[PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Chen Yu]

Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective masks")
leverages cpuset's cpus_allowed and its parent's effective_cpus to calculate
the new_cpus:
cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
since cpus_allowed will also be updated after the CPU's offline on
non-dfl platform, - that is in hotplug_update_tasks_legacy,
so when the CPU is online again, it will never get the correct
new_cpus, and new_cpus will get smaller and smaller after each round
of offline/online(because cs->cpus_allowed is getting smaller and
smaller).
This problem can be reproduced by:

1. echo 0 > /sys/devices/system/cpu/cpu2/online
2. echo 1 > /sys/devices/system/cpu/cpu2/online
3. taskset -c 2 ls
taskset: failed to set pid 0's affinity: Invalid argument

This patch works around this problem by introducing a new
mask cpumask_var_t cpus_sysfs inside struct cpuset,
which will only be updated by writing value to sysfs.cpuset.cpus,
and CPU offline/online will use this mask to set the new cpumask
for a cpuset.

Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
 kernel/cpuset.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 02a8ea5..49c9cd5 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -106,6 +106,12 @@ struct cpuset {
 	nodemask_t effective_mems;
 
 	/*
+	 * This cpumask can only be modified by sysfs - cpuset.cpus,
+	 * and will not change during cpu online/offline.
+	 */
+	cpumask_var_t cpus_sysfs;
+
+	/*
 	 * This is old Memory Nodes tasks took on.
 	 *
 	 * - top_cpuset.old_mems_allowed is initialized to mems_allowed.
@@ -963,6 +969,7 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 
 	spin_lock_irq(&callback_lock);
 	cpumask_copy(cs->cpus_allowed, trialcs->cpus_allowed);
+	cpumask_copy(cs->cpus_sysfs, trialcs->cpus_allowed);
 	spin_unlock_irq(&callback_lock);
 
 	/* use trialcs->cpus_allowed as a temp variable */
@@ -1922,17 +1929,22 @@ cpuset_css_alloc(struct cgroup_subsys_state *parent_css)
 		goto free_cs;
 	if (!alloc_cpumask_var(&cs->effective_cpus, GFP_KERNEL))
 		goto free_cpus;
+	if (!alloc_cpumask_var(&cs->cpus_sysfs, GFP_KERNEL))
+		goto free_effective;
 
 	set_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);
 	cpumask_clear(cs->cpus_allowed);
 	nodes_clear(cs->mems_allowed);
 	cpumask_clear(cs->effective_cpus);
 	nodes_clear(cs->effective_mems);
+	cpumask_clear(cs->cpus_sysfs);
 	fmeter_init(&cs->fmeter);
 	cs->relax_domain_level = -1;
 
 	return &cs->css;
 
+free_effective:
+	free_cpumask_var(cs->effective_cpus);
 free_cpus:
 	free_cpumask_var(cs->cpus_allowed);
 free_cs:
@@ -1997,6 +2009,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
 	cs->effective_mems = parent->mems_allowed;
 	cpumask_copy(cs->cpus_allowed, parent->cpus_allowed);
 	cpumask_copy(cs->effective_cpus, parent->cpus_allowed);
+	cpumask_copy(cs->cpus_sysfs, parent->cpus_allowed);
 	spin_unlock_irq(&callback_lock);
 out_unlock:
 	mutex_unlock(&cpuset_mutex);
@@ -2030,6 +2043,7 @@ static void cpuset_css_free(struct cgroup_subsys_state *css)
 
 	free_cpumask_var(cs->effective_cpus);
 	free_cpumask_var(cs->cpus_allowed);
+	free_cpumask_var(cs->cpus_sysfs);
 	kfree(cs);
 }
 
@@ -2078,11 +2092,14 @@ int __init cpuset_init(void)
 		BUG();
 	if (!alloc_cpumask_var(&top_cpuset.effective_cpus, GFP_KERNEL))
 		BUG();
+	if (!alloc_cpumask_var(&top_cpuset.cpus_sysfs, GFP_KERNEL))
+		BUG();
 
 	cpumask_setall(top_cpuset.cpus_allowed);
 	nodes_setall(top_cpuset.mems_allowed);
 	cpumask_setall(top_cpuset.effective_cpus);
 	nodes_setall(top_cpuset.effective_mems);
+	cpumask_setall(top_cpuset.cpus_sysfs);
 
 	fmeter_init(&top_cpuset.fmeter);
 	set_bit(CS_SCHED_LOAD_BALANCE, &top_cpuset.flags);
@@ -2213,7 +2230,10 @@ retry:
 		goto retry;
 	}
 
-	cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
+	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys))
+		cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
+	else
+		cpumask_and(&new_cpus, cs->cpus_sysfs, parent_cs(cs)->effective_cpus);
 	nodes_and(new_mems, cs->mems_allowed, parent_cs(cs)->effective_mems);
 
 	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
@@ -2354,6 +2374,8 @@ void __init cpuset_init_smp(void)
 	cpumask_copy(top_cpuset.effective_cpus, cpu_active_mask);
 	top_cpuset.effective_mems = node_states[N_MEMORY];
 
+	cpumask_copy(top_cpuset.cpus_sysfs, cpu_active_mask);
+
 	register_hotmemory_notifier(&cpuset_track_online_nodes_nb);
 }
 
-- 
1.8.4.2

[PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Chen Yu]

Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective masks")
leverages cpuset's cpus_allowed and its parent's effective_cpus to calculate
the new_cpus by:

cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);

However cpus_allowed will also be updated after the CPU is offline, in
hotplug_update_tasks_legacy, so when the CPU is online again, it will use
the old cpus_allowed mask to calculate the new_cpus, thus new_cpus will get
incorrect value after each round of offline/online.

This problem is found on ubuntu 15.10 with cpuset mounted:

1. echo 0 > /sys/devices/system/cpu/cpu2/online
2. echo 1 > /sys/devices/system/cpu/cpu2/online
3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
   0-3
4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
   0-1,3
5. taskset -c 2 ls
   taskset: failed to set pid 0's affinity: Invalid argument

This patch works around this problem by introducing a new
mask cpumask_var_t cpus_sysfs inside struct cpuset,
which will only be updated by writing value to sysfs.cpuset.cpus,
and CPU offline/online will use this mask to set the new cpumask
for a cpuset.

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vishnu Pratap Singh <vishnu.ps@samsung.com>
Cc: Pintu Kumar <pintu.k@samsung.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: cgroups@vger.kernel.org
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
 kernel/cpuset.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 02a8ea5..49c9cd5 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -106,6 +106,12 @@ struct cpuset {
 	nodemask_t effective_mems;
 
 	/*
+	 * This cpumask can only be modified by sysfs - cpuset.cpus,
+	 * and will not change during cpu online/offline.
+	 */
+	cpumask_var_t cpus_sysfs;
+
+	/*
 	 * This is old Memory Nodes tasks took on.
 	 *
 	 * - top_cpuset.old_mems_allowed is initialized to mems_allowed.
@@ -963,6 +969,7 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 
 	spin_lock_irq(&callback_lock);
 	cpumask_copy(cs->cpus_allowed, trialcs->cpus_allowed);
+	cpumask_copy(cs->cpus_sysfs, trialcs->cpus_allowed);
 	spin_unlock_irq(&callback_lock);
 
 	/* use trialcs->cpus_allowed as a temp variable */
@@ -1922,17 +1929,22 @@ cpuset_css_alloc(struct cgroup_subsys_state *parent_css)
 		goto free_cs;
 	if (!alloc_cpumask_var(&cs->effective_cpus, GFP_KERNEL))
 		goto free_cpus;
+	if (!alloc_cpumask_var(&cs->cpus_sysfs, GFP_KERNEL))
+		goto free_effective;
 
 	set_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);
 	cpumask_clear(cs->cpus_allowed);
 	nodes_clear(cs->mems_allowed);
 	cpumask_clear(cs->effective_cpus);
 	nodes_clear(cs->effective_mems);
+	cpumask_clear(cs->cpus_sysfs);
 	fmeter_init(&cs->fmeter);
 	cs->relax_domain_level = -1;
 
 	return &cs->css;
 
+free_effective:
+	free_cpumask_var(cs->effective_cpus);
 free_cpus:
 	free_cpumask_var(cs->cpus_allowed);
 free_cs:
@@ -1997,6 +2009,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
 	cs->effective_mems = parent->mems_allowed;
 	cpumask_copy(cs->cpus_allowed, parent->cpus_allowed);
 	cpumask_copy(cs->effective_cpus, parent->cpus_allowed);
+	cpumask_copy(cs->cpus_sysfs, parent->cpus_allowed);
 	spin_unlock_irq(&callback_lock);
 out_unlock:
 	mutex_unlock(&cpuset_mutex);
@@ -2030,6 +2043,7 @@ static void cpuset_css_free(struct cgroup_subsys_state *css)
 
 	free_cpumask_var(cs->effective_cpus);
 	free_cpumask_var(cs->cpus_allowed);
+	free_cpumask_var(cs->cpus_sysfs);
 	kfree(cs);
 }
 
@@ -2078,11 +2092,14 @@ int __init cpuset_init(void)
 		BUG();
 	if (!alloc_cpumask_var(&top_cpuset.effective_cpus, GFP_KERNEL))
 		BUG();
+	if (!alloc_cpumask_var(&top_cpuset.cpus_sysfs, GFP_KERNEL))
+		BUG();
 
 	cpumask_setall(top_cpuset.cpus_allowed);
 	nodes_setall(top_cpuset.mems_allowed);
 	cpumask_setall(top_cpuset.effective_cpus);
 	nodes_setall(top_cpuset.effective_mems);
+	cpumask_setall(top_cpuset.cpus_sysfs);
 
 	fmeter_init(&top_cpuset.fmeter);
 	set_bit(CS_SCHED_LOAD_BALANCE, &top_cpuset.flags);
@@ -2213,7 +2230,10 @@ retry:
 		goto retry;
 	}
 
-	cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
+	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys))
+		cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
+	else
+		cpumask_and(&new_cpus, cs->cpus_sysfs, parent_cs(cs)->effective_cpus);
 	nodes_and(new_mems, cs->mems_allowed, parent_cs(cs)->effective_mems);
 
 	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
@@ -2354,6 +2374,8 @@ void __init cpuset_init_smp(void)
 	cpumask_copy(top_cpuset.effective_cpus, cpu_active_mask);
 	top_cpuset.effective_mems = node_states[N_MEMORY];
 
+	cpumask_copy(top_cpuset.cpus_sysfs, cpu_active_mask);
+
 	register_hotmemory_notifier(&cpuset_track_online_nodes_nb);
 }
 
-- 
1.8.4.2

RE: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Chen, Yu C]

Previous one seems to be dropped by maillist so I resend it,
sorry for any inconvenience and happy new year : - )

thanks,
Yu


> -----Original Message-----
> From: Chen, Yu C
> Sent: Friday, January 01, 2016 8:09 PM
> To: cgroups@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org; Chen, Yu C; Vlastimil Babka; Rik van Riel;
> Joonsoo Kim; David Rientjes; Vishnu Pratap Singh; Pintu Kumar; Michal
> Nazarewicz; Mel Gorman; Gortmaker, Paul (Wind River); Peter Zijlstra; Tim
> Chen; Hugh Dickins; Li Zefan; Tejun Heo
> Subject: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs
> 
> Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective
> masks") leverages cpuset's cpus_allowed and its parent's effective_cpus to
> calculate the new_cpus by:
> 
> cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)-
> >effective_cpus);
> 
> However cpus_allowed will also be updated after the CPU is offline, in
> hotplug_update_tasks_legacy, so when the CPU is online again, it will use
> the old cpus_allowed mask to calculate the new_cpus, thus new_cpus will
> get incorrect value after each round of offline/online.
> 
> This problem is found on ubuntu 15.10 with cpuset mounted:
> 
> 1. echo 0 > /sys/devices/system/cpu/cpu2/online
> 2. echo 1 > /sys/devices/system/cpu/cpu2/online
> 3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
>    0-3
> 4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
>    0-1,3
> 5. taskset -c 2 ls
>    taskset: failed to set pid 0's affinity: Invalid argument
> 
> This patch works around this problem by introducing a new mask
> cpumask_var_t cpus_sysfs inside struct cpuset, which will only be updated
> by writing value to sysfs.cpuset.cpus, and CPU offline/online will use this
> mask to set the new cpumask for a cpuset.
> 
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Vishnu Pratap Singh <vishnu.ps@samsung.com>
> Cc: Pintu Kumar <pintu.k@samsung.com>
> Cc: Michal Nazarewicz <mina86@mina86.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: cgroups@vger.kernel.org
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> ---
>  kernel/cpuset.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c index 02a8ea5..49c9cd5 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -106,6 +106,12 @@ struct cpuset {
>  	nodemask_t effective_mems;
> 
>  	/*
> +	 * This cpumask can only be modified by sysfs - cpuset.cpus,
> +	 * and will not change during cpu online/offline.
> +	 */
> +	cpumask_var_t cpus_sysfs;
> +
> +	/*
>  	 * This is old Memory Nodes tasks took on.
>  	 *
>  	 * - top_cpuset.old_mems_allowed is initialized to mems_allowed.
> @@ -963,6 +969,7 @@ static int update_cpumask(struct cpuset *cs, struct
> cpuset *trialcs,
> 
>  	spin_lock_irq(&callback_lock);
>  	cpumask_copy(cs->cpus_allowed, trialcs->cpus_allowed);
> +	cpumask_copy(cs->cpus_sysfs, trialcs->cpus_allowed);
>  	spin_unlock_irq(&callback_lock);
> 
>  	/* use trialcs->cpus_allowed as a temp variable */ @@ -1922,17
> +1929,22 @@ cpuset_css_alloc(struct cgroup_subsys_state *parent_css)
>  		goto free_cs;
>  	if (!alloc_cpumask_var(&cs->effective_cpus, GFP_KERNEL))
>  		goto free_cpus;
> +	if (!alloc_cpumask_var(&cs->cpus_sysfs, GFP_KERNEL))
> +		goto free_effective;
> 
>  	set_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);
>  	cpumask_clear(cs->cpus_allowed);
>  	nodes_clear(cs->mems_allowed);
>  	cpumask_clear(cs->effective_cpus);
>  	nodes_clear(cs->effective_mems);
> +	cpumask_clear(cs->cpus_sysfs);
>  	fmeter_init(&cs->fmeter);
>  	cs->relax_domain_level = -1;
> 
>  	return &cs->css;
> 
> +free_effective:
> +	free_cpumask_var(cs->effective_cpus);
>  free_cpus:
>  	free_cpumask_var(cs->cpus_allowed);
>  free_cs:
> @@ -1997,6 +2009,7 @@ static int cpuset_css_online(struct
> cgroup_subsys_state *css)
>  	cs->effective_mems = parent->mems_allowed;
>  	cpumask_copy(cs->cpus_allowed, parent->cpus_allowed);
>  	cpumask_copy(cs->effective_cpus, parent->cpus_allowed);
> +	cpumask_copy(cs->cpus_sysfs, parent->cpus_allowed);
>  	spin_unlock_irq(&callback_lock);
>  out_unlock:
>  	mutex_unlock(&cpuset_mutex);
> @@ -2030,6 +2043,7 @@ static void cpuset_css_free(struct
> cgroup_subsys_state *css)
> 
>  	free_cpumask_var(cs->effective_cpus);
>  	free_cpumask_var(cs->cpus_allowed);
> +	free_cpumask_var(cs->cpus_sysfs);
>  	kfree(cs);
>  }
> 
> @@ -2078,11 +2092,14 @@ int __init cpuset_init(void)
>  		BUG();
>  	if (!alloc_cpumask_var(&top_cpuset.effective_cpus, GFP_KERNEL))
>  		BUG();
> +	if (!alloc_cpumask_var(&top_cpuset.cpus_sysfs, GFP_KERNEL))
> +		BUG();
> 
>  	cpumask_setall(top_cpuset.cpus_allowed);
>  	nodes_setall(top_cpuset.mems_allowed);
>  	cpumask_setall(top_cpuset.effective_cpus);
>  	nodes_setall(top_cpuset.effective_mems);
> +	cpumask_setall(top_cpuset.cpus_sysfs);
> 
>  	fmeter_init(&top_cpuset.fmeter);
>  	set_bit(CS_SCHED_LOAD_BALANCE, &top_cpuset.flags); @@ -2213,7
> +2230,10 @@ retry:
>  		goto retry;
>  	}
> 
> -	cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)-
> >effective_cpus);
> +	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys))
> +		cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)-
> >effective_cpus);
> +	else
> +		cpumask_and(&new_cpus, cs->cpus_sysfs,
> +parent_cs(cs)->effective_cpus);
>  	nodes_and(new_mems, cs->mems_allowed, parent_cs(cs)-
> >effective_mems);
> 
>  	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
> @@ -2354,6 +2374,8 @@ void __init cpuset_init_smp(void)
>  	cpumask_copy(top_cpuset.effective_cpus, cpu_active_mask);
>  	top_cpuset.effective_mems = node_states[N_MEMORY];
> 
> +	cpumask_copy(top_cpuset.cpus_sysfs, cpu_active_mask);
> +
>  	register_hotmemory_notifier(&cpuset_track_online_nodes_nb);
>  }
> 
> --
> 1.8.4.2

Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Tejun Heo]

On Fri, Jan 01, 2016 at 08:09:13PM +0800, Chen Yu wrote:
> Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective masks")
> leverages cpuset's cpus_allowed and its parent's effective_cpus to calculate
> the new_cpus by:
> 
> cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
> 
> However cpus_allowed will also be updated after the CPU is offline, in
> hotplug_update_tasks_legacy, so when the CPU is online again, it will use
> the old cpus_allowed mask to calculate the new_cpus, thus new_cpus will get
> incorrect value after each round of offline/online.
> 
> This problem is found on ubuntu 15.10 with cpuset mounted:
> 
> 1. echo 0 > /sys/devices/system/cpu/cpu2/online
> 2. echo 1 > /sys/devices/system/cpu/cpu2/online
> 3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
>    0-3
> 4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
>    0-1,3
> 5. taskset -c 2 ls
>    taskset: failed to set pid 0's affinity: Invalid argument
> 
> This patch works around this problem by introducing a new
> mask cpumask_var_t cpus_sysfs inside struct cpuset,
> which will only be updated by writing value to sysfs.cpuset.cpus,
> and CPU offline/online will use this mask to set the new cpumask
> for a cpuset.

Li?

-- 
tejun

Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Zefan Li]

On 2016/1/1 20:09, Chen Yu wrote:
> Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective masks")
> leverages cpuset's cpus_allowed and its parent's effective_cpus to calculate
> the new_cpus by:
>
> cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
>
> However cpus_allowed will also be updated after the CPU is offline, in
> hotplug_update_tasks_legacy, so when the CPU is online again, it will use
> the old cpus_allowed mask to calculate the new_cpus, thus new_cpus will get
> incorrect value after each round of offline/online.
>
> This problem is found on ubuntu 15.10 with cpuset mounted:
>
> 1. echo 0 > /sys/devices/system/cpu/cpu2/online
> 2. echo 1 > /sys/devices/system/cpu/cpu2/online
> 3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
>     0-3
> 4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
>     0-1,3
> 5. taskset -c 2 ls
>     taskset: failed to set pid 0's affinity: Invalid argument
>

This is the expected behavior...In legacy hierachy onlining an offlined cpu
won't restore cpuset configurations automatically.

Commit be4c9dd7aee5 changed this behavior, but only for unified hierachy.

RE: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Chen, Yu C]

Hi Zefan,

> -----Original Message-----
> From: Zefan Li [mailto:lizefan@huawei.com]
> Sent: Monday, January 04, 2016 9:52 AM
> To: Chen, Yu C; cgroups@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org; Vlastimil Babka; Rik van Riel; Joonsoo Kim;
> David Rientjes; Vishnu Pratap Singh; Pintu Kumar; Michal Nazarewicz; Mel
> Gorman; Gortmaker, Paul (Wind River); Peter Zijlstra; Tim Chen; Hugh Dickins;
> Tejun Heo
> Subject: Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs
> 
> On 2016/1/1 20:09, Chen Yu wrote:
> > Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective
> > masks") leverages cpuset's cpus_allowed and its parent's
> > effective_cpus to calculate the new_cpus by:
> >
> > cpumask_and(&new_cpus, cs->cpus_allowed,
> > parent_cs(cs)->effective_cpus);
> >
> > However cpus_allowed will also be updated after the CPU is offline, in
> > hotplug_update_tasks_legacy, so when the CPU is online again, it will
> > use the old cpus_allowed mask to calculate the new_cpus, thus new_cpus
> > will get incorrect value after each round of offline/online.
> >
> > This problem is found on ubuntu 15.10 with cpuset mounted:
> >
> > 1. echo 0 > /sys/devices/system/cpu/cpu2/online
> > 2. echo 1 > /sys/devices/system/cpu/cpu2/online
> > 3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
> >     0-3
> > 4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
> >     0-1,3
> > 5. taskset -c 2 ls
> >     taskset: failed to set pid 0's affinity: Invalid argument
> >
> 
> This is the expected behavior...In legacy hierachy onlining an offlined cpu
> won't restore cpuset configurations automatically.
[Yu] Ah, I see, we were just a little confused with it before.
 So if we want to online an offlined cpu in legacy hierarchy, we
should not only echo 1 to 'online' sysfs, but also restore the mask manually? 
It seems that we should not rely on /sys/devices/system/cpu/cpu2/online,
if cpuset has been mounted.
> 
> Commit be4c9dd7aee5 changed this behavior, but only for unified hierachy.

thanks,
yu

Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Zefan Li]

>> On 2016/1/1 20:09, Chen Yu wrote:
>>> Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective
>>> masks") leverages cpuset's cpus_allowed and its parent's
>>> effective_cpus to calculate the new_cpus by:
>>>
>>> cpumask_and(&new_cpus, cs->cpus_allowed,
>>> parent_cs(cs)->effective_cpus);
>>>
>>> However cpus_allowed will also be updated after the CPU is offline, in
>>> hotplug_update_tasks_legacy, so when the CPU is online again, it will
>>> use the old cpus_allowed mask to calculate the new_cpus, thus new_cpus
>>> will get incorrect value after each round of offline/online.
>>>
>>> This problem is found on ubuntu 15.10 with cpuset mounted:
>>>
>>> 1. echo 0 > /sys/devices/system/cpu/cpu2/online
>>> 2. echo 1 > /sys/devices/system/cpu/cpu2/online
>>> 3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
>>>      0-3
>>> 4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
>>>      0-1,3
>>> 5. taskset -c 2 ls
>>>      taskset: failed to set pid 0's affinity: Invalid argument
>>>
>>
>> This is the expected behavior...In legacy hierachy onlining an offlined cpu
>> won't restore cpuset configurations automatically.
> [Yu] Ah, I see, we were just a little confused with it before.
>   So if we want to online an offlined cpu in legacy hierarchy, we
> should not only echo 1 to 'online' sysfs, but also restore the mask manually?

Right. You have to take care of this by yourself.

> It seems that we should not rely on /sys/devices/system/cpu/cpu2/online,
> if cpuset has been mounted.