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From:   Jonathan Corbet <corbet@lwn.net>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api <linux-api@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Florian Weimer <fw@deneb.enyo.de>,
        David Laight <David.Laight@ACULAB.COM>,
        carlos <carlos@redhat.com>, Peter Oskolkov <posk@posk.io>
Subject: Re: [RFC PATCH v2 09/11] sched: Introduce per memory space current
 virtual cpu id
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Date:   Fri, 25 Feb 2022 11:15:11 -0700
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Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:

> Some effective upper bounds for the number of vcpu ids observable in a process:
>
> - sysconf(3) _SC_NPROCESSORS_CONF,
> - the number of threads which exist concurrently in the process,
> - the number of cpus in the cpu affinity mask applied by sched_setaffinity,
>   except in corner-case situations such as cpu hotplug removing all cpus from
>   the affinity set,
> - cgroup cpuset "partition" limits,
>
> Note that AFAIR non-partition cgroup cpusets allow a cgroup to "borrow"
> additional cores from the rest of the system if they are idle, therefore
> allowing the number of concurrent threads to go beyond the specified limit.
>
> AFAIR the sched affinity mask is tweaked independently of the cgroup cpuset.
> Those are two mechanisms both affecting the scheduler task placement.
>
> I would expect the user-space code to use some sensible upper bound as a
> hint about how many per-vcpu data structure elements to expect (and how many
> to pre-allocate), but have a "lazy initialization" fall-back in case the
> vcpu id goes up to the number of configured processors - 1. And I suspect
> that even the number of configured processors may change with CRIU.
>
> If the above explanation makes sense (please let me know if I am wrong
> or missed something), I suspect I should add it to the commit message.

That helps, thanks.  I do think that something like this belongs in the
changelog - or, even better, in the upcoming restartable-sequences
section in the userspace-api documentation :)

Thanks,

jon