Re: NetBSD thread model: a few questions

[Jean-Yves Migeon <jeanyves.migeon%free.fr@localhost>]

Andrew Doran wrote:
> Yes it's valid. Ideally you would take the lock first and then disable
> preemption, especially if it's known that the lock could cause the caller to
> spin-wait for some time. I realise that's not always possible or easy; it's
> the ideal pattern.

Alright.

> You want to do complete CPU offline, right? I will describe it as 'pause'
> because we already have an 'offline' :-)

Yes, exactly, a complete CPU shutdown. I did not look at the device 
interrupts/event channels code yet, but I guess that they will be easier 
to handle by masking them.

> . For userspace threads that have
> affinity or are part of a processor set any pause would need to be prevented
> and an error returned.
>
> For kernel activity and bound threads here is my suggestion:
>
> - Offline the cpu when beginning pause (SPFC_OFFLINE).
>
> - Scan the callout table on the target CPU and migrate all its callouts to a
>   CPU that will still be online+unpaused. Ensure that callouts will not be
>   migrated back onto the target CPU while it is _offline_. That could be as
>   simple as a check for the SPCF_OFFLINE flag. See kern_timeout.c.
> [...]
> - When starting the CPU back up, clear SPCF_PAUSELOOP with a unicast xcall.

Nice! I will take a close look at this.

Many many thanks for the help and guidance, Andrew.

Cheers,

--
Jean-Yves Migeon
jeanyves.migeon%free.fr@localhost