On Mon, Jun 8, 2020 at 7:56 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > 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 for taking this patch, Peter. I just wanted to double-check if you considered the race I mentioned in the above paragraph and decided it's a non-issue. If it is an issue I can send a small follow-up patch to close the race or I can send a new version of the whole patch with the fix if that's preferable. Please LMK. > > > > Thanks, > > Suren. > > > > > > > > -- > > > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. > > >
