On 04/29/2013 10:12 AM, li guang wrote: > 在 2013-04-29一的 10:00 +0530，Srivatsa S. Bhat写道： >> On 04/29/2013 08:19 AM, liguang wrote: >>> in cpu_down(), _cpu_down() will do >>> " >>> if (num_online_cpus() == 1) >>> return -EBUSY; >>> " >>> when cpu_hotplug_disabled was set, num_online_cpus >>> will return 1 for there's only 1 boot cpu. >>> so, it's unnecessary to check cpu_hotplug_disabled >>> here. >>> >> >> The 2 checks serve very different purposes; they are not the same! > > purposes are different, but I think effects are same for this case, > the statement 'if ((num_online_cpus() == 1)' seems > have same effect with cpu_hotplug_disabled here, > because when cpu_hotplug_disabled, only boot cpu is online >
Why do you keep saying that? They are *two* *different* checks! Whether they happen to have the same effect or not completely depends on the particular *usecase*. And for example, in the suspend/resume case, when the flag 'cpu_hotplug_disabled' is set, *all* CPUs are online! Only much later do we actually call disable_nonboot_cpus() to offline the non-boot CPUs. Take a look at the following functions, then you'll see what scenario I'm referring to: cpu_hotplug_disable_before_freeze(), cpu_hotplug_disable_after_thaw() Also, recently, there was another usecase for the cpu_hotplug_disabled flag, in the reboot code. Here is the link to that discussion: http://thread.gmane.org/gmane.linux.kernel/1480380 Regards, Srivatsa S. Bhat >> >> The num_online_cpus() check is to ensure that the user doesn't do >> something insane like trying to offline the last online CPU in the >> system. >> >> Whereas, the flag 'cpu_hotplug_disabled' is used to prevent user- >> triggered CPU hotplug (such as those initiated through sysfs). >> This is useful in cases where the system itself wants to initiate CPU >> hotplug and it doesn't want annoying races with CPU hotplug going >> on in parallel due to other reasons. One such case is suspend/resume. >> That's why, if you have noticed, the suspend/resume code invokes the >> _cpu_down() version, in order to bypass the flag and get its job done. >> >> So, no, I think the check needs to stay. >> >> Regards, >> Srivatsa S. Bhat >> >>> Signed-off-by: liguang <lig.f...@cn.fujitsu.com> >>> --- >>> kernel/cpu.c | 6 ------ >>> 1 files changed, 0 insertions(+), 6 deletions(-) >>> >>> diff --git a/kernel/cpu.c b/kernel/cpu.c >>> index b5e4ab2..cd166d3 100644 >>> --- a/kernel/cpu.c >>> +++ b/kernel/cpu.c >>> @@ -330,14 +330,8 @@ int __ref cpu_down(unsigned int cpu) >>> >>> cpu_maps_update_begin(); >>> >>> - if (cpu_hotplug_disabled) { >>> - err = -EBUSY; >>> - goto out; >>> - } >>> - >>> err = _cpu_down(cpu, 0); >>> >>> -out: >>> cpu_maps_update_done(); >>> return err; >>> } >>> >> > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
