Messages in this thread | | | Subject | Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling | From | Chris Hyser <> | Date | Wed, 24 Feb 2021 08:02:58 -0500 |
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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
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