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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/