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

[Li, Aubrey]

On 2020/8/14 12:04, benbjiang(蒋彪) wrote:
> 
> 
>> On Aug 14, 2020, at 9:36 AM, Li, Aubrey <aubrey.li@xxxxxxxxxxxxxxx> wrote:
>>
>> On 2020/8/14 8:26, benbjiang(蒋彪) wrote:
>>>
>>>
>>>> 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. :)
>>
>> From uperf point of review, it can trust cookie-0(I assume we still need
>> some modifications to change cookie-match to cookie-compatible to allow
>> ZERO and NONZERO run together).
>>
>> But from kernel thread point of review, it can NOT trust uperf, unless
>> we set uperf's cookie to 0.
> That’s right. :)
> Could we set the cookie of cgroup where uperf lies to 0?
> 
IMHO the disadvantage is that if there are two or more cgroups set cookie-0,
then the user applications in these cgroups could run concurrently on a core,
though all of them are set as trusted, we made a hole of user->user isolation.

Thanks,
-Aubrey