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

[Li, Aubrey]

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.

Thanks,
-Aubrey