Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 25 May 2020 16:38:59 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Qian Cai <cai@lca.pw>, "Paul E. McKenney" <paulmck@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: Endless soft-lockups for compiling workload since next-20200519
Message-ID: <20200525143859.GX325280@hirez.programming.kicks-ass.net>
References: <CAG=TAF6jUsQrW-fjbS3vpjkMfn8=MUDsuQxjk3NMfvQa250RHA@mail.gmail.com>
 <20200520125056.GC325280@hirez.programming.kicks-ass.net>
 <20200521004035.GA15455@lenoir>
 <20200521093938.GG325280@hirez.programming.kicks-ass.net>
 <20200521104937.GB325303@hirez.programming.kicks-ass.net>
 <20200521110027.GC325303@hirez.programming.kicks-ass.net>
 <20200521124113.GC15455@lenoir>
 <20200525132105.GW325280@hirez.programming.kicks-ass.net>
 <20200525140541.GA28923@lenoir>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200525140541.GA28923@lenoir>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, May 25, 2020 at 04:05:49PM +0200, Frederic Weisbecker wrote:
> On Mon, May 25, 2020 at 03:21:05PM +0200, Peter Zijlstra wrote:
> > @@ -2320,7 +2304,7 @@ static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
> >  
> >  	if (llist_add(&p->wake_entry, &rq->wake_list)) {
> >  		if (!set_nr_if_polling(rq->idle))
> > -			smp_call_function_single_async(cpu, &rq->wake_csd);
> > +			smp_call_function_single_async(cpu, &p->wake_csd);
> >  		else
> >  			trace_sched_wake_idle_without_ipi(cpu);
> 
> Ok that's of course very unlikely but could it be possible to have the
> following:
> 
> CPU 0                         CPU 1                                     CPU 2
> -----       
> 
> //Wake up A
> ttwu_queue(TASK A, CPU 1)     idle_loop {
>                                   ttwu_queue_pending {
>                                       ....
>                                       raw_spin_unlock_irqrestore(rq)
>                                       # VMEXIT (with IPI still pending)
>                                                                         //task A migrates here
>                                                                         wait_event(....)
>                                                                         //sleep
> 
> //Wake up A
> ttwu_queue(TASK A, CPU 2) {
>     //IPI on CPU 2 ignored
>     // due to csd->flags == CSD_LOCK
> 

Right you are.

Bah!

More thinking....