linux-kernel - Re: [PATCH] sched/core: Schedule new worker even if PI-blocked

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Message-ID: <20190820160217.GR2369@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Aug 2019 18:02:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        tglx@...utronix.de
Subject: Re: [PATCH] sched/core: Schedule new worker even if PI-blocked

On Tue, Aug 20, 2019 at 05:54:01PM +0200, Sebastian Andrzej Siewior wrote:
> On 2019-08-20 17:20:25 [+0200], Peter Zijlstra wrote:

> > And am I right in thinking that that, again, is specific to the
> > sleeping-spinlocks from PREEMPT_RT? Is there really nothing else that
> > identifies those more specifically? It's been a while since I looked at
> > them.
> 
> Not really. I hacked "int sleeping_lock" into task_struct which is
> incremented each time a "sleeping lock" version of rtmutex is requested.
> We have two users as of now:
> - RCU, which checks if we schedule() while holding rcu_read_lock() which
>   is okay if it is a sleeping lock.
> 
> - NOHZ's pending softirq detection while going to idle. It is possible
>   that "ksoftirqd" and "current" are blocked on locks and the CPU goes
>   to idle (because nothing else is runnable) with pending softirqs.
> 
> I wanted to let rtmutex invoke another schedule() function in case of a
> sleeping lock to avoid the RCU warning. This would avoid incrementing
> "sleeping_lock" in the fast path. But then I had no idea what to do with
> the NOHZ thing.

Once upon a time there was also a shadow task->state thing, that was
specific to the sleeping locks, because normally spinlocks don't muck
with task->state and so we have code relying on it not getting trampled.

Can't we use that somewhow? Or is that gone?

> > Also, I suppose it would be really good to put that in a comment.
> So, what does that mean for that patch. According to my inbox it has
> applied to an "urgent" branch. Do I resubmit the whole thing or just a
> comment on top?

Yeah, I'm not sure. I was surprised by that, because afaict all this is
PREEMPT_RT specific and not really /urgent material in the first place.
Ingo?