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

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Date:   Sun, 21 Jan 2018 17:11:17 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Levin Alexander <alexander.levin@...izon.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Dmitry Safonov <dima@...sta.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Radu Rendec <rrendec@...sta.com>,
        Ingo Molnar <mingo@...nel.org>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH 2/4] softirq: Per vector deferment to workqueue

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.

> 
> (2) Ksoftirqd thread gets parked when a CPU is hotplugged out. So
> there is a gaurantee that the softirq handling never happens on
> another CPU. Where as a bounded worker gets detached and the queued
> work can run on another CPU. I guess, some special handling is
> needed to handle hotplug.

Good catch. Funny, I worried a bit about CPU hotplug but I assumed
the pending CPU-bound worklets would be simply sync'ed before CPU gets down.

Breaking their CPU-bound properties doesn't look sane to me.

Anyway, I'll need to make a CPU hotplug hook.

Thanks for reporting that!