* 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.
move_task_off_dead_cpu() does not seem to respect this though: it could
potentially move away a process while it is in a critical section by
forcing a __migrate_task. I guess I should put a test in __migrate_task
also so move_task_off_dead_cpu() can try to repeat the migration.
> What if the task does copy_process() under migration_disable() ? Child
> gets a copy of ->migration_count.
>
Right. Neighter does copy_process set the preemption count. It seems to
be dealt by sched_fork() when the thread is first scheduled in to make
sure that the preempt count is 1. I guess is would be safer to set the
migration count to 0 there too. (or set it to one if we ever decide to
make it the kernel default).
> Also, cpu_down() still can migrate this task to another CPU.
>
Yes, just noted this earlier. I first thought it was using
can_migrate_task(), but it does not seem so.
Thanks,
Mathieu
> Oleg.
>
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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