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Date:   Fri, 25 Feb 2022 13:39:20 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Jonathan Corbet <corbet@lwn.net>,
        linux-man <linux-man@vger.kernel.org>
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>
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Subject: Re: [RFC PATCH v2 09/11] sched: Introduce per memory space current
 virtual cpu id
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Thread-Topic: sched: Introduce per memory space current virtual cpu id
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----- On Feb 25, 2022, at 1:15 PM, Jonathan Corbet corbet@lwn.net wrote:

> 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 :)

Just to confirm, when you say "userspace-api documentation" do you refer to
man pages ?

I did a few attempts at upstreaming a rseq.2 man page in 2020, but I have been
stuck waiting for feedback from Michael Kerrisk since then.

So for the moment I'm maintaining a rseq.2 man page here:

https://git.kernel.org/pub/scm/libs/librseq/librseq.git/tree/doc/man/rseq.2

I'd gladly accept some help to improve the documentation of rseq.

Thanks,

Mathieu

> 
> Thanks,
> 
> jon

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com