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Date: Wed, 23 Jul 2014 13:21:30 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Chintan Pandya <cpandya@...eaurora.org>
Cc: hughd@...gle.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
lauraa@...eaurora.org
Subject: Re: [PATCH] ksm: Provide support to use deferred timers for scanner
thread
On Wed, 23 Jul 2014 14:41:32 +0530 Chintan Pandya <cpandya@...eaurora.org> wrote:
> KSM thread to scan pages is getting schedule on definite timeout.
> That wakes up CPU from idle state and hence may affect the power
> consumption. Provide an optional support to use deferred timer
> which suites low-power use-cases.
Do you have any data on the effectiveness of this patch? Because if it
makes no useful difference, we shouldn't merge it!
> To enable deferred timers,
> $ echo 1 > /sys/kernel/mm/ksm/deferred_timer
It would be preferable to do this unconditionally (or automatically
somehow) rather than adding yet another weird knob.
> Signed-off-by: Chintan Pandya <cpandya@...eaurora.org>
> ---
> Documentation/vm/ksm.txt | 7 ++++++
> mm/ksm.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 71 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
> index f34a8ee..f40b965 100644
> --- a/Documentation/vm/ksm.txt
> +++ b/Documentation/vm/ksm.txt
> @@ -87,6 +87,13 @@ pages_sharing - how many more sites are sharing them i.e. how much saved
> pages_unshared - how many pages unique but repeatedly checked for merging
> pages_volatile - how many pages changing too fast to be placed in a tree
> full_scans - how many times all mergeable areas have been scanned
> +deferred_timer - whether to use deferred timers or not
> + e.g. "echo 1 > /sys/kernel/mm/ksm/deferred_timer"
> + Default: 0 (means, we are not using deferred timers. Users
> + might want to set deferred_timer option if they donot want
> + ksm thread to wakeup CPU to carryout ksm activities thus
> + gaining on battery while compromising slightly on memory
> + that could have been saved.)
>
> A high ratio of pages_sharing to pages_shared indicates good sharing, but
> a high ratio of pages_unshared to pages_sharing indicates wasted effort.
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 346ddc9..e26ec3b 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -223,6 +223,9 @@ static unsigned int ksm_thread_pages_to_scan = 100;
> /* Milliseconds ksmd should sleep between batches */
> static unsigned int ksm_thread_sleep_millisecs = 20;
>
> +/* Boolean to indicate whether to use deferred timer or not */
> +static bool use_deferred_timer;
> +
> #ifdef CONFIG_NUMA
> /* Zeroed when merging across nodes is not allowed */
> static unsigned int ksm_merge_across_nodes = 1;
> @@ -1705,6 +1708,41 @@ static void ksm_do_scan(unsigned int scan_npages)
> }
> }
>
> +static void process_timeout(unsigned long __data)
> +{
> + wake_up_process((struct task_struct *)__data);
> +}
> +
> +static signed long __sched deferred_schedule_timeout(signed long timeout)
Should be called schedule_timeout_deferrable_interruptible() for
consistency.
And let's not start mixing "deferred" and "deferrable".
> +{
> + struct timer_list timer;
> + unsigned long expire;
> +
> + __set_current_state(TASK_INTERRUPTIBLE);
> + if (timeout < 0) {
> + pr_err("schedule_timeout: wrong timeout value %lx\n",
^^^^^^^^^^^^^^^^ copy-n-paste?
> + timeout);
> + __set_current_state(TASK_RUNNING);
> + goto out;
> + }
> +
> + expire = timeout + jiffies;
> +
> + setup_deferrable_timer_on_stack(&timer, process_timeout,
> + (unsigned long)current);
> + mod_timer(&timer, expire);
> + schedule();
> + del_singleshot_timer_sync(&timer);
> +
> + /* Remove the timer from the object tracker */
> + destroy_timer_on_stack(&timer);
> +
> + timeout = expire - jiffies;
> +
> +out:
> + return timeout < 0 ? 0 : timeout;
> +}
Methinks all this should be in kernel/timer.c (kernel/time/timer.c in
linux-next). And it should be documented. That means a separate patch
and review by Thomas Gleixner and probably others.
I haven't looked, but I expect a lot of schedule_timeout() callsites
could be converted to use such a thing.
> static int ksmd_should_run(void)
> {
> return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
> @@ -1725,7 +1763,11 @@ static int ksm_scan_thread(void *nothing)
> try_to_freeze();
>
> if (ksmd_should_run()) {
> - schedule_timeout_interruptible(
> + if (use_deferred_timer)
> + deferred_schedule_timeout(
> + msecs_to_jiffies(ksm_thread_sleep_millisecs));
> + else
> + schedule_timeout_interruptible(
> msecs_to_jiffies(ksm_thread_sleep_millisecs));
> } else {
> wait_event_freezable(ksm_thread_wait,
> @@ -2181,6 +2223,26 @@ static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
> }
> KSM_ATTR(run);
>
> +static ssize_t deferred_timer_show(struct kobject *kobj,
> + struct kobj_attribute *attr, char *buf)
> +{
> + return snprintf(buf, 8, "%d\n", use_deferred_timer);
> +}
> +
> +static ssize_t deferred_timer_store(struct kobject *kobj,
> + struct kobj_attribute *attr,
> + const char *buf, size_t count)
> +{
> + unsigned long enable;
> + int err;
> +
> + err = kstrtoul(buf, 10, &enable);
> + use_deferred_timer = enable;
We should check for legitimate values here. ie: 0 or 1 only.
> + return count;
> +}
> +KSM_ATTR(deferred_timer);
> +
> #ifdef CONFIG_NUMA
> static ssize_t merge_across_nodes_show(struct kobject *kobj,
> struct kobj_attribute *attr, char *buf)
> @@ -2293,6 +2355,7 @@ static struct attribute *ksm_attrs[] = {
> &pages_unshared_attr.attr,
> &pages_volatile_attr.attr,
> &full_scans_attr.attr,
> + &deferred_timer_attr.attr,
> #ifdef CONFIG_NUMA
> &merge_across_nodes_attr.attr,
> #endif
--
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