Date:   Fri, 14 Jul 2023 17:42:26 -0500
From:   "Moger, Babu" <babu.moger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Cc:     fenghua.yu@...el.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, paulmck@...nel.org, akpm@...ux-foundation.org,
        quic_neeraju@...cinc.com, rdunlap@...radead.org,
        damien.lemoal@...nsource.wdc.com, songmuchun@...edance.com,
        peterz@...radead.org, jpoimboe@...nel.org, pbonzini@...hat.com,
        chang.seok.bae@...el.com, pawan.kumar.gupta@...ux.intel.com,
        jmattson@...gle.com, daniel.sneddon@...ux.intel.com,
        sandipan.das@....com, tony.luck@...el.com, james.morse@....com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        bagasdotme@...il.com, eranian@...gle.com,
        christophe.leroy@...roup.eu, jarkko@...nel.org,
        adrian.hunter@...el.com, quic_jiles@...cinc.com,
        peternewman@...gle.com
Subject: Re: [PATCH v5 7/8] x86/resctrl: Move default control group creation
 during mount

Hi Reinette,

On 7/14/23 16:54, Reinette Chatre wrote:
> Hi Babu,
> 
> On 7/14/2023 9:26 AM, Moger, Babu wrote:
>> Hi Reinette,
>> Sorry.. Took a while to respond. I had to recreate the issue to refresh my
>> memory.
> 
> No problem!
> 
>> On 7/7/23 16:46, Reinette Chatre wrote:
>>> Hi Babu,
>>>
>>> On 6/1/2023 12:02 PM, Babu Moger wrote:
> 
> 
>>>>  	ctx = kzalloc(sizeof(struct rdt_fs_context), GFP_KERNEL);
>>>> -	if (!ctx)
>>>> +	if (!ctx) {
>>>> +		kernfs_destroy_root(rdt_root);
>>>>  		return -ENOMEM;
>>>> +	}
>>>>  
>>>>  	ctx->kfc.root = rdt_root;
>>>>  	ctx->kfc.magic = RDTGROUP_SUPER_MAGIC;
>>>> @@ -2845,6 +2860,9 @@ static void rdt_kill_sb(struct super_block *sb)
>>>>  	static_branch_disable_cpuslocked(&rdt_alloc_enable_key);
>>>>  	static_branch_disable_cpuslocked(&rdt_mon_enable_key);
>>>>  	static_branch_disable_cpuslocked(&rdt_enable_key);
>>>> +	/* Remove the default group and cleanup the root */
>>>> +	list_del(&rdtgroup_default.rdtgroup_list);
>>>> +	kernfs_destroy_root(rdt_root);
>>>
>>> Why not just add kernfs_remove(rdtgroup_default.kn) to rmdir_all_sub()?
>>
>> List rdtgroup_default.rdtgroup_list is added during the mount and had to
>> be removed during umount and rdt_root is destroyed here.
> 
> I do not think it is required for default resource group management to
> be tied with the resctrl files associated with default resource group.
> 
> I think rdtgroup_setup_root can be split in two, one for all the
> resctrl files that should be done at mount/unmount and one for the
> default group init done at __init.

Ok.
> 
>>>>  	kernfs_kill_sb(sb);
>>>>  	mutex_unlock(&rdtgroup_mutex);
>>>>  	cpus_read_unlock();
>>>> @@ -3598,10 +3616,8 @@ static struct kernfs_syscall_ops rdtgroup_kf_syscall_ops = {
>>>>  	.show_options	= rdtgroup_show_options,
>>>>  };
>>>>  
>>>> -static int __init rdtgroup_setup_root(void)
>>>> +static int rdtgroup_setup_root(void)
>>>>  {
>>>> -	int ret;
>>>> -
>>>>  	rdt_root = kernfs_create_root(&rdtgroup_kf_syscall_ops,
>>>>  				      KERNFS_ROOT_CREATE_DEACTIVATED |
>>>>  				      KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK,
>>>> @@ -3618,19 +3634,11 @@ static int __init rdtgroup_setup_root(void)
>>>>  
>>>>  	list_add(&rdtgroup_default.rdtgroup_list, &rdt_all_groups);
>>>>  
>>>> -	ret = rdtgroup_add_files(kernfs_root_to_node(rdt_root), RFTYPE_CTRL_BASE);
>>>> -	if (ret) {
>>>> -		kernfs_destroy_root(rdt_root);
>>>> -		goto out;
>>>> -	}
>>>> -
>>>>  	rdtgroup_default.kn = kernfs_root_to_node(rdt_root);
>>>> -	kernfs_activate(rdtgroup_default.kn);
>>>>  
>>>> -out:
>>>>  	mutex_unlock(&rdtgroup_mutex);
>>>>  
>>>> -	return ret;
>>>> +	return 0;
>>>>  }
>>>>  
>>>>  static void domain_destroy_mon_state(struct rdt_domain *d)
>>>> @@ -3752,13 +3760,9 @@ int __init rdtgroup_init(void)
>>>>  	seq_buf_init(&last_cmd_status, last_cmd_status_buf,
>>>>  		     sizeof(last_cmd_status_buf));
>>>>  
>>>> -	ret = rdtgroup_setup_root();
>>>> -	if (ret)
>>>> -		return ret;
>>>> -
>>>>  	ret = sysfs_create_mount_point(fs_kobj, "resctrl");
>>>>  	if (ret)
>>>> -		goto cleanup_root;
>>>> +		return ret;
>>>>  
>>>
>>> It is not clear to me why this change is required, could you
>>> please elaborate? It seems that all that is needed is for 
>>> rdtgroup_add_files() to move to rdt_get_tree() (which you have done)
>>> and then an additional call to kernfs_remove() in rmdir_all_sub().
>>> I must be missing something, could you please help me understand?
>>>
>>
>> Yes. I started with that approach. But there are issues with that approach.
>>
>> Currently, rdt_root(which is rdtgroup_default.kn) is created during
>> rdtgroup_init. At the same time the root files are created. Also, default
>> group is added to rdt_all_groups. Basically, the root files and
>> rdtgroup_default group is always there even though filesystem is never
>> mounted. Also mbm_over and cqm_limbo workqueues are always running even
>> though filesystem is not mounted.
>>
>> I changed rdtgroup_add_files() to move to rdt_get_tree() and added
>> kernfs_remove() in rmdir_all_sub(). This caused problems. The
>> kernfs_remove(rdtgroup_default.kn) removes all the reference counts and
>> releases the root. When we mount again, we hit this this problem below.
>>
>> [  404.558461] ------------[ cut here ]------------
>> [  404.563631] WARNING: CPU: 35 PID: 7728 at fs/kernfs/dir.c:522
>> kernfs_new_node+0x63/0x70
>>
>> 404.778793]  ? __warn+0x81/0x140
>> [  404.782535]  ? kernfs_new_node+0x63/0x70
>> [  404.787036]  ? report_bug+0x102/0x200
>> [  404.791247]  ? handle_bug+0x3f/0x70
>> [  404.795269]  ? exc_invalid_op+0x13/0x60
>> [  404.799671]  ? asm_exc_invalid_op+0x16/0x20
>> [  404.804461]  ? kernfs_new_node+0x63/0x70
>> [  404.808954]  ? snprintf+0x49/0x70
>> [  404.812762]  __kernfs_create_file+0x30/0xc0
>> [  404.817534]  rdtgroup_add_files+0x6c/0x100
>>
>> Basically kernel says your rdt_root is not initialized. That is the reason
>> I had to move everything to mount time. The rdt_root is created and
>> initialized during the mount and also destroyed during the umount.
>> And I had to move rdt_enable_key check during rdt_root creation.
>>
> 
> ok, thank you for the additional details. I see now how this patch evolved.
> I understand how rdt_root needs to be created/destroyed
> during mount/unmount. If I understand correctly the changes to
> rdt_init_fs_context() was motivated by this line:
> 
> 	ctx->kfc.root = rdt_root;
> 
> ... that prompted you to move rdt_root creation there in order to have
> it present for this assignment and that prompted the
> rdt_enable_key check to follow. Is this correct?

That is correct.

> 
> I am concerned about the changes to rdt_init_fs_context() since it further
> separates the resctrl file management, it breaks the symmetry of the
> key checked and set, and finally these new actions seem unrelated to a function
> named "init_fs_context". I looked at other examples and from what I can tell
> it is not required that ctx->kfc.root be initialized within
> rdt_init_fs_context(). Looks like the value is required by kernfs_get_tree()
> that is called from rdt_get_tree(). For comparison I found cgroup_do_get_tree().
> Note how cgroup_do_get_tree(), within the .get_tree callback,
> initializes kernfs_fs_context.root and then call kernfs_get_tree()? 

Yes. I see that. Thanks for pointing.

> 
> It thus looks to me as though things can be simplified significantly
> if the kernfs_fs_context.root assignment is moved from rdt_init_fs_context()
> to rdt_get_tree(). rdt_get_tree() can then create rdt_root (and add all needed
> files), assign it to kernfs_fs_context.root and call kernfs_get_tree().
> 
> What do you think?

Yes. I think we can do that. Let me try it. Will let you know how it goes.
Thanks for the suggestion.
-- 
Thanks
Babu Moger

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