Re: [PATCH v3] locking/rtmutex: Limit # of lock stealing for non-RT waiters

[Boqun Feng]

On Wed, Jul 06, 2022 at 10:03:10AM -0400, Waiman Long wrote:
> On 7/6/22 09:59, Waiman Long wrote:
> > Commit 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock
> > stealing") allows unlimited number of lock stealing's for non-RT
> > tasks. That can lead to lock starvation of non-RT top waiter tasks if
> > there is a constant incoming stream of non-RT lockers. This can cause
> > rcu_preempt self-detected stall or even task lockup in PREEMPT_RT kernel.
> > For example,
> > 
> > [77107.424943] rcu: INFO: rcu_preempt self-detected stall on CPU
> > [ 1249.921363] INFO: task systemd:2178 blocked for more than 622 seconds.
> > 
> > Avoiding this problem and ensuring forward progress by limiting the
> > number of times that a lock can be stolen from each waiter. This patch
> > sets a threshold of 32. That number is arbitrary and can be changed
> > if needed.
> > 
> > Fixes: 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock stealing")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   kernel/locking/rtmutex.c        | 9 ++++++---
> >   kernel/locking/rtmutex_common.h | 8 ++++++++
> >   2 files changed, 14 insertions(+), 3 deletions(-)
> > 
> >   [v3: Increase threshold to 32 and add rcu_preempt self-detected stall]
> 
> Note that I decided to increase the threshold to 32 from 10 to reduce the
> potential performance impact of this change, if any. We also found out that
> this patch can fix some of the rcu_preempt self-detected stall problems that
> we saw with the PREEMPT_RT kernel. So I added that information in the patch
> description.
> 

Have you considered (and tested) whether we can set the threshold
directly proportional to nr_cpu_ids? Because IIUC, the favorable case
for lock stealing is that every CPU gets a chance to steal once. If one
CPU can steal twice, 1) either there is a context switch between two
tasks, which costs similarly as waking up the waiter, or 2) a task drops
and re-graps a lock, which means the task wants to yield to other
waiters of the lock.

Regards,
Boqun

> Cheers,
> Longman
>