Re: [RFC PATCH 00/16] Core scheduling v6

[Li, Aubrey]

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.

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).
>>>>>
>>>>
>>