On 07/06, Mathieu Desnoyers wrote:
>
> * Oleg Nesterov ([email protected]) wrote:
> > Mathieu Desnoyers wrote:
> > >
> > > This patch adds the ability to protect critical sections from migration to
> > > another CPU without disabling preemption.
> > >
> > > Typical use:
> > >
> > > migration_disable();
> > > local_inc(&__get_cpu_var(&my_local_t_var));
> > > migration_enable();
> > >
> > > --- linux-2.6-lttng.orig/kernel/sched.c 2007-07-05 16:28:15.000000000 -0400
> > > +++ linux-2.6-lttng/kernel/sched.c 2007-07-05 16:53:24.000000000 -0400
> > > @@ -1996,6 +1996,7 @@
> > > * 1) running (obviously), or
> > > * 2) cannot be migrated to this CPU due to cpus_allowed, or
> > > * 3) are cache-hot on their current CPU.
> > > + * 4) migration preemption is non 0 for this non running task.
> > > */
> > > if (!cpu_isset(this_cpu, p->cpus_allowed))
> > > return 0;
> > > @@ -2003,6 +2004,8 @@
> > >
> > > if (task_running(rq, p))
> > > return 0;
> > > + if (task_thread_info(p)->migration_count)
> > > + return 0;
> >
>
> Hi Oleg,
>
> > Question: This means that the task could be preempted, but can't sleep, yes?
> >
>
> No exactly. This means that the task can be preempted, and can therefore
> sleep, but we do not allow this sleeping task to be migrated to a
> different CPU.
>
> > Because try_to_wake_up() can change ->cpu. Shouldn't might_sleep() check
> > ->migration_count then? Or we can change try_to_wake_up().
> >
>
> If we look at try_to_wake_up, it seems to take the new_cpu from
> task_cpu(), which comes from the thread info ->cpu, updated by
> set_task_cpu(). The *_move_tasks seems to be responsible for calling
> can_migrate_task() to see if, in a deferred way, the migration_thread
> can use set_task_cpu() to change the task's cpu.
Hmm. Perhaps I missed something, perhaps you... Or we both :)
try_to_wake_up() can choose new_cpu which differs from initial ->cpu.
Then it does set_task_cpu() which just sets ->cpu, it doesn't check
can_migrate_task(). This means that the task could be waken on another
CPU, even if it sleeps under migration_disable().
Btw, I think you should also fix debug_smp_processor_id(), it won't
like migration_disable() + __get_cpu_var().
Oleg.
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