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SubjectRe: [PATCH RT v2] sched/migrate_disable: fallback to preempt_disable() instead barrier()
On Fri, Jul 06, 2018 at 12:58:57PM +0200, Sebastian Andrzej Siewior wrote:
> On SMP + !RT migrate_disable() is still around. It is not part of spin_lock()
> anymore so it has almost no users. However the futex code has a workaround for
> the !in_atomic() part of migrate disable which fails because the matching
> migrade_disable() is no longer part of spin_lock().
>
> On !SMP + !RT migrate_disable() is reduced to barrier(). This is not optimal
> because we few spots where a "preempt_disable()" statement was replaced with
> "migrate_disable()".
>
> We also used the migration_disable counter to figure out if a sleeping lock is
> acquired so RCU does not complain about schedule() during rcu_read_lock() while
> a sleeping lock is held. This changed, we no longer use it, we have now a
> sleeping_lock counter for the RCU purpose.
>
> This means we can now:
> - for SMP + RT_BASE
> full migration program, nothing changes here
>
> - for !SMP + RT_BASE
> the migration counting is no longer required. It used to ensure that the task
> is not migrated to another CPU and that this CPU remains online. !SMP ensures
> that already.
> Move it to CONFIG_SCHED_DEBUG so the counting is done for debugging purpose
> only.
>
> - for all other cases including !RT
> fallback to preempt_disable(). The only remaining users of migrate_disable()
> are those which were converted from preempt_disable() and the futex
> workaround which is already in the preempt_disable() section due to the
> spin_lock that is held.
>
> Cc: stable-rt@vger.kernel.org
> Reported-by: joe.korty@concurrent-rt.com
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> v1???v2: limit migrate_disable to RT only. Use preempt_disable() for !RT
> if migrate_disable() is used.
>
> include/linux/preempt.h | 6 +++---
> include/linux/sched.h | 4 ++--
> kernel/sched/core.c | 23 +++++++++++------------
> kernel/sched/debug.c | 2 +-
> 4 files changed, 17 insertions(+), 18 deletions(-)


Hi Sebastian,
v2 works for me.

I compiled and booted both smp+preempt+!rt and
smp+preempt+rt kernels, no splats on boot for either.

I ran the futex selftests on both kernels, both passed.

I ran a selection of posix tests from an old version of
the Linux Test Project, both kernels passed all tests.

Regards, and thanks,
Joe

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