Re: [PATCH v3 1/7] x86/resctrl: Add multiple tasks to the resctrl group at once

["Moger, Babu" <babu.moger@amd.com>]

Hi Reinette,

On 3/20/23 11:52, Reinette Chatre wrote:
> Hi Babu,
> 
> On 3/20/2023 8:07 AM, Moger, Babu wrote:
>> On 3/16/23 15:33, Reinette Chatre wrote:
>>> On 3/16/2023 12:51 PM, Moger, Babu wrote:
>>>> On 3/16/23 12:12, Reinette Chatre wrote:
>>>>> On 3/16/2023 9:27 AM, Moger, Babu wrote:
>>>>>>> -----Original Message-----
>>>>>>> From: Reinette Chatre <reinette.chatre@intel.com>
>>>>>>> Sent: Wednesday, March 15, 2023 1:33 PM
>>>>>>> To: Moger, Babu <Babu.Moger@amd.com>; corbet@lwn.net;
>>>>>>> tglx@linutronix.de; mingo@redhat.com; bp@alien8.de
>>>>>>> Cc: fenghua.yu@intel.com; dave.hansen@linux.intel.com; x86@kernel.org;
>>>>>>> hpa@zytor.com; paulmck@kernel.org; akpm@linux-foundation.org;
>>>>>>> quic_neeraju@quicinc.com; rdunlap@infradead.org;
>>>>>>> damien.lemoal@opensource.wdc.com; songmuchun@bytedance.com;
>>>>>>> peterz@infradead.org; jpoimboe@kernel.org; pbonzini@redhat.com;
>>>>>>> chang.seok.bae@intel.com; pawan.kumar.gupta@linux.intel.com;
>>>>>>> jmattson@google.com; daniel.sneddon@linux.intel.com; Das1, Sandipan
>>>>>>> <Sandipan.Das@amd.com>; tony.luck@intel.com; james.morse@arm.com;
>>>>>>> linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org;
>>>>>>> bagasdotme@gmail.com; eranian@google.com; christophe.leroy@csgroup.eu;
>>>>>>> jarkko@kernel.org; adrian.hunter@intel.com; quic_jiles@quicinc.com;
>>>>>>> peternewman@google.com
>>>>>>> Subject: Re: [PATCH v3 1/7] x86/resctrl: Add multiple tasks to the resctrl group
>>>>>>> at once
>>>>>>>
>>>>>>> Hi Babu,
>>>>>>>
>>>>>>> On 3/2/2023 12:24 PM, Babu Moger wrote:
>>>>>>>> The resctrl task assignment for MONITOR or CONTROL group needs to be
>>>>>>>> done one at a time. For example:
>>>>>>>>
>>>>>>>>   $mount -t resctrl resctrl /sys/fs/resctrl/
>>>>>>>>   $mkdir /sys/fs/resctrl/clos1
>>>>>>>>   $echo 123 > /sys/fs/resctrl/clos1/tasks
>>>>>>>>   $echo 456 > /sys/fs/resctrl/clos1/tasks
>>>>>>>>   $echo 789 > /sys/fs/resctrl/clos1/tasks
>>>>>>>>
>>>>>>>> This is not user-friendly when dealing with hundreds of tasks. Also,
>>>>>>>> there is a syscall overhead for each command executed from user space.
>>>>>>>
>>>>>>> To support this change it may also be helpful to add that moving tasks take the
>>>>>>> mutex so attempting to move tasks in parallel will not achieve a significant
>>>>>>> performance gain.
>>>>>>
>>>>>> Agree. It may not be significant performance gain.  Will remove this line. 
>>>>>
>>>>> It does not sound as though you are actually responding to my comment.
>>>>
>>>> I am confused. I am already saying there is syscall overhead for each
>>>> command if we move the tasks one by one. Now do you want me to add "moving
>>>> tasks take the mutex so attempting to move tasks in parallel will not
>>>> achieve a significant performance gain".
>>>>
>>>> It is contradictory, So, I wanted to remove the line about performance.
>>>> Did I still miss something?
>>>
>>> Where is the contradiction?
>>>
>>> Consider your example:
>>>    $echo 123 > /sys/fs/resctrl/clos1/tasks
>>>    $echo 456 > /sys/fs/resctrl/clos1/tasks
>>>    $echo 789 > /sys/fs/resctrl/clos1/tasks
>>>
>>> Yes, there is syscall overhead for each of the above lines. My statement was in
>>> support of this work by stating that a user aiming to improve performance by
>>> attempting the above in parallel would not be able to see achieve significant
>>> performance gain since the calls would end up being serialized.
>>
>> ok. Sure. Will add the text. I may modify little bit.
>>>
>>> You are providing two motivations (a) "user-friendly when dealing with
>>> hundreds of tasks", and (b) syscall overhead. Have you measured the
>>> improvement this solution provides?
>>
>> No. I have not measured the performance improvement.
> 
> The changelog makes a claim that the current implementation has overhead
> that is removed with this change. There is no data to support this claim.

My main motivation for this change is to make it user-friendly. So that
users can search the pid's and assign multiple tasks at a time. Originally
I did not have the line for performance. Actually, I don't want to claim
performance benefits. I will remove the performance claims.

> 
> ...
> 
>>>>>>>> +
>>>>>>>> +	buf[nbytes - 1] = '\0';
>>>>>>>> +
>>>>>>>>  	rdtgrp = rdtgroup_kn_lock_live(of->kn);
>>>>>>>>  	if (!rdtgrp) {
>>>>>>>>  		rdtgroup_kn_unlock(of->kn);
>>>>>>>>  		return -ENOENT;
>>>>>>>>  	}
>>>>>>>> +
>>>>>>>> +next:
>>>>>>>> +	if (!buf || buf[0] == '\0')
>>>>>>>> +		goto unlock;
>>>>>>>> +
>>>>>>>> +	pid_str = strim(strsep(&buf, ","));
>>>>>>>> +
>>>>>>>
>>>>>>> Could lib/cmdline.c:get_option() be useful?
>>>>>>
>>>>>> Yes. We could that also. May not be required for the simple case like this.
>>>>>
>>>>> Please keep an eye out for how much of it you end up duplicating ....
>>>>
>>>> Using the get_options will require at least two calls(one to get the
>>>> length and then read the integers). Also need to allocate the integers
>>>> array dynamically. That is lot code if we are going that route.
>>>>
>>>
>>> I did not ask about get_options(), I asked about get_option().
>>
>> If you insist, will use get_option. But we still have to loop thru all the
>> string till get_option returns 0. I can try that.
> 
> 
> I just asked whether get_option() could be useful. Could you please point out what
> I said that made you think that I insist on this change being made? If it matches
> your usage, then know it is available, if it does not, then don't use it.

Ok. I dont see a major benefit using get_option here. So, not planning to
to use it.

> 
> ...
> 
>>>> I can say "The failure pid will be logged in
>>>> /sys/fs/resctrl/info/last_cmd_status file."
>>>
>>> That will not be accurate. Not all errors include the pid.
>>
>> Can you please suggest?
> 
> last_cmd_status provides a 512 char buffer to communicate details
> to the user. The buffer is cleared before the loop that moves all the
> tasks start. If an error is encountered, a detailed message is written
> to the buffer. One option may be to append a string to the buffer that
> includes the pid? Perhaps something like:
> 	rdt_last_cmd_printf("Error encountered while moving task %d\n", pid);

ok. Will try to add and test it.

> 
> Please feel free to improve.
> 
> Reinette
> 
> 

-- 
Thanks
Babu Moger