Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling

[Chris Hyser <chris.hyser@oracle.com>]

On 2/24/21 12:15 AM, Josh Don wrote:
> On Tue, Feb 23, 2021 at 11:26 AM Chris Hyser <chris.hyser@oracle.com> wrote:
>>
>> On 2/23/21 4:05 AM, Peter Zijlstra wrote:
>>> On Mon, Feb 22, 2021 at 11:00:37PM -0500, Chris Hyser wrote:
>>>> On 1/22/21 8:17 PM, Joel Fernandes (Google) wrote:
>>>> While trying to test the new prctl() code I'm working on, I ran into a bug I
>>>> chased back into this v10 code. Under a fair amount of stress, when the
>>>> function __sched_core_update_cookie() is ultimately called from
>>>> sched_core_fork(), the system deadlocks or otherwise non-visibly crashes.
>>>> I've not had much success figuring out why/what. I'm running with LOCKDEP on
>>>> and seeing no complaints. Duplicating it only requires setting a cookie on a
>>>> task and forking a bunch of threads ... all of which then want to update
>>>> their cookie.
>>>
>>> Can you share the code and reproducer?
>>
>> Attached is a tarball with c code (source) and scripts. Just run ./setup_bug which will compile the source and start a
>> bash with a cs cookie. Then run ./show_bug which dumps the cookie and then fires off some processes and threads. Note
>> the cs_clone command is not doing any core sched prctls for this test (not needed and currently coded for a diff prctl
>> interface). It just creates processes and threads. I see this hang almost instantly.
>>
>> Josh, I did verify that this occurs on Joel's coresched tree both with and w/o the kprot patch and that should exactly
>> correspond to these patches.
>>
>> -chrish
>>
> 
> I think I've gotten to the root of this. In the fork code, our cases
> for inheriting task_cookie are inverted for CLONE_THREAD vs
> !CLONE_THREAD. As a result, we are creating a new cookie per-thread,
> rather than inheriting from the parent. Now this is actually ok; I'm
> not observing a scalability problem with creating this many cookies.

This isn't the issue. The test code generates cases for both THREAD_CLONE and not and both paths call the cookie update 
code. The new code I was testing when I discovered this, fixed the problem you noted.


> However, it means that overall throughput of your binary is cut in
> ~half, since none of the threads can share a core. Note that I never
> saw an indefinite deadlock, just ~2x runtime for your binary vs th > control. I've verified that both a) manually hardcoding all threads to
> be able to share regardless of cookie, and b) using a machine with 6
> cores instead of 2, both allow your binary to complete in the same
> amount of time as without the new API.

This was on a 24 core box. When I run the test, I definitely hangs. I'll answer your other email as wwll.

-chrish