linux-kernel - Re: [PATCH] time/tick-broadcast: Fix tick_broadcast

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Message-Id: <20190621174104.GA7519@linux.ibm.com>
Date:   Fri, 21 Jun 2019 10:41:04 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        frederic@...nel.org, tglx@...utronix.de
Subject: Re: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline()
 lockdep complaint

On Fri, Jun 21, 2019 at 06:34:14AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 21, 2019 at 02:29:27PM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 21, 2019 at 05:16:30AM -0700, Paul E. McKenney wrote:
> > > A pair of full hangs at boot (TASKS03 and TREE04), no console output
> > > whatsoever.  Not sure how these changes could cause that, but suspicion
> > > falls on sched_tick_offload_init().  Though even that is a bit strange
> > > because if so, why didn't TREE01 and TREE07 also hang?  Again, looking
> > > into it.
> > 
> > Pesky details ;-)
> 
> And backing out to the earlier patch removes the hangs, though statistical
> insignificance and all that.

And purists might argue that four failures out of four attempts does not
constitute true statistical significance, but too bad.  If I interpose
a twork pointer in sched_tick_offload_init()'s initialization, it seems
to work fine, give or take lack of statistical significance.  This is
surprising, so I am rerunning with added parentheses in the atomic_set()
expression.

			Thanx, Paul "I hate initialization" McKenney

> Ah, in answer to your earlier question, if you want it in v5.3, you
> will need to take it (but I do humbly request that you wait until it
> actually works).  If you don't take it, I won't be submitting it earlier
> than v5.4.  Either way, your choice!
> 
> 							Thanx, Paul