lkml.org 
[lkml]   [2011]   [Oct]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 2/2] CPU hotplug, Freezer: Synchronize CPU hotplug and Freezer
    On 10/28/2011 05:27 PM, Rafael J. Wysocki wrote:
    > On Friday, October 28, 2011, Srivatsa S. Bhat wrote:
    >> On 10/28/2011 01:43 AM, Rafael J. Wysocki wrote:
    >>> Hi,
    >>>
    >>> On Thursday, October 27, 2011, Srivatsa S. Bhat wrote:
    >>>> Prevent CPU hotplug and the freezer from racing with each other, to ensure
    >>>> that during the *entire duration* for which the callbacks for CPU hotplug
    >>>> notifications such as CPU_ONLINE[_FROZEN], CPU_DEAD[_FROZEN] etc are being
    >>>> executed, the state of the system (with respect to the tasks being frozen
    >>>> or not) remains constant.
    >>>>
    >>>> This patches hooks the CPU hotplug infrastructure onto the freezer
    >>>> notifications (PM_FREEZE_PREPARE and PM_POST_THAW) and thus synchronizes
    >>>> with the freezer.
    >>>>
    >>>> Specifically,
    >>>>
    >>>> * Upon the PM_FREEZE_PREPARE notification, the CPU hotplug callback disables
    >>>> future (regular) CPU hotplugging and also ensures that any currently running
    >>>> CPU hotplug operation is completed before allowing the freezer to continue
    >>>> any further.
    >>>>
    >>>> * Upon the PM_POST_THAW notification, the CPU hotplug callback re-enables
    >>>> regular CPU hotplug.
    >>>>
    >>>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    >>>> ---
    >>>>
    >>>> kernel/cpu.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    >>>> 1 files changed, 76 insertions(+), 0 deletions(-)
    >>>>
    >>>> diff --git a/kernel/cpu.c b/kernel/cpu.c
    >>>> index 12b7458..61985ce 100644
    >>>> --- a/kernel/cpu.c
    >>>> +++ b/kernel/cpu.c
    >>>> @@ -15,6 +15,7 @@
    >>>> #include <linux/stop_machine.h>
    >>>> #include <linux/mutex.h>
    >>>> #include <linux/gfp.h>
    >>>> +#include <linux/suspend.h>
    >>>>
    >>>> #ifdef CONFIG_SMP
    >>>> /* Serializes the updates to cpu_online_mask, cpu_present_mask */
    >>>> @@ -478,6 +479,81 @@ static int alloc_frozen_cpus(void)
    >>>> core_initcall(alloc_frozen_cpus);
    >>>> #endif /* CONFIG_PM_SLEEP_SMP */
    >>>>
    >>>> +
    >>>> +#ifdef CONFIG_FREEZER
    >>>> +
    >>>> +/*
    >>>> + * Avoid CPU hotplug racing with the freezer subsystem, by disabling CPU
    >>>> + * hotplug when tasks are about to be frozen.
    >>>> + *
    >>>> + * Also, don't allow the freezer subsystem to continue until any currently
    >>>> + * running CPU hotplug operation gets completed.
    >>>> + * To modify the 'cpu_hotplug_disabled' flag, we need to acquire the
    >>>> + * 'cpu_add_remove_lock'. And this same lock is also taken by the regular
    >>>> + * CPU hotplug path and released only after it is complete. Thus, we
    >>>> + * (and hence the freezer) will block here until any currently running CPU
    >>>> + * hotplug operation is completed.
    >>>> + */
    >>>> +static void cpu_hotplug_freezer_block_begin(void)
    >>>> +{
    >>>> + cpu_maps_update_begin();
    >>>> + cpu_hotplug_disabled = 1;
    >>>> + cpu_maps_update_done();
    >>>> +}
    >>>> +
    >>>> +
    >>>> +/*
    >>>> + * When thawing of tasks is complete, re-enable CPU hotplug (which had been
    >>>> + * disabled while beginning to freeze tasks).
    >>>> + */
    >>>> +static void cpu_hotplug_freezer_block_done(void)
    >>>> +{
    >>>> + cpu_maps_update_begin();
    >>>> + cpu_hotplug_disabled = 0;
    >>>> + cpu_maps_update_done();
    >>>> +}
    >>>> +
    >>>
    >>> I wonder if the new PM notifier events are really necessary?
    >>>
    >>> Why don't you just call cpu_hotplug_freezer_block_begin() (perhaps
    >>> with a better name?) directly from freeze_processes()? And analogously
    >>> for cpu_hotplug_freezer_block_done() and thaw_processes()?
    >>>
    >>
    >> Yes, we can definitely do that.
    >>
    >> But the reason why I chose to introduce new notifiers was to make this
    >> more extensible (because we know that at least 2 subsystems would benefit
    >> from mutually excluding themselves from the freezer, namely CPU hotplug
    >> and x86 microcode).
    >> http://thread.gmane.org/gmane.linux.kernel/1198291/focus=1200591
    >>
    >> But now that I think of it, hooking onto the freezer notifiers wouldn't
    >> solve the microcode cases since usermodehelper_disable() is called
    >> _before_ freezing tasks... :(
    >>
    >> So we should probably call the functions directly like you suggested..
    >>
    >> But I really didn't want to clutter the freezer call path because of problems
    >> elsewhere. So I felt freezer notifiers would be a cleaner way of dealing with
    >> such things. Also, since freezer is a generic subsystem that could be used
    >> for purposes other than S3/S4 as well (I have heard of attempts to use freezer
    >> during tracing), wouldn't it be better to introduce new notifiers to
    >> announce the begin and end of freezer activity to interested subsystems?
    >> (and then use them to solve the CPU hotplug issue like this patch does...)
    >>
    >> Please let me know your suggestions.
    >
    > The freeze_processes() and thaw_processes() functions are only used for
    > system suspend and hibernation, as far as I can tell, and I don't think there
    > will be any other users in predictable future.
    >
    > Also, adding the calls directly to those functions will show exactly what
    > the dependecies are, while doing that through a notifier kind of obfuscates
    > things. So, please make direct calls from there.
    >

    Ok, thank you for the clarification. I'll post the next version of the patch
    with direct function calls.

    --
    Regards,
    Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    Linux Technology Center,
    IBM India Systems and Technology Lab


    \
     
     \ /
      Last update: 2011-10-28 14:01    [W:0.040 / U:30.568 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site