linux-kernel - Re: [RFC PATCH v2 1/4] rseq: Add sched

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Message-ID: <87o7hmdnnv.ffs@tglx>
Date:   Thu, 28 Sep 2023 22:54:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, Christian Brauner <brauner@...nel.org>,
        Florian Weimer <fw@...eb.enyo.de>, David.Laight@...LAB.COM,
        carlos@...hat.com, Peter Oskolkov <posk@...k.io>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Chris Kennelly <ckennelly@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        André Almeida 
        <andrealmeid@...lia.com>, libc-alpha@...rceware.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Jonathan Corbet <corbet@....net>,
        Noah Goldstein <goldstein.w.n@...il.com>,
        Daniel Colascione <dancol@...gle.com>, longman@...hat.com,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Florian Weimer <fweimer@...hat.com>
Subject: Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq

On Mon, May 29 2023 at 15:14, Mathieu Desnoyers wrote:
> +/*
> + * rseq_sched_state should be aligned on the cache line size.
> + */
> +struct rseq_sched_state {
> +	/*
> +	 * Version of this structure. Populated by the kernel, read by
> +	 * user-space.
> +	 */
> +	__u32 version;
> +	/*
> +	 * The state is updated by the kernel. Read by user-space with
> +	 * single-copy atomicity semantics. This field can be read by any
> +	 * userspace thread. Aligned on 32-bit. Contains a bitmask of enum
> +	 * rseq_sched_state_flags. This field is provided as a hint by the
> +	 * scheduler, and requires that the page holding this state is
> +	 * faulted-in for the state update to be performed by the scheduler.
> +	 */
> +	__u32 state;
> +	/*
> +	 * Thread ID associated with the thread registering this structure.
> +	 * Initialized by user-space before registration.
> +	 */
> +	__u32 tid;
> +};
> +
>  /*
>   * struct rseq is aligned on 4 * 8 bytes to ensure it is always
>   * contained within a single cache-line.
> @@ -148,6 +180,15 @@ struct rseq {
>  	 */
>  	__u32 mm_cid;
>  
> +	__u32 padding1;
> +
> +	/*
> +	 * Restartable sequences sched_state_ptr field. Initialized by
> +	 * userspace to the address at which the struct rseq_sched_state is
> +	 * located. Read by the kernel on rseq registration.
> +	 */
> +	__u64 sched_state_ptr;
> +

Why is this a separate entity instead of being simply embedded into
struct rseq?

Neither the code comment nor the changelog tells anything about that.

If your intention was to provide a solution for process shared futexes
then you completely failed to understand how process shared futexes
work. If not, then please explain why you need a pointer and the
associated hackery to deal with it.

Thanks,

        tglx