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Message-ID: <20111203231429.GB18961@linux.vnet.ibm.com>
Date: Sat, 3 Dec 2011 15:14:29 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Josh Triplett <josh@...htriplett.org>
Cc: linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
patches@...aro.org, "Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 6/7] driver-core/cpu: Add
cpu_is_hotpluggable() for rcutorture error analysis
On Sat, Dec 03, 2011 at 01:06:50PM -0800, Josh Triplett wrote:
> On Sat, Dec 03, 2011 at 10:34:41AM -0800, Paul E. McKenney wrote:
> > The rcutorture test now can automatically exercise CPU hotplug and
> > collect success statistics, which can be correlated with other rcutorture
> > activity. This permits rcutorture to completely exercise RCU regardless
> > of what sort of userspace and filesystem layout is in use. Unfortunately,
> > rcutorture is happy to attempt to offline CPUs that cannot be offlined,
> > for example, CPU 0 in both the x86 and ARM architectures. Although this
> > allows rcutorture testing to proceed normally, it confounds attempts at
> > error analysis due to the resulting flood of spurious CPU-hotplug errors.
> >
> > Therefore, this commit creates a cpu_is_hotpluggable() function that
> > allows rcutorture to avoid attempting to offline CPUs that are not
> > hotpluggable, which in turn allows rcutorture to avoid reporting spurious
> > CPU-hotplug errors. Note that this function is EXPORT_SYMBOL_GPL()
> > to allow rcutorture to use it when compiled as a kernel module.
> > This commit also includes modifications to rcutorture to use this
> > new function.
>
> I'd suggest writing this as a two-patch series: add the API, then use
> it.
>
> I'd also suggest making the more general case for this API, beyond just
> rcutorture.
>
> > --- a/drivers/base/cpu.c
> > +++ b/drivers/base/cpu.c
> > @@ -23,6 +23,7 @@ struct sysdev_class cpu_sysdev_class = {
> > EXPORT_SYMBOL(cpu_sysdev_class);
> >
> > static DEFINE_PER_CPU(struct sys_device *, cpu_sys_devices);
> > +static DEFINE_PER_CPU(bool, is_hotpluggable);
> [...]
> > @@ -224,8 +226,10 @@ int __cpuinit register_cpu(struct cpu *cpu, int num)
> >
> > error = sysdev_register(&cpu->sysdev);
> >
> > - if (!error && cpu->hotpluggable)
> > + if (!error && cpu->hotpluggable) {
> > register_cpu_control(cpu);
> > + per_cpu(is_hotpluggable, num) = 1;
> > + }
>
> This information already exists in the ->hotpluggable field of "struct
> cpu". I'd suggest an alternate approach (included as a patch below),
> which avoids the need to add a new per_cpu property redundant with
> ->hotpluggable.
I do like your patch better, as it is smaller, less intrusive, and
makes better use of existing facilities. I have therefore replaced
mine with it. I also created a separate patch containing the rcutorture
modifications from my original.
> Note that you can get to the patch below via "git am --scissors", or
> just by copy/pasting it.
Very cool -- wasn't aware of --scissors before. ;-)
Thanx, Paul
> ---8<---
> From: Josh Triplett <josh@...htriplett.org>
> Subject: [PATCH] driver-core/cpu: Expose hotpluggability to the rest of the kernel
>
> When architectures register CPUs, they indicate whether the CPU allows
> hotplugging; notably, x86 and ARM don't allow hotplugging CPU 0.
> Userspace can easily query the hotpluggability of a CPU via sysfs;
> however, the kernel has no convenient way of accessing that property in
> an architecture-independent way. While the kernel can simply try it and
> see, some code needs to distinguish between "hotplug failed" and
> "hotplug has no hope of working on this CPU"; for example, rcutorture's
> CPU hotplug tests want to avoid drowning out real hotplug failures with
> expected failures.
>
> Expose this property via a new cpu_is_hotpluggable function, so that the
> rest of the kernel can access it in an architecture-independent way.
>
> Signed-off-by: Josh Triplett <josh@...htriplett.org>
> ---
> drivers/base/cpu.c | 7 +++++++
> include/linux/cpu.h | 1 +
> 2 files changed, 8 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index 251acea..3991502 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -247,6 +247,13 @@ struct sys_device *get_cpu_sysdev(unsigned cpu)
> }
> EXPORT_SYMBOL_GPL(get_cpu_sysdev);
>
> +bool cpu_is_hotpluggable(unsigned cpu)
> +{
> + struct sys_device *dev = get_cpu_sysdev(cpu);
> + return dev && container_of(dev, struct cpu, sysdev)->hotpluggable;
> +}
> +EXPORT_SYMBOL_GPL(cpu_is_hotpluggable);
> +
> int __init cpu_dev_init(void)
> {
> int err;
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 6cb60fd..305c263 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -27,6 +27,7 @@ struct cpu {
>
> extern int register_cpu(struct cpu *cpu, int num);
> extern struct sys_device *get_cpu_sysdev(unsigned cpu);
> +extern bool cpu_is_hotpluggable(unsigned cpu);
>
> extern int cpu_add_sysdev_attr(struct sysdev_attribute *attr);
> extern void cpu_remove_sysdev_attr(struct sysdev_attribute *attr);
> --
> 1.7.7.3
>
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