psi: eliminate kthread_worker from psi trigger scheduling mechanism

Each psi group requires a dedicated kthread_delayed_work and kthread_worker. Since no other work can be performed using psi_group's kthread_worker, the same result can be obtained using a task_struct and a timer directly. This makes psi triggering simpler by removing lists and locks involved with kthread_worker usage and eliminates the need for poll_scheduled atomic use in the hot path.

Signed-off-by Suren Baghdasaryan <[email protected]>

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This patch is meant to address Peter's request in [1] to pull kthread_queue_delayed_work() out from under rq->lock. This should also address the lockdep warning about possibility of a circular dependency described in [2]

Quoted 7 lines · Suren Baghdasaryan

Each psi group requires a dedicated kthread_delayed_work and kthread_worker. Since no other work can be performed using psi_group's kthread_worker, the same result can be obtained using a task_struct and a timer directly. This makes psi triggering simpler by removing lists and locks involved with kthread_worker usage and eliminates the need for poll_scheduled atomic use in the hot path.

Quoted 5 lines

Signed-off-by Suren Baghdasaryan <[email protected]>

---
This patch is meant to address Peter's request in [1] to pull kthread_queue_delayed_work() out from under rq->lock. This should also address the lockdep warning about possibility of a circular dependency described in [2]

I think you could've just fixed kthread_queue_delayed_work(), that code is sub-optimal.

But I suppose this works too.

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Quoted 9 lines · Peter Zijlstra

Quoted 7 lines · Suren Baghdasaryan

Each psi group requires a dedicated kthread_delayed_work and kthread_worker. Since no other work can be performed using psi_group's kthread_worker, the same result can be obtained using a task_struct and a timer directly. This makes psi triggering simpler by removing lists and locks involved with kthread_worker usage and eliminates the need for poll_scheduled atomic use in the hot path.

Quoted 8 lines

Quoted 5 lines

Signed-off-by Suren Baghdasaryan <[email protected]>

---
This patch is meant to address Peter's request in [1] to pull kthread_queue_delayed_work() out from under rq->lock. This should also address the lockdep warning about possibility of a circular dependency described in [2]

I think you could've just fixed kthread_queue_delayed_work(), that code is sub-optimal.

Ok, let me look into it some more. My understanding was that the worker->lock in kthread_queue_delayed_work() was needed to synchronize worker->delayed_work_list access. But maybe I'm missing something... I assume you are talking about optimizing this beyond what
https://lkml.org/lkml/2020/5/4/1148 was doing?

BTW, any objections against taking https://lkml.org/lkml/2020/5/4/1148
? It's not the ultimate fix but it is an improvement since it gets some of the operations that were unnecessarily under worker->lock out of it.

Quoted 2 lines

But I suppose this works too.

In PSI's case there is always one work for each worker, so the delayed_work_list and work_list are not needed and therefore I can replace kthread_worker machinery with a task and a timer. I think I can simplify this a bit further. For example group->poll_wakeup doesn't have to be an atomic. Originally I wanted to avoid a possibility of a race when poll_timer_fn sets it and psi_poll_worker resets it and as a result misses a wakeup, however if psi_poll_worker resets it before calling psi_poll_work then there is no harm in missing a wakeup because we called psi_poll_work and did the required work anyway.

One question about this patch I'm not sure about and wanted to ask you Peter is whether it's ok to call mod_timer from within a hotpath (while holding rq->lock). As I described in the additional comment, there is a possibility of a race between when I check timer_pending and the call to mod_timer, so it's possible that mod_timer might be called both from psi_poll_work (psi poll work handler) and from psi_task_change (hotpath under rq->lock). I see that mod_timer takes base->lock spinlock, and IIUC such a race might block the hotpath and therefore is unacceptable. If this is true I'll need to revive the poll_scheduled atomic to close this race and then I can change mod_timer into add_timer.
WDYT? And sorry for my ignorance if this is a trivial question. I'm not sure about the rules when it comes to rq->locks.

Thanks,
Suren.

Quoted 4 lines

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Quoted 11 lines · Suren Baghdasaryan

Quoted 9 lines · Peter Zijlstra

Quoted 7 lines · Suren Baghdasaryan

Each psi group requires a dedicated kthread_delayed_work and kthread_worker. Since no other work can be performed using psi_group's kthread_worker, the same result can be obtained using a task_struct and a timer directly. This makes psi triggering simpler by removing lists and locks involved with kthread_worker usage and eliminates the need for poll_scheduled atomic use in the hot path.

Quoted 8 lines

Quoted 8 lines

Quoted 5 lines

Signed-off-by Suren Baghdasaryan <[email protected]>

---
This patch is meant to address Peter's request in [1] to pull kthread_queue_delayed_work() out from under rq->lock. This should also address the lockdep warning about possibility of a circular dependency described in [2]

I think you could've just fixed kthread_queue_delayed_work(), that code is sub-optimal.

After some more staring into kthread code I think I understand what Peter's comment meant about delayed_work_list. worker->delayed_work_list seems to be unnecessary because each kthread_delayed_work has its own timer which will add the work into worker->work_list when the time comes. So there is no need to store the delayed work in an intermediate worker->delayed_work_list. However I think kthread_destroy_worker() has an issue if it's called while worker->delayed_work_list is non-empty. The issue is that kthread_destroy_worker() does not stop all the kthread_delayed_work->timers scheduled on the worker->delayed_work_list. So if such a timer fires after a call to kthread_destroy_worker(), timer's handler will dereference the already destroyed worker.

If I'm right and this is indeed an issue then I think we do need worker->delayed_work_list to cancel all the scheduled timers. The issue can be avoided if we assume that the caller will alway call kthread_cancel_delayed_work_sync() for each delayed_work scheduled on worker->delayed_work_list before calling kthread_destroy_worker(). If that's what we expect I think this expectation should be reflected in the comments and a WARN_ON(!list_empty(&worker->delayed_work_list)) be added in kthread_destroy_worker(). WDYT?

Quoted 39 lines

Ok, let me look into it some more. My understanding was that the worker->lock in kthread_queue_delayed_work() was needed to synchronize worker->delayed_work_list access. But maybe I'm missing something... I assume you are talking about optimizing this beyond what
https://lkml.org/lkml/2020/5/4/1148 was doing?

BTW, any objections against taking https://lkml.org/lkml/2020/5/4/1148
? It's not the ultimate fix but it is an improvement since it gets some of the operations that were unnecessarily under worker->lock out of it.

Quoted 2 lines

But I suppose this works too.

In PSI's case there is always one work for each worker, so the delayed_work_list and work_list are not needed and therefore I can replace kthread_worker machinery with a task and a timer. I think I can simplify this a bit further. For example group->poll_wakeup doesn't have to be an atomic. Originally I wanted to avoid a possibility of a race when poll_timer_fn sets it and psi_poll_worker resets it and as a result misses a wakeup, however if psi_poll_worker resets it before calling psi_poll_work then there is no harm in missing a wakeup because we called psi_poll_work and did the required work anyway.

One question about this patch I'm not sure about and wanted to ask you Peter is whether it's ok to call mod_timer from within a hotpath (while holding rq->lock). As I described in the additional comment, there is a possibility of a race between when I check timer_pending and the call to mod_timer, so it's possible that mod_timer might be called both from psi_poll_work (psi poll work handler) and from psi_task_change (hotpath under rq->lock). I see that mod_timer takes base->lock spinlock, and IIUC such a race might block the hotpath and therefore is unacceptable. If this is true I'll need to revive the poll_scheduled atomic to close this race and then I can change mod_timer into add_timer.
WDYT? And sorry for my ignorance if this is a trivial question. I'm not sure about the rules when it comes to rq->locks.

Quoted 8 lines

Thanks,
Suren.

Quoted 4 lines

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