Messages in this thread |  | | Subject | Re: [PATCH] sched/core: Create new task with twice disabled preemption | From | Kirill Tkhai <> | Date | Mon, 17 Feb 2014 18:43:39 +0400 |
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В Птн, 14/02/2014 в 15:49 +0000, Catalin Marinas пишет: > On Fri, Feb 14, 2014 at 12:44:01PM +0000, Kirill Tkhai wrote: > > В Птн, 14/02/2014 в 12:35 +0000, Catalin Marinas пишет: > > > On Thu, Feb 13, 2014 at 07:51:56PM +0400, Kirill Tkhai wrote: > > > > Preemption state on enter in finish_task_switch() is different > > > > in cases of context_switch() and schedule_tail(). > > > > > > > > In the first case we have it twice disabled: at the start of > > > > schedule() and during spin locking. In the second it is only > > > > once: the value which was set in init_task_preempt_count(). > > > > > > > > For archs without __ARCH_WANT_UNLOCKED_CTXSW set this means > > > > that all newly created tasks execute finish_arch_post_lock_switch() > > > > and post_schedule() with preemption enabled. > > > > > > > > It seems there is possible a problem in rare situations on arm64, > > > > when one freshly created thread preempts another before > > > > finish_arch_post_lock_switch() has finished. If mm is the same, > > > > then TIF_SWITCH_MM on the second won't be set. > > > > > > > > The second rare but possible issue is zeroing of post_schedule() > > > > on a wrong cpu. > > > > > > > > So, lets fix this and unify preempt_count state. > > > > > > An alternative to your patch: > > > > It looks better, than the initial. > > > > You may add my Acked-by if you want. > > Thanks for the ack. But apart from arm64, are there any other problems > with running part of finish_task_switch() and post_schedule() with > preemption enabled?
1)We have architecture-dependent finish_arch_post_lock_switch() which is possible(?) to be fixed for every arch at the moment, but someone may run into it in the future.
2)The second is fire_sched_in_preempt_notifiers(). It's generic interface which is currently unused. It notifies about preemption, so it's bad if additional preemption happens when it has not finished.
3)tick_nohz_task_switch() seems to be without problems. Just very-very slightly performance.
4)If post_schedule() happens on wrong CPU, the system may occur imbalanced for a short period. This happens, when post_schedule() of wrong class is executed.
If we fix that once in scheduler we'll decide everything above now and in the future. Also we'll decrease number of rare situations.
> The finish_arch_post_lock_switch() is currently only used by arm and > arm64 (the former UP only) and arm no longer has the preemption problem > (see commit bdae73cd374e2). So I can either disable the preemption > around schedule_tail() call in arm64 or do it globally as per yours or > my patch. > > Peter, Ingo, any thoughts? Do we care about preempt count consistency > across finish_task_switch() and post_schedule()? > > Thanks. >
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