Messages in this thread | | | Date | Thu, 14 Mar 2024 21:39:45 -0700 | Subject | [PATCH v9 0/7] Preparatory changes for Proxy Execution v9 | From | John Stultz <> |
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As mentioned last time[1], after previous submissions of the Proxy Execution series, I got feedback that the patch series was getting a bit unwieldy to review, and Qais suggested I break out just the cleanups/preparatory components of the patch series and submit them on their own in the hope we can start to merge the less complex bits and discussion can focus on the more complicated portions afterwards. This so far has not been very successful, with the submission & RESEND of the v8 preparatory changes not getting much in the way of review.
Nonetheless, for v9 of this series, I’m again only submitting those early cleanup/preparatory changes here (which have not changed since the v8 submissions, but to avoid confusion with the git branch names, I’m labeling it as v9). In the meantime, I’ve continued to put a lot of effort into the full series, mostly focused on polishing the series for correctness, and fixing some hard to trip races.
If you are interested, the full v9 series, it can be found here: https://github.com/johnstultz-work/linux-dev/commits/proxy-exec-v9-6.8 https://github.com/johnstultz-work/linux-dev.git proxy-exec-v9-6.8
New in v9: (In the git tree. Again, none of the preparatory patches submitted here have changed since v8) --------- * Change to force mutex lock handoff when we have a blocked donor (preserves optimistic spinning elsewhere, but still prioritizes donor when present on unlock)
* Do return migration whenever we’re not on the wake_cpu (should address placement concerns brought up earlier by Xuewen Yan)
* Closed hole where we might mark a task as BO_RUNNABLE without doing return migration
* Much improved handling of balance callbacks when we need to pick_again
* Fixes for cases where we put_prev_task() but left a dangling pointer to rq_selected() when deactivating a task (as it could then be migrated away while we still have a reference to it), by selecting idle before deactivating next.
* Fixes for dangling references to rq->curr (which had been put_prev_task’ed) when we drop rq lock for proxy_migration
* Fixes for ttwu / find_proxy_task() races if the lock owner was being return migrated, and ttwu hadn’t yet set_task_cpu() and activated it, which allowed that task to be scheduled on two cpus at the same time.
* Fix for live-lock between activate_blocked_tasks() and proxy_enqueue_on_owner() if activated owner went right back to sleep (which also simplifies the locking in activate_blocked_tasks())
* Cleanups to avoid locked BO_WAKING->BO_RUNNABLE transition in try_to_wake_up() if proxy execution isn't enabled
* Fix for psi_dequeue, as proxy changes assumptions around voluntary sleeps.
* Numerous typos, comment improvements, and other fixups suggested by Metin
* And more!
Performance: --------- K Prateek Nayak provided some feedback on the v8 series here[2]. Given the potential extra overhead of doing rq migrations/return migrations/etc for the proxy case, it’s not completely surprising a few of K Prateek’s test cases saw ~3-5% regressions, but I’m hoping to look into this soon to see if we can reduce those further. The donor mutex handoff in this revision may help some.
Issues still to address: --------- * The chain migration functionality needs further iterations and better validation to ensure it truly maintains the RT/DL load balancing invariants.
* CFS load balancing. There was concern that blocked tasks may carry forward load (PELT) to the lock owner's CPU, so the CPU may look like it is overloaded. Needs investigation.
* The sleeping owner handling (where we deactivate waiting tasks and enqueue them onto a list, then reactivate them when the owner wakes up) doesn’t feel great. This is in part because when we want to activate tasks, we’re already holding a task.pi_lock and a rq_lock, just not the locks for the task we’re activating, nor the rq we’re enqueuing it onto. So there has to be a bit of lock juggling to drop and acquire the right locks (in the right order). It feels like there’s got to be a better way. Also needs some rework to get rid of the recursion.
Credit/Disclaimer: —-------------------- As mentioned previously, this Proxy Execution series has a long history: First described in a paper[3] by Watkins, Straub, Niehaus, then from patches from Peter Zijlstra, extended with lots of work by Juri Lelli, Valentin Schneider, and Connor O'Brien. (and thank you to Steven Rostedt for providing additional details here!)
So again, many thanks to those above, as all the credit for this series really is due to them - while the mistakes are likely mine.
Thanks so much! -john
[1] https://lore.kernel.org/lkml/20240224001153.2584030-1-jstultz@google.com/ [2] https://lore.kernel.org/lkml/c26251d2-e1bf-e5c7-0636-12ad886e1ea8@amd.com/ [3] https://static.lwn.net/images/conf/rtlws11/papers/proc/p38.pdf
Cc: Joel Fernandes <joelaf@google.com> Cc: Qais Yousef <qyousef@google.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Juri Lelli <juri.lelli@redhat.com> Cc: Vincent Guittot <vincent.guittot@linaro.org> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> Cc: Valentin Schneider <vschneid@redhat.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Ben Segall <bsegall@google.com> Cc: Zimuzo Ezeozue <zezeozue@google.com> Cc: Youssef Esmat <youssefesmat@google.com> Cc: Mel Gorman <mgorman@suse.de> Cc: Daniel Bristot de Oliveira <bristot@redhat.com> Cc: Will Deacon <will@kernel.org> Cc: Waiman Long <longman@redhat.com> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: "Paul E. McKenney" <paulmck@kernel.org> Cc: Metin Kaya <Metin.Kaya@arm.com> Cc: Xuewen Yan <xuewen.yan94@gmail.com> Cc: K Prateek Nayak <kprateek.nayak@amd.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: kernel-team@android.com
Connor O'Brien (2): sched: Add do_push_task helper sched: Consolidate pick_*_task to task_is_pushable helper
John Stultz (1): sched: Split out __schedule() deactivate task logic into a helper
Juri Lelli (2): locking/mutex: Make mutex::wait_lock irq safe locking/mutex: Expose __mutex_owner()
Peter Zijlstra (2): locking/mutex: Remove wakeups from under mutex::wait_lock sched: Split scheduler and execution contexts
kernel/locking/mutex.c | 60 +++++++---------- kernel/locking/mutex.h | 25 +++++++ kernel/locking/rtmutex.c | 26 +++++--- kernel/locking/rwbase_rt.c | 4 +- kernel/locking/rwsem.c | 4 +- kernel/locking/spinlock_rt.c | 3 +- kernel/locking/ww_mutex.h | 49 ++++++++------ kernel/sched/core.c | 122 +++++++++++++++++++++-------------- kernel/sched/deadline.c | 53 ++++++--------- kernel/sched/fair.c | 18 +++--- kernel/sched/rt.c | 59 +++++++---------- kernel/sched/sched.h | 44 ++++++++++++- 12 files changed, 268 insertions(+), 199 deletions(-)
-- 2.44.0.291.gc1ea87d7ee-goog
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