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Re: NetBSD thread model: a few questions

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.


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.


Jean-Yves Migeon

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