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Date: Sun, 24 Jun 2012 22:25:40 +0530
From: Aaditya Kumar <aaditya.kumar.30@...il.com>
To: KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
rostedt@...dmis.org, mingo@...nel.org, C.Emde@...dl.org,
jkacur@...hat.com, frank.rowand@...sony.com, tim.bird@...sony.com,
takuzo.ohara@...sony.com, kan.iibuchi@...sony.com,
aaditya.kumar@...sony.com,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC][RT][PATCH] mm: Do not use stop_machine() for
__zone_pcp_udpate() for CONFIG_PREEMPT_RT_FULL
On Sat, Jun 23, 2012 at 9:07 AM, KOSAKI Motohiro
<kosaki.motohiro@...il.com> wrote:
> (6/19/12 2:32 PM), Aaditya Kumar wrote:
>> The code path of __zone_pcp_update() has following locks, which in
>> CONFIG_PREEMPT_RT_FULL=y are rt-mutex.
>> - pa_lock locked by cpu_lock_irqsave()
>> - zone->lock locked by free_pcppages_bulk()
>>
>> Since __zone_pcp_update() is called from stop_machine(), so with
>> CONFIG_PREEMPT_RT_FULL=y
>> we get following backtrace when __zone_pcp_update() is called during
>> memory hot plugging while
>> doing heavy file I/O.
>>
>> I think stop_machine() may not be required for calling __zone_pcp_update()
>> in case of CONFIG_PREEMPT_RT_FULL=y as acquiring pa_lock in __zone_pcp_update()
>> should be sufficient to isolate pcp pages and to setup per cpu pagesets.
>>
>> Can someone please let me know if am missing anything here?
>
Hello Kosaki-san,
> First off, you should cc memory hotplug experts when discussing memory
> hotplug topic.
Sorry for that.
> Second, stop_machine() is required because usually zone->pageset is
> per-cpu variable.
> the regular access rule is, 1) owner cpu can always access their own
> pcp, 2) offlined cpu's
> pcp can be accessed from any cpus because is has no race chance 3)
> otherwise caller must
> use stop_machine for preventing owner cpu accesses pcp.
Thank you very much for your explanation, yes, my approach was not correct.
Since "mm: page_alloc: rt-friendly per-cpu pages" from RT patch set
introduces a preemptible lock
(pa_lock which becomes an rt-mutex) for accessing pcp,
(http://www.kernel.org/pub/linux/kernel/projects/rt/3.4/patch-3.4.3-rt12-rc1.patch.xz)
So while calling zone_pcp_update() (with RT-patch set applied and with
CONFIG_PREEMPT_RT_FULL=y),
we have possibly two options to fix the BUG caused by taking a
sleeping lock in stop_machine.
Option 1. Revert the patch which introduces the sleeping(pa_lock) lock.
Option 2. Fix the calling frame work.(Use another framework)
Since usually memory hot plug is not that frequent an activity in the system,
So a little more overhead occurred while taking option 2, I think is acceptable.
My approach in my below patch for zone_pcp_update() is:
1. For each online cpu, setup pageset of a cpu by scheduling work on that cpu.
2. For each offline cpu, setup pageset of a cpu from current cpu.
3. Flush the all the work spawned in step1.
I will re-send this as a formal patch if there are no objections to
this approach.
---
mm/page_alloc.c | 74 74 + 0 - 0 !
1 file changed, 74 insertions(+)
Index: b/mm/page_alloc.c
===================================================================
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -111,6 +111,16 @@ unsigned long totalreserve_pages __read_
int percpu_pagelist_fraction;
gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
+#ifdef CONFIG_PREEMPT_RT_FULL
+struct zone_pcp_work {
+ int cpu;
+ struct zone *zone;
+ struct work_struct work;
+};
+
+static DEFINE_PER_CPU(struct zone_pcp_work, zone_pcp_update_work);
+#endif
+
#ifdef CONFIG_PM_SLEEP
/*
* The following functions are used by the suspend/hibernate code to
temporarily
@@ -3926,6 +3936,7 @@ int zone_wait_table_init(struct zone *zo
return 0;
}
+#ifndef CONFIG_PREEMPT_RT_FULL
static int __zone_pcp_update(void *data)
{
struct zone *zone = data;
@@ -3949,13 +3960,69 @@ static int __zone_pcp_update(void *data)
return 0;
}
+#else
+
+static void __zone_cpu_pcp_update(struct work_struct *work)
+{
+ struct zone_pcp_work *work_data =
+ container_of(work, struct zone_pcp_work, work);
+ struct zone *zone = work_data->zone;
+ int cpu = work_data->cpu;
+ unsigned long batch = zone_batchsize(zone), flags;
+ struct per_cpu_pageset *pset;
+ struct per_cpu_pages *pcp;
+ LIST_HEAD(dst);
+
+ pset = per_cpu_ptr(zone->pageset, cpu);
+ pcp = &pset->pcp;
+
+ cpu_lock_irqsave(cpu, flags);
+ isolate_pcp_pages(pcp->count, pcp, &dst);
+ free_pcppages_bulk(zone, pcp->count, &dst);
+ setup_pageset(pset, batch);
+ cpu_unlock_irqrestore(cpu, flags);
+
+}
+#endif
+
void zone_pcp_update(struct zone *zone)
{
+
+#ifdef CONFIG_PREEMPT_RT_FULL
+ int cpu;
+
+ get_online_cpus();
+ for_each_possible_cpu(cpu) {
+ struct zone_pcp_work *zone_pcp_work =
+ &per_cpu(zone_pcp_update_work, cpu);
+ zone_pcp_work->zone = zone;
+ zone_pcp_work->cpu = cpu;
+
+ if (cpu_online(cpu))
+ schedule_work_on(cpu, &zone_pcp_work->work);
+ else
+ __zone_cpu_pcp_update(&zone_pcp_work->work);
+ }
+
+ for_each_online_cpu(cpu) {
+ struct zone_pcp_work *zone_pcp_work =
+ &per_cpu(zone_pcp_update_work, cpu);
+
+ flush_work(&zone_pcp_work->work);
+ }
+ put_online_cpus();
+
+#else
+
stop_machine(__zone_pcp_update, zone, NULL);
+#endif
}
static __meminit void zone_pcp_init(struct zone *zone)
{
+#ifdef CONFIG_PREEMPT_RT_FULL
+ int cpu;
+#endif
/*
* per cpu subsystem is not up at this point. The following code
* relies on the ability of the linker to provide the
@@ -3967,6 +4034,13 @@ static __meminit void zone_pcp_init(stru
printk(KERN_DEBUG " %s zone: %lu pages, LIFO batch:%u\n",
zone->name, zone->present_pages,
zone_batchsize(zone));
+#ifdef CONFIG_PREEMPT_RT_FULL
+ for_each_possible_cpu(cpu) {
+ struct zone_pcp_work *zone_pcp_work =
+ &per_cpu(zone_pcp_update_work, cpu);
+ INIT_WORK(&zone_pcp_work->work, __zone_cpu_pcp_update);
+ }
+#endif
}
__meminit int init_currently_empty_zone(struct zone *zone,
>
> stop_machine and send ipi are common technique for per-cpu area hack.
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