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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org,
        Christian Brauner <christian.brauner@ubuntu.com>,
        David.Laight@ACULAB.COM, carlos@redhat.com,
        Peter Oskolkov <posk@posk.io>
Subject: Re: [RFC PATCH 2/3] rseq: extend struct rseq with per thread group
 vcpu id
References: <20220201192540.10439-1-mathieu.desnoyers@efficios.com>
        <20220201192540.10439-2-mathieu.desnoyers@efficios.com>
Date:   Tue, 01 Feb 2022 21:03:13 +0100
In-Reply-To: <20220201192540.10439-2-mathieu.desnoyers@efficios.com> (Mathieu
        Desnoyers's message of "Tue, 1 Feb 2022 14:25:39 -0500")
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* Mathieu Desnoyers:

> If a thread group has fewer threads than cores, or is limited to run on
> few cores concurrently through sched affinity or cgroup cpusets, the
> virtual cpu ids will be values close to 0, thus allowing efficient use
> of user-space memory for per-cpu data structures.

>From a userspace programmer perspective, what's a good way to obtain a
reasonable upper bound for the possible tg_vcpu_id values?

I believe not all users of cgroup cpusets change the affinity mask.

> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 13f6d0419f31..37b43735a400 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -86,10 +86,14 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
>  	struct rseq __user *rseq = t->rseq;
>  	u32 cpu_id = raw_smp_processor_id();
>  	u32 node_id = cpu_to_node(cpu_id);
> +	u32 tg_vcpu_id = task_tg_vcpu_id(t);
>  
>  	if (!user_write_access_begin(rseq, t->rseq_len))
>  		goto efault;
>  	switch (t->rseq_len) {
> +	case offsetofend(struct rseq, tg_vcpu_id):
> +		unsafe_put_user(tg_vcpu_id, &rseq->tg_vcpu_id, efault_end);
> +		fallthrough;
>  	case offsetofend(struct rseq, node_id):
>  		unsafe_put_user(node_id, &rseq->node_id, efault_end);
>  		fallthrough;

Is the switch really useful?  I suspect it's faster to just write as
much as possible all the time.  The switch should be well-predictable
if running uniform userspace, but still …