On Thu, Jun 4, 2020 at 12:20 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Thu, Jun 4, 2020 at 6:12 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > On Thu, May 28, 2020 at 12:54:42PM -0700, Suren Baghdasaryan wrote: > > > 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 <surenb@xxxxxxxxxx> > > > --- > > > 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? > > 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. > > > > > 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. > > > > > -- > > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. > >
