Re: [RFC PATCH v2 09/11] sched: Introduce per memory space current virtual cpu id

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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