linux-kernel - Re: [PATCH v2 03/11] sched,idle,rcu: Push rcu

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Message-ID: <875z94le33.fsf@nanos.tec.linutronix.de>
Date:   Thu, 27 Aug 2020 09:53:20 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     peterz@...radead.org, Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org, will@...nel.org,
        npiggin@...il.com, elver@...gle.com, jgross@...e.com,
        paulmck@...nel.org, rostedt@...dmis.org, rjw@...ysocki.net,
        svens@...ux.ibm.com
Subject: Re: [PATCH v2 03/11] sched,idle,rcu: Push rcu_idle deeper into the idle path

On Thu, Aug 27 2020 at 09:47, peterz wrote:
> On Wed, Aug 26, 2020 at 09:24:19PM -0400, Joel Fernandes wrote:
>> On Wed, Aug 26, 2020 at 09:18:26PM -0400, Joel Fernandes wrote:
>> > On Fri, Aug 21, 2020 at 10:47:41AM +0200, Peter Zijlstra wrote:
>> > > Lots of things take locks, due to a wee bug, rcu_lockdep didn't notice
>> > > that the locking tracepoints were using RCU.
>> > > 
>> > > Push rcu_idle_{enter,exit}() as deep as possible into the idle paths,
>> > > this also resolves a lot of _rcuidle()/RCU_NONIDLE() usage.
>> > > 
>> > > Specifically, sched_clock_idle_wakeup_event() will use ktime which
>> > > will use seqlocks which will tickle lockdep, and
>> > > stop_critical_timings() uses lock.
>> > 
>> > I was wondering if those tracepoints should just use _rcuidle variant of the
>> > trace call. But that's a terrible idea considering that would add unwanted
>> > overhead I think.
>> > 
>> > Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>> 
>> BTW, if tracepoint is converted to use RCU-trace flavor, then these kinds of
>> issues go away, no? That RCU flavor is always watching.
>
> All trace_*_rcuidle() and RCU_NONIDLE() usage is a bug IMO.

It's the same problem as low level entry/exit. And that stuff is a hack
which papers over the problem instead of fixing it from ground up. But
we are talking about tracing, right?

Thanks,

        tglx