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From:   =?UTF-8?B?WWl3ZWkgWmhhbmfigI4=?= <zzyiwei@android.com>
Date:   Tue, 23 Feb 2021 14:29:37 -0800
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Subject: Re: [PATCH] kthread: add kthread_mod_pending_delayed_work api
To:     Petr Mladek <pmladek@suse.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Jens Axboe <axboe@kernel.dk>,
        "J. Bruce Fields" <bfields@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Ilias Stamatis <stamatis.iliass@gmail.com>,
        Rob Clark <robdclark@chromium.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Liang Chen <cl@rock-chips.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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> > which is not cool because it will make the
> > asynchronous effort a no-op. Is there a way we can include caller
> > thread metadata(task_struct pointer?) when it enqueues the work so
> > that the RT worker thread won't preempt the caller thread when that
> > queued work gets scheduled? Probably require the CPU scheduler to poke
> > at the next work...or any other ideas will be very appreciated,
> > thanks!
>
> This sounds like a very strange use case.
> Why is the worker kthread RT when the work can be delayed?
>
> If the kthread has to be RT because of another work then
> your proposal will not work. The delayed processing of
> low priority work might block and delay any pending
> high priority work.
>
> You should consider handling the less important work in a separate
> kthread worker with a lower priority or by the system workqueue.

Just want to clarify that it's not about delayed_work any more. In my
latest question, it's a RT thread with normal work queued and
scheduled to be run immediately. However, I simply don't want the
worker to preempt the thread that queues the work.

It's a high prio work that we don't want other random tasks to preempt
it. Meanwhile, we don't want it to preempt the called thread. In
addition, assume we can't raise the priority of those caller
threads(otherwise I'd be fine with using a workqueue).