linux-kernel - Re: [PATCH v8 1/3] smpboot: allow excluding cpus from the smpboot threads

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Message-ID: <20150416164848.GC16270@lerouge>
Date:	Thu, 16 Apr 2015 18:48:50 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Jones <drjones@...hat.com>,
	chai wen <chaiw.fnst@...fujitsu.com>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Fabian Frederick <fabf@...net.be>,
	Aaron Tomlin <atomlin@...hat.com>,
	Ben Zhang <benzh@...omium.org>,
	Christoph Lameter <cl@...ux.com>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
	linux-doc@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v8 1/3] smpboot: allow excluding cpus from the smpboot
 threads

On Thu, Apr 16, 2015 at 11:50:06AM -0400, Chris Metcalf wrote:
> On 4/16/2015 11:28 AM, Frederic Weisbecker wrote:
> >>+	/* Unpark any threads that were voluntarily parked. */
> >>>+	if (ht->cpumask) {
> >>>+		cpumask_andnot(&tmp_mask, cpu_online_mask, ht->cpumask);
> >>>+		for_each_cpu(cpu, &tmp_mask) {
> >>>+			struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
> >>>+			if (tsk)
> >>>+				kthread_unpark(tsk);
> >>>+		}
> >>>+	}
> >Why do you need to do that? smpboot_destroy_threads() doesn't work on parked threads?
> >But kthread_stop() does an explicit unparking.
> 
> Yes, this part left me scratching my head.  Experimentally, this was necessary.
> I saw the unpark in kthread_stop() but it didn't make things work properly.
> Currently it looks like parked threads are only in that state while cores are
> being offlined, and then they are killed individually, so it seems likely that
> this particular path hasn't been tested before.

I'm not sure I understand. You mean that kthreads can be parked only when cores they
are affine to are offline?

Also I'm scratching my head around kthread_stop() when called on kthreads that are parked
on offline cores. I don't see how they can wake up and do the kthread->exited completion since
they are only affine to that offline core. But I likely overlooked something.

> 
> >+/* Statically allocated and used under smpboot_threads_lock. */
> >+static struct cpumask tmp_mask;
> >+
> >Better allocate the cpumask on need rather than have it resident on memory.
> >struct cpumask can be large. Plus we need to worry about locking it.
> >
> 
> I was trying to avoid the need to make functions return errors for the
> extremely unlikely case of ENOMEM.  No one is going to check that error
> return in practice anyway; programmers are lazy.  It seemed easy to
> allocate one mask statically and use it under the lock; even large systems aren't
> likely to burn more than a couple hundred bytes of .bss for this.

Sure, but I guess it's a common practice to allocate temporary cpumasks. I can't
see much "static struct cpumask" around that are used for temporary stuffs.

> 
> But, if you'd prefer using allocation and the error-return model, I can
> certainly change the code to do that.

There is always a caller to return -ENOMEM to ;-)

> 
> -- 
> Chris Metcalf, EZChip Semiconductor
> http://www.ezchip.com
> 
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