Re: [RFC PATCH 00/16] Core scheduling v6(Internet mail)

[benbjiang(蒋彪)]

> On Aug 13, 2020, at 12:28 PM, Li, Aubrey <aubrey.li@xxxxxxxxxxxxxxx> wrote:
> 
> On 2020/8/13 7:08, Joel Fernandes wrote:
>> On Wed, Aug 12, 2020 at 10:01:24AM +0800, Li, Aubrey wrote:
>>> Hi Joel,
>>> 
>>> On 2020/8/10 0:44, Joel Fernandes wrote:
>>>> Hi Aubrey,
>>>> 
>>>> Apologies for replying late as I was still looking into the details.
>>>> 
>>>> On Wed, Aug 05, 2020 at 11:57:20AM +0800, Li, Aubrey wrote:
>>>> [...]
>>>>> +/*
>>>>> + * Core scheduling policy:
>>>>> + * - CORE_SCHED_DISABLED: core scheduling is disabled.
>>>>> + * - CORE_COOKIE_MATCH: tasks with same cookie can run
>>>>> + *                     on the same core concurrently.
>>>>> + * - CORE_COOKIE_TRUST: trusted task can run with kernel
>>>>> 			thread on the same core concurrently. 
>>>>> + * - CORE_COOKIE_LONELY: tasks with cookie can run only
>>>>> + *                     with idle thread on the same core.
>>>>> + */
>>>>> +enum coresched_policy {
>>>>> +       CORE_SCHED_DISABLED,
>>>>> +       CORE_SCHED_COOKIE_MATCH,
>>>>> +	CORE_SCHED_COOKIE_TRUST,
>>>>> +       CORE_SCHED_COOKIE_LONELY,
>>>>> +};
>>>>> 
>>>>> We can set policy to CORE_COOKIE_TRUST of uperf cgroup and fix this kind
>>>>> of performance regression. Not sure if this sounds attractive?
>>>> 
>>>> Instead of this, I think it can be something simpler IMHO:
>>>> 
>>>> 1. Consider all cookie-0 task as trusted. (Even right now, if you apply the
>>>>   core-scheduling patchset, such tasks will share a core and sniff on each
>>>>   other. So let us not pretend that such tasks are not trusted).
>>>> 
>>>> 2. All kernel threads and idle task would have a cookie 0 (so that will cover
>>>>   ksoftirqd reported in your original issue).
>>>> 
>>>> 3. Add a config option (CONFIG_SCHED_CORE_DEFAULT_TASKS_UNTRUSTED). Default
>>>>   enable it. Setting this option would tag all tasks that are forked from a
>>>>   cookie-0 task with their own cookie. Later on, such tasks can be added to
>>>>   a group. This cover's PeterZ's ask about having 'default untrusted').
>>>>   (Users like ChromeOS that don't want to userspace system processes to be
>>>>   tagged can disable this option so such tasks will be cookie-0).
>>>> 
>>>> 4. Allow prctl/cgroup interfaces to create groups of tasks and override the
>>>>   above behaviors.
>>> 
>>> How does uperf in a cgroup work with ksoftirqd? Are you suggesting I set uperf's
>>> cookie to be cookie-0 via prctl?
>> 
>> Yes, but let me try to understand better. There are 2 problems here I think:
>> 
>> 1. ksoftirqd getting idled when HT is turned on, because uperf is sharing a
>> core with it: This should not be any worse than SMT OFF, because even SMT OFF
>> would also reduce ksoftirqd's CPU time just core sched is doing. Sure
>> core-scheduling adds some overhead with IPIs but such a huge drop of perf is
>> strange. Peter any thoughts on that?
>> 
>> 2. Interface: To solve the performance problem, you are saying you want uperf
>> to share a core with ksoftirqd so that it is not forced into idle.  Why not
>> just keep uperf out of the cgroup?
> 
> I guess this is unacceptable for who runs their apps in container and vm.
IMHO,  just as Joel proposed, 
1. Consider all cookie-0 task as trusted.
2. All kernel threads and idle task would have a cookie 0 
In that way, all tasks with cookies(including uperf in a cgroup) could run
concurrently with kernel threads.
That could be a good solution for the issue. :)

If with CONFIG_SCHED_CORE_DEFAULT_TASKS_UNTRUSTED enabled,
maybe we should set ksoftirqd’s cookie to be cookie-0 to solve the issue. 

Thx.
Regards,
Jiang
> 
> Thanks,
> -Aubrey
> 
>> Then it will have cookie 0 and be able to
>> share core with kernel threads. About user-user isolation that you need, if
>> you tag any "untrusted" threads by adding it to CGroup, then there will
>> automatically isolated from uperf while allowing uperf to share CPU with
>> kernel threads.
>> 
>> Please let me know your thoughts and thanks,
>> 
>> - Joel
>> 
>>> 
>>> Thanks,
>>> -Aubrey
>>>> 
>>>> 5. Document everything clearly so the semantics are clear both to the
>>>>   developers of core scheduling and to system administrators.
>>>> 
>>>> Note that, with the concept of "system trusted cookie", we can also do
>>>> optimizations like:
>>>> 1. Disable STIBP when switching into trusted tasks.
>>>> 2. Disable L1D flushing / verw stuff for L1TF/MDS issues, when switching into
>>>>   trusted tasks.
>>>> 
>>>> At least #1 seems to be biting enabling HT on ChromeOS right now, and one
>>>> other engineer requested I do something like #2 already.
>>>> 
>>>> Once we get full-syscall isolation working, threads belonging to a process
>>>> can also share a core so those can just share a core with the task-group
>>>> leader.
>>>> 
>>>>>> Is the uperf throughput worse with SMT+core-scheduling versus no-SMT ?
>>>>> 
>>>>> This is a good question, from the data we measured by uperf,
>>>>> SMT+core-scheduling is 28.2% worse than no-SMT, :(
>>>> 
>>>> This is worrying for sure. :-(. We ought to debug/profile it more to see what
>>>> is causing the overhead. Me/Vineeth added it as a topic for LPC as well.
>>>> 
>>>> Any other thoughts from others on this?
>>>> 
>>>> thanks,
>>>> 
>>>> - Joel
>>>> 
>>>> 
>>>>>> thanks,
>>>>>> 
>>>>>> - Joel
>>>>>> PS: I am planning to write a patch behind a CONFIG option that tags
>>>>>> all processes (default untrusted) so everything gets a cookie which
>>>>>> some folks said was how they wanted (have a whitelist instead of
>>>>>> blacklist).
>>>>>> 
>>>>> 
>>> 
> 
>