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

["Li, Aubrey" <aubrey.li@linux.intel.com>]

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?

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).
>>>
>>