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SubjectRe: [PATCH v2] PM: Add a switch for disabling/enabling sync() before suspend
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Hey Rafael,

thanks a lot for taking the time for this thorough review. It's much
appreciated!

I tried to address all your concerns and will resubmit the patch shortly.

Rafael J. Wysocki:
> On Fri, Dec 20, 2019 at 5:03 PM Jonas Meurer <jonas@freesources.org> wrote:
>>
>> The switch allows to enable or disable the final sync() from the suspend.c
>> Linux Kernel system suspend implementation. This is useful to avoid race
>> conditions if block devices have been suspended before. Be aware that you
>> have to take care of sync() yourself before suspending the system if you
>> disable it here.
>>
>> Congruously, the behaviour of build-time switch CONFIG_SUSPEND_SKIP_SYNC
>> is slightly changed: It now configures the default for fun-time switch
>> '/sys/power/sync_on_suspend'.
>>
>> Signed-off-by: Jonas Meurer <jonas@freesources.org>
>
> First, I would use a slightly different subject. What about "PM:
> suspend: Add sysfs attribute to control the "sync on suspend"
> behavior"?

Sure, much more appropriate. Changed :)

> Then, I would write in the changelog that whether or not filesystems
> are synced before system suspend by the kernel depends on a Kconfig
> option which is not flexible enough, because there are cases in which
> user space may want to decide whether or not the sync in the kernel
> should be carried out.
>
> Actually, in the example case given by you, it is not clear enough why
> you cannot build the kernel with CONFIG_SUSPEND_SKIP_SYNC set, so it
> would be good to explain that. Or give a different example.
>
> Then I would say that the existing CONFIG_SUSPEND_SKIP_SYNC will be
> used to set the default value of the new sysfs attribute after the
> patch.

Tried to address that now. Don't hesitate to sugest further improvements
if you feel it's still not properly explained.

>> ---
>> Documentation/ABI/testing/sysfs-power | 15 ++++++++++++
>> include/linux/suspend.h | 2 ++
>> kernel/power/Kconfig | 5 +++-
>> kernel/power/main.c | 33 +++++++++++++++++++++++++++
>> kernel/power/suspend.c | 2 +-
>> 5 files changed, 55 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-power b/Documentation/ABI/testing/sysfs-power
>> index 6f87b9dd384b..783b3ce8eb06 100644
>> --- a/Documentation/ABI/testing/sysfs-power
>> +++ b/Documentation/ABI/testing/sysfs-power
>> @@ -407,3 +407,18 @@ Contact: Kalesh Singh <kaleshsingh96@gmail.com>
>> Description:
>> The /sys/power/suspend_stats/last_failed_step file contains
>> the last failed step in the suspend/resume path.
>> +
>> +What: /sys/power/sync_on_suspend
>> +Date: October 2019
>> +Contact: Jonas Meurer <jonas@freesources.org>
>> +Description:
>> + This file controls the switch to enable or disable the final
>> + sync() before system suspend. This is useful to avoid race
>> + conditions if block devices have been suspended before. Be
>> + aware that you have to take care of sync() yourself before
>> + suspending the system if you disable it here.
>
> I'm not entirely happy with the paragraph above.
>
> It basically doesn't have to explain what the feature is useful for,
> but if you waht to give an example, I would write something like this:
>
> "This file controls whether or not the kernel will sync() filesystems
> during system suspend (after freezing user space and before suspending
> devices). For example, user space may want to prevent the kernel from
> syncing filesystems at that time in cases when system suspend works
> reliably and is carried out relatively often, so doing a system-wide
> filesystem sync every time it happens is not really useful."

On a second thought I agree that the example/reasoning doesn't belong
here. I removed it altogether.

>> +
>> + Writing a "1" to this file enables the sync() and writing a
>> + "0" disables it. Reads from the file return the current value.
>> + The default is "1" but can be configured with the build-time
>> + config flag "SUSPEND_SKIP_SYNC".
>> diff --git a/include/linux/suspend.h b/include/linux/suspend.h
>> index 6fc8843f1c9e..4a230c2f1c31 100644
>> --- a/include/linux/suspend.h
>> +++ b/include/linux/suspend.h
>> @@ -329,6 +329,7 @@ extern void arch_suspend_disable_irqs(void);
>> extern void arch_suspend_enable_irqs(void);
>>
>> extern int pm_suspend(suspend_state_t state);
>> +extern bool sync_on_suspend_enabled;
>> #else /* !CONFIG_SUSPEND */
>> #define suspend_valid_only_mem NULL
>>
>> @@ -342,6 +343,7 @@ static inline bool pm_suspend_default_s2idle(void) { return false; }
>>
>> static inline void suspend_set_ops(const struct platform_suspend_ops *ops) {}
>> static inline int pm_suspend(suspend_state_t state) { return -ENOSYS; }
>> +static inline bool sync_on_suspend_enabled(void) { return true; }
>> static inline bool idle_should_enter_s2idle(void) { return false; }
>> static inline void __init pm_states_init(void) {}
>> static inline void s2idle_set_ops(const struct platform_s2idle_ops *ops) {}
>> diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
>> index d3667b4075c1..7cbfbeacd68a 100644
>> --- a/kernel/power/Kconfig
>> +++ b/kernel/power/Kconfig
>> @@ -27,7 +27,10 @@ config SUSPEND_SKIP_SYNC
>> Skip the kernel sys_sync() before freezing user processes.
>> Some systems prefer not to pay this cost on every invocation
>> of suspend, or they are content with invoking sync() from
>> - user-space before invoking suspend. Say Y if that's your case.
>> + user-space before invoking suspend. There's a run-time switch
>> + at '/sys/power/sync_on_suspend' to configure this behaviour.
>> + This setting changes the default for the run-tim switch. Say Y
>> + to change the default to disable the kernel sys_sync().
>>
>> config HIBERNATE_CALLBACKS
>> bool
>> diff --git a/kernel/power/main.c b/kernel/power/main.c
>> index e26de7af520b..69b7a8aeca3b 100644
>> --- a/kernel/power/main.c
>> +++ b/kernel/power/main.c
>> @@ -190,6 +190,38 @@ static ssize_t mem_sleep_store(struct kobject *kobj, struct kobj_attribute *attr
>> }
>>
>> power_attr(mem_sleep);
>> +
>> +/*
>> + * sync_on_suspend: invoke ksys_sync_helper() before suspend.
>> + *
>> + * show() returns whether ksys_sync_helper() is invoked before suspend.
>> + * store() accepts 0 or 1. 0 disables ksys_sync_helper() and 1 enables it.
>> + */
>> +bool sync_on_suspend_enabled = !IS_ENABLED(CONFIG_SUSPEND_SKIP_SYNC);
>> +
>> +static ssize_t sync_on_suspend_show(struct kobject *kobj,
>> + struct kobj_attribute *attr, char *buf)
>> +{
>> + return sprintf(buf, "%d\n", sync_on_suspend_enabled);
>> +}
>> +
>> +static ssize_t sync_on_suspend_store(struct kobject *kobj,
>> + struct kobj_attribute *attr,
>> + const char *buf, size_t n)
>> +{
>> + unsigned long val;
>> +
>> + if (kstrtoul(buf, 10, &val))
>> + return -EINVAL;
>> +
>> + if (val > 1)
>> + return -EINVAL;
>> +
>> + sync_on_suspend_enabled = !!val;
>> + return n;
>> +}
>> +
>> +power_attr(sync_on_suspend);
>> #endif /* CONFIG_SUSPEND */
>>
>> #ifdef CONFIG_PM_SLEEP_DEBUG
>> @@ -855,6 +887,7 @@ static struct attribute * g[] = {
>> &wakeup_count_attr.attr,
>> #ifdef CONFIG_SUSPEND
>> &mem_sleep_attr.attr,
>> + &sync_on_suspend_attr.attr,
>> #endif
>> #ifdef CONFIG_PM_AUTOSLEEP
>> &autosleep_attr.attr,
>> diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
>> index f3b7239f1892..2c47280fbfc7 100644
>> --- a/kernel/power/suspend.c
>> +++ b/kernel/power/suspend.c
>> @@ -564,7 +564,7 @@ static int enter_state(suspend_state_t state)
>> if (state == PM_SUSPEND_TO_IDLE)
>> s2idle_begin();
>>
>> - if (!IS_ENABLED(CONFIG_SUSPEND_SKIP_SYNC)) {
>> + if (sync_on_suspend_enabled) {
>> trace_suspend_resume(TPS("sync_filesystems"), 0, true);
>> ksys_sync_helper();
>> trace_suspend_resume(TPS("sync_filesystems"), 0, false);
>> --
>
> The patch itself looks fine to me.

Yay, that's great to hear :)

Cheers,
jonas

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