linux-kernel - Re: [RFC PATCH 3/3 v0.2] sched/umcg: RFC: implement UMCG syscalls

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Date:   Sun, 11 Jul 2021 09:35:22 -0700
From:   Peter Oskolkov <posk@...k.io>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org
Cc:     Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andrei Vagin <avagin@...gle.com>,
        Jim Newsome <jnewsome@...project.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [RFC PATCH 3/3 v0.2] sched/umcg: RFC: implement UMCG syscalls

On Thu, Jul 8, 2021 at 12:46 PM Peter Oskolkov <posk@...k.io> wrote:
[...]

> Pretty much everything works, with one issue: when a worker
> blocks, we need to wake its server in umcg_wq_worker_sleeping
> called from sched_submit_work within preempt_disable block.
> As the server_tid is set by the userspace, it can point to
> a running/spinning task, and so wake_server will hang waiting
> for ttwu() to succeed. I do not think this is appropriate,
> but I am not sure at the moment how to resolve this. Any sugestions?

[...]

I think I can solve this by carefully ordering state changes (both
umcg state and task state) and maybe sending a signal to the wakee if
not enough. I'll try this approach in v0.3.