Messages in this thread | | | Date | Mon, 20 Dec 2010 16:57:42 +0100 | From | Frederic Weisbecker <> | Subject | Re: [RFC PATCH 15/15] nohz_task: Procfs interface |
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On Mon, Dec 20, 2010 at 04:42:24PM +0100, Peter Zijlstra wrote: > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote: > > This implements the /proc/pid/nohz file that enables the > > nohz attribute of a task. > > > > Synchronization is enforced so that: > > > > - A CPU can have only one nohz task > > Why?
This is because of the hooks we have with entering/exiting userspace. The "wants to enter extended quiescent" variable (nohz_task_ext_qs) is per CPU and applies to any nohz task.
If A and B are nohz task bound to the same CPU,
A enters userspace, says it can enter extended quiescent state (nohz_task_ext_qs = 1). B preempts it and enters kernel, hence saying that it doesn't want extended quiescent state (nohz_task_ext_qs = 0). B sleeps, we return to A which said that it wants extended quiescent state but the per cpu var has been screwed (nohz_task_ext_qs == 0).
But this can be solved using a per task variable. I just thought it wouldn't be very useful to have two nohz task on a same CPU, but actually why not.
> > - A nohz task can be only affine to a single CPU > > Why?
Same problem, we need to make some things per task. That's fixable, This will may be add a bit of complexity and since I couldn't find a usecase for migratable nohz tasks, I did not handled that case.
Should I?
> > For now this is only possible to write on /proc/self but probably > > allowing it from another task would be a good idea and wouldn't > > increase so much the complexity of the code. > > ptrace rules might match that.
You think I should use the ptrace interface? Hmm, dunno if it's appropriate.
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