Re: [PATCH -rt 2/5] Thread Migration Preemption - v2

Peter Zijlstra
2007-07-14T18:45:27Z

On Sat, 2007-07-14 at 21:16 +0400, Oleg Nesterov wrote:
> On 07/14, Peter Zijlstra wrote:
> >
> > From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> >
> > This patch adds the ability to protect critical sections from migration to
> > another CPU without disabling preemption.
> >
> > This will be useful to minimize the amount of preemption disabling for the -rt
> > patch. It will help leveraging improvements brought by the local_t types in
> > asm/local.h (see Documentation/local_ops.txt). Note that the updates done to
> > variables protected by migrate_disable must be either atomic or protected from
> > concurrent updates done by other threads.
> >
> > Typical use:
> >
> > migrate_disable();
> > local_inc(&__get_cpu_var(&my_local_t_var));
> > migrate_enable();
> >
> > Which will increment the variable atomically wrt the local CPU.
>
> I still think this patch is buggy. Perhaps I am wrong. Please look at
>
> http://marc.info/?l=linux-kernel&m=118417177818825
>
> there was no any reply.

Looking at it, I tend to agree that the migration_thread thing is indeed
quite broken. Not quite sure on how to solve it nicely though, one would
need to stick the migration request structure somewhere in task_struct
or thread_info, and not back on the migration_queue. But given that
people already complained about the adding ov migration_counter,. I'm
not daring to propose such things.