Messages in this thread | | | From | Neil Zhang <> | Date | Wed, 29 Oct 2014 19:03:20 -0700 | Subject | RE: [PATCH V2] Driver cpu: update online when cpu_up/down besides sysfs |
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Dan,
> -----Original Message----- > From: ddstreet@gmail.com [mailto:ddstreet@gmail.com] On Behalf Of Dan > Streetman > Sent: 2014年10月30日 5:52 > To: Rafael J. Wysocki > Cc: Rafael J. Wysocki; Neil Zhang; linux-kernel; Greg Kroah-Hartman; > nfont@linux.vnet.ibm.com > Subject: Re: [PATCH V2] Driver cpu: update online when cpu_up/down besides > sysfs > > On Wed, Oct 29, 2014 at 5:46 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote: > > On Monday, October 27, 2014 08:46:08 PM Dan Streetman wrote: > >> On Mon, Oct 27, 2014 at 5:38 PM, Rafael J. Wysocki > >> <rafael.j.wysocki@intel.com> wrote: > >> > On 10/27/2014 3:59 AM, Neil Zhang wrote: > >> >> > >> >> The current per-cpu offline info won't be updated when we use any > >> >> other method besides sysfs to call cpu_up/down. > >> >> Thus the cpu/online can't reflect the real online status. > >> >> > >> >> This patch is going to fix the issue introduced by commit > >> >> 0902a9044fa5b7a0456ea4daacec2c2b3189ba8c (Driver core: > >> >> Use generic offline/online for CPU offline/online) > >> >> > >> >> CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > >> >> Tested-by: Dan Streetman <ddstreet@ieee.org> > >> >> Signed-off-by: Neil Zhang <zhangwm@marvell.com> > >> > > >> > > >> > Oh dear, no. > >> > > >> > Please first tell me what exactly the problem you're seeing is. > >> > >> For some background, here is my last comment on the first email thread on > this: > >> https://lkml.org/lkml/2014/10/27/595 > >> > >> I didn't create this patch, but the problem essentially is that > >> before your commit the individual cpu online nodes > >> (/sys/devices/system/cpu/cpuN/online) stayed in sync during > >> cpu_down/up, because they used the cpu_online_mask; while after the > >> commit, they are tracked by the cpu's generic dev->offline flag, > >> which isn't updated during cpu_down/up. > > > > Which is not triggered from sysfs. > > > >> So now, any place in the kernel > >> that brings a cpu up or down must also update the cpu->dev->offline > >> flag. > > > > Not any place. In particular, system suspend-resume doesn't need to > > do that, because it takes CPUs offline and then brings them back > > online. > > > > If there's a place in the kernel where CPUs are taken offline and left > > in that state, then it needs to be updated. > > The only place I know of is ppc's dlpar code, as mentioned below. > > Neil, as you crafted the original patch, I assume you know of some other place > in the kernel doing cpu_up/down directly, where you're seeing this problem? >
As I replied to Rafael many ARM SoCs will use an in kernel profiler to romove/add a core via cpu_up /down for power and performance consideration. But actually we can fix these issues as Rafael suggested. Thanks for the discussion in these days.
> > > >> My interest in the patch was coincidental because I was seeing the > >> same problem when using dlpar operations to hotplug cpus, which uses > >> the arch/powerpc/platform/pseries/dlpar.c code; that code brings a > >> cpu offline when it's hot-removed (and the cpu online when it's > >> hot-added), but it hasn't been changed to also update the cpu's > >> dev->offline flag. > > > > It should be modified to do that. > > > >> As I said in the previous email to the first thread, the ppc dlpar > >> operation might be changed in the future to fully unregister a cpu > >> when it's hot-removed, which would remove the entire sysfs cpuN > >> directory. Alternately and/or until then, it could be updated to > >> simply update the cpu'd dev->offline flag (that's what I originally > >> did for my own testing). However, without a central place to update > >> the cpu's dev->offline field, like this, or possibly in > >> set_cpu_online(), or elsewhere during cpu_down/up, each place in the > >> kernel that calls cpu_down() or cpu_up() also needs to update the > >> dev->offline flag. It's possible that the ppc dlpar code is the only > >> place in the kernel that has this problem; I haven't searched. > > > > It is quite likely to be the only place like that. > > > > While I'm not familiar with the code in question, the most > > straightforward way to fix the problem would be to replace cpu_down() > > in there with device_offline(get_cpu_device(cpu)), but that needs to > > be called under device_hotplug_lock. > > Ok, will do. > > > > > -- > > I speak only for myself. > > Rafael J. Wysocki, Intel Open Source Technology Center.
Best Regards, Neil Zhang
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