Re: [PATCH] mm,page_alloc: PF_WQ_WORKER threads must sleep at should_reclaim_retry().

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, Jul 31, 2018 at 12:25:04AM +0900, Tetsuo Handa wrote:
> WQ_MEM_RECLAIM guarantees that "struct task_struct" is preallocated. But
> WQ_MEM_RECLAIM does not guarantee that the pending work is started as soon
> as an item was queued. Same rule applies to both WQ_MEM_RECLAIM workqueues 
> and !WQ_MEM_RECLAIM workqueues regarding when to start a pending work (i.e.
> when schedule_timeout_*() is called).
> 
> Is this correct?

WQ_MEM_RECLAIM guarantees that there's always gonna exist at least one
kworker running the workqueue.  But all per-cpu kworkers are subject
to concurrency limiting execution - ie. if there are any per-cpu
actively running on a cpu, no futher kworkers will be scheduled.

> >              We can add timeout mechanism to workqueue so that it
> > kicks off other kworkers if one of them is in running state for too
> > long, but idk, if there's an indefinite busy loop condition in kernel
> > threads, we really should get rid of them and hung task watchdog is
> > pretty effective at finding these cases (at least with preemption
> > disabled).
> 
> Currently the page allocator has a path which can loop forever with
> only cond_resched().

Yeah, workqueue can choke on things like that and kthread indefinitely
busy looping doesn't do anybody any good.

Thanks.

-- 
tejun