: Return if dst_cgrp equals to src_cgrp

[Michal Koutný]

Hello.

On Sat, May 21, 2022 at 01:02:00AM +0800, shisiyuan <shisiyuan19870131@gmail.com> wrote:
> In function cgroup_migrate_add_src(), if dst_cgrp
> equals to src_cgroup which the tasks link, dont
> go on migrating tasks to another css_set.

Do you have an example which operations lead to this situation?

> This can save the cost of unnecessary migration.

How much are the savings visible?

(Note, I suspect the shortcut on `src_cgrp == dst_cgrp` might not be
always correct when called from cgroup_update_dfl_csses() but not 100%
sure now.)

Thanks,
Michal

Re: [PATCH] cgroup: Return if dst_cgrp equals to src_cgrp

[Michal Koutný]

On Thu, May 26, 2022 at 12:40:17PM +0800, 史思远 <shisiyuan19870131@gmail.com> wrote:
> echo 1413 >  /dev/cpuset/foreground/cgroup.procs
> echo 1413 >  /dev/cpuset/foreground/cgroup.procs
> echo 1413 >  /dev/cpuset/foreground/cgroup.procs

Thank you.

> In this situation, since the second echo, dst_cgrp equals to src_cgrp(both
> cgroups are the 'foreground' cpuset).
> cgroup1_procs_write（）-> __cgroup1_procs_write()
> -> cgrp = cgroup_kn_lock_live()
> -> task = cgroup_procs_write_start(buf...)
> -> cgroup_attach_task(cgrp, task, threadgroup);
>   -> 1) cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);  //
> detect the (src_cgrp == dst_cgrp), don't need to continue the following
> migration flow.
>   -> 2) cgroup_migrate_prepare_dst(&mgctx)

Here, I'd like to let you know about [1]

>   -> 3) cgroup_migrate(leader, threadgroup, &mgctx)
>   -> 4) cgroup_migrate_finish(&mgctx) // cleanup
> -> cgroup_procs_write_finish();  // cleanup


> >> The main purpose for this patch is to handle the situation above, and
> just noticed that cgroup_update_dfl_csses() also
> calls cgroup_migrate_add_src(). So I assume it's a similar situation. We
> can discuss more about it.

So I think the patch doesn't save that much and identical css_set is
better condition to check on (covers cgroup_update_dfl_csses too).

Michal

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/cgroup/cgroup.c?h=v5.18#n2710

Re: [PATCH] cgroup: Return if dst_cgrp equals to src_cgrp

[Michal Koutný]

On Thu, May 26, 2022 at 12:40:17PM +0800, 史思远 <shisiyuan19870131-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> echo 1413 >  /dev/cpuset/foreground/cgroup.procs
> echo 1413 >  /dev/cpuset/foreground/cgroup.procs
> echo 1413 >  /dev/cpuset/foreground/cgroup.procs

Thank you.

> In this situation, since the second echo, dst_cgrp equals to src_cgrp(both
> cgroups are the 'foreground' cpuset).
> cgroup1_procs_write（）-> __cgroup1_procs_write()
> -> cgrp = cgroup_kn_lock_live()
> -> task = cgroup_procs_write_start(buf...)
> -> cgroup_attach_task(cgrp, task, threadgroup);
>   -> 1) cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);  //
> detect the (src_cgrp == dst_cgrp), don't need to continue the following
> migration flow.
>   -> 2) cgroup_migrate_prepare_dst(&mgctx)

Here, I'd like to let you know about [1]

>   -> 3) cgroup_migrate(leader, threadgroup, &mgctx)
>   -> 4) cgroup_migrate_finish(&mgctx) // cleanup
> -> cgroup_procs_write_finish();  // cleanup


> >> The main purpose for this patch is to handle the situation above, and
> just noticed that cgroup_update_dfl_csses() also
> calls cgroup_migrate_add_src(). So I assume it's a similar situation. We
> can discuss more about it.

So I think the patch doesn't save that much and identical css_set is
better condition to check on (covers cgroup_update_dfl_csses too).

Michal

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/cgroup/cgroup.c?h=v5.18#n2710