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. 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. >
