Messages in this thread | | | Date | Thu, 7 Jul 2022 12:04:12 -0700 | From | Boqun Feng <> | Subject | Re: [PATCH v3] locking/rtmutex: Limit # of lock stealing for non-RT waiters |
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On Thu, Jul 07, 2022 at 02:45:10PM -0400, Waiman Long wrote: > On 7/7/22 14:22, Boqun Feng wrote: > > 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@redhat.com> > > > > --- > > > > 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. > > There is no inherent restriction on not allowing the same cpu stealing the > lock twice or more. With rtmutex, the top waiter may be sleeping and the
Well, I'm not saying we need to restrict the same cpu to steal a lock twice or more. Think about this, when there is a task running on CPU 1 already steals a lock once, for example:
<lock release> {task C is the top waiter}
CPU 1 ===== <now task A running> lock(); // steal the lock ... unlock(): // set owner to NULL <switch task B> // similar cost to wake up A lock(); // steal the lock
, which means if a CPU steals a lock twice or more, it's almost certain that a context happened between two steals ("almost" because there could be a case where task A lock()+unlock() twice, but as I said, it means that task A is willing to yield.).
Therefore if there are @nr_cpu_ids lock steals, it means either there is a context switch somewhere or a task has been willing to yield. And I think it's a reasonable signal to stop lock stealing.
Thoughts?
Regards, Boqun
> wakeup latency can be considerable. By allowing another ready lock waiter to > steal the lock for productive use, it can improve system throughput. There > is no fairness in lock stealing and I don't believe it is a worthwhile goal > to allow each cpu to steal the lock once. It will just complicate the code. > > On the other hand, unlimited lock stealing is bad and we have a put a limit > somehow to ensure forward progress. > > Cheers, > Longman >
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