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SubjectRe: [RFC PATCH 00/16] Core scheduling v6(Internet mail)
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> On Aug 13, 2020, at 12:28 PM, Li, Aubrey <aubrey.li@linux.intel.com> 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).
>>>>>>
>>>>>
>>>
>
>

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