Re: [RFC PATCH 2/4] softirq: Per vector deferment to workqueue

[Frederic Weisbecker]

On Sun, Jan 21, 2018 at 11:20:40PM +0530, Pavan Kondeti wrote:
> On Sun, Jan 21, 2018 at 05:11:17PM +0100, Frederic Weisbecker wrote:
> > On Sat, Jan 20, 2018 at 02:11:39PM +0530, Pavan Kondeti wrote:
> > 
> > Hi Pavan,
> > 
> > 
> > > I have couple questions/comments.
> > > 
> > > (1) Since the work is queued on a bounded per-cpu worker, we may run
> > > into a deadlock if a TASKLET is killed from another work running on
> > > the same bounded per-cpu worker.
> > > 
> > > For example,
> > > 
> > > (1) Schedule a TASKLET on CPU#0 from IRQ.
> > > (2) Another IRQ comes on the same CPU and we queue a work to kill
> > > the TASKLET. 
> > > (3) The TASKLET vector is deferred to workqueue.
> > > (4) We run the TASKLET kill work and wait for the TASKLET to finish,
> > > which won't happen.
> > > 
> > > We can fix this by queueing the TASKLET kill work on an unbounded
> > > workqueue so that this runs in parallel with TASKLET vector work.
> > > 
> > > Just wanted to know if we have to be aware of this *condition*.
> > 
> > But IIRC the workqueues have several workers per CPU so the tasklet to
> > be killed can run while the tasklet killer yields.
> > 
> AFAIK, the work items queued via schedule_work_on() goes to the system_wq
> which is bounded to a CPU with concurrency restrictions. If any work
> item (in this case tasklet kill) is getting executed on this bounded
> worker, the next items have to wait. The forward progress happens only
> when the current work is finished or enters sleep.

Workqueues have multiple workers to handle the pending works, unless they
are __WQ_ORDERED.

The worker manager in a pool is supposed to create workers on-demand
when necessary to make sure that no work is blocking the others.

Thanks.

> 
> This also makes me wonder what happens if a CPU hogging work gets executed
> on the system_wq while softirq work is pending? The softirq work gets
> starved which won't happen now with ksoftirqd design. Ideally the CPU
> hogging work should not be queued on the system_wq and instead should
> be queued on CPU intenstive workqueue (WQ_CPU_INTENSIVE) to exempt
> from concurrency management. May be we need some special workqueue
> which is bounded but not subjected to concurrency management.
> 
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