linux-kernel - Re: [PATCH] softirq: Provide a handshake for canceling tasklets via polling on PREEMPT

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Message-ID: <aKdTEkK5MBz_Fj47@slm.duckdns.org>
Date: Thu, 21 Aug 2025 07:10:42 -1000
From: Tejun Heo <tj@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] softirq: Provide a handshake for canceling tasklets via
 polling on PREEMPT_RT

Hello, Sebastian.

On Thu, Aug 21, 2025 at 11:28:27AM +0200, Sebastian Andrzej Siewior wrote:
...
> It is not wrong as in something will explode. The priority-inheritance
> boost is meant to give the lower priority task runtime in order to leave
> its critical section. So the task with the higher priority can continue
> to make progress instead of sitting around. Spinning while waiting for
> completion will not succeed.
> In this case "leaving the critical section" would mean complete the one
> work item. But instead we flush all of them. It is more of semantics in
> this case. That is why the looping-cancel in tasklet cancels just that
> one workitem.

Understood. However, given that these pools are per-cpu, BHs are usually not
heavily contended and canceling itself is a low frequency operation, the
practical difference likely won't be noticeable.

> > I think the main focus is keeping the
> > semantics matching on RT, right?
> 
> Yes, but having the semantics with busy waiting on a BH work is kind of
> the problem. And there is no need for it which makes it a bit difficult.
> The previous patch would match the !RT bits but we flush all work, have
> and the lock for no reason. That is why I don't like it. The majority of
> tasklet users don't need it. It is in my opinion bad semantics.
> 
> But if you insist on it, the previous patch will make it work and has
> been tested. There is not much I can do.

Oh, I'm not insisting, don't know enough to do so. Just trying to understand
the situation.

> > I'm most likely missing something about RT but wouldn't the above still lead
> > to deadlocks if the caller is non-hardirq but higher priority thread?
> 
> Not sure what you refer to. Right now there is this lock in
> local_bh_disable() which forces PI.
> Removing the whole section for RT as in this snippet gets us to the
> wait_for_completion() below. It lets the task with higher priority
> schedule out allowing task with lower priority to run. Eventually the
> barrier item completes and with the wakeup the high priority task will
> continue.
> So the high-priority task will lose runtime (allowing task with lower
> priority to run). I don't think it will be a problem because it is
> mostly used in "quit" scenarios (not during normal workload) and matches
> tasklet_disable().

Okay, so, on !RT, that busy loop section is there to allow
cancel_work_sync() to be called from BH-disabled contexts and the caller is
responsible for ensuring there's no recursion. It's not great but matches
the existing behavior. Obviously, in !RT, we can't go to
wait_for_completion() there because we can be in a non-sleepable context.

Are you saying that, in RT, it'd be fine to call wait_for_completion()
inside local_bh_disable() and won't trip any of the context warnings? If so,
yeah, we don't need any of the looping.

Thanks.

-- 
tejun