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Date: Wed, 16 Jun 2010 23:59:07 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Arjan van de Ven <arjan@infradead.org>, "Rafael J. Wysocki" <rjw@sisk.pl>,
       Maxim Levitsky <maximlevitsky@gmail.com>,
       Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
       Jiri Slaby <jslaby@suse.cz>, linux-pm@lists.linux-foundation.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] cpuidle: avoid using smp_processor_id() in preemptible
 code (nr_iowait_cpu) v4
Message-Id: <20100616235907.eb11dfcc.akpm@linux-foundation.org>
In-Reply-To: <20100617062950.GA3979@swordfish>
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On Thu, 17 Jun 2010 09:29:50 +0300 Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote:

> Fix
>    
>  BUG: using smp_processor_id() in preemptible [00000000] code: s2disk/3392
>  caller is nr_iowait_cpu+0xe/0x1e
>  Pid: 3392, comm: s2disk Not tainted 2.6.35-rc3-dbg-00106-ga75e02b #2
>  Call Trace:
>  [<c1184c55>] debug_smp_processor_id+0xa5/0xbc
>  [<c10282a5>] nr_iowait_cpu+0xe/0x1e
>  [<c104ab7c>] update_ts_time_stats+0x32/0x6c
>  [<c104ac73>] get_cpu_idle_time_us+0x36/0x58
>  [<c124229b>] get_cpu_idle_time+0x12/0x74   
>  [<c1242963>] cpufreq_governor_dbs+0xc3/0x2dc
>  [<c1240437>] __cpufreq_governor+0x51/0x85   
>  [<c1241190>] __cpufreq_set_policy+0x10c/0x13d
>  [<c12413d3>] cpufreq_add_dev_interface+0x212/0x233
>  [<c1241b1e>] ? handle_update+0x0/0xd
>  [<c1241a18>] cpufreq_add_dev+0x34b/0x35a
>  [<c103c973>] ? schedule_delayed_work_on+0x11/0x13
>  [<c12c14db>] cpufreq_cpu_callback+0x59/0x63
>  [<c1042f39>] notifier_call_chain+0x26/0x48 
>  [<c1042f7d>] __raw_notifier_call_chain+0xe/0x10
>  [<c102efb9>] __cpu_notify+0x15/0x29
>  [<c102efda>] cpu_notify+0xd/0xf
>  [<c12bfb30>] _cpu_up+0xaf/0xd2 
>  [<c12b3ad4>] enable_nonboot_cpus+0x3d/0x94
>  [<c1055eef>] hibernation_snapshot+0x104/0x1a2
>  [<c1058b49>] snapshot_ioctl+0x24b/0x53e
>  [<c1028ad1>] ? sub_preempt_count+0x7c/0x89
>  [<c10ab91d>] vfs_ioctl+0x2e/0x8c
>  [<c10588fe>] ? snapshot_ioctl+0x0/0x53e
>  [<c10ac2c7>] do_vfs_ioctl+0x42f/0x45a  
>  [<c10a0ba5>] ? fsnotify_modify+0x4f/0x5a
>  [<c11e9dc3>] ? tty_write+0x0/0x1d0
>  [<c10a12d6>] ? vfs_write+0xa2/0xda
>  [<c10ac333>] sys_ioctl+0x41/0x62  
>  [<c10027d3>] sysenter_do_call+0x12/0x2d
> 
> The initial fix was to use get_cpu/put_cpu in nr_iowait_cpu.  However,
> Arjan stated that "the bug is that it needs to be nr_iowait_cpu(int cpu)".
> 
> This patch introduces nr_iowait_cpu(int cpu) and changes to its callers.
> 
> Arjan also pointed out that we can't use get_cpu/put_cpu in update_ts_time_stats
> since we "pick the current cpu, rather than the one denoted by ts" in that case.
> To match given *ts and cpu denoted by *ts we use new field in the struct tick_sched: int cpu.
> 
>
> ...
>
>  struct tick_sched *tick_get_tick_sched(int cpu)
>  {
> +	/*FIXME: Arjan van de Ven:
> +	 can we do this bit once, when the ts structure gets initialized?*/
> +	per_cpu(tick_cpu_sched, cpu).cpu = cpu;
>  	return &per_cpu(tick_cpu_sched, cpu);
>  }

That's just weird.  And by doing a write it does require that this
cahcheline be probably-read and written back regularly, which is more
bus traffic.

It should be OK to initialise these guys with a for_each_possible_cpu()
loop in a new module_init() function in tick-sched.c - if someone runs
update_ts_time_stats() before the initcalls then conceivably the
`swapper' process's accounting will go a little bit wrong, but I doubt
it.

Still, it'd be better to do it earlier, I guess.  tick_init() is called
super-early and that would be a good place.  tick_init() is presently a
no-op if !CONFIG_GENERIC_CLOCKEVENTS, but all this code depends on
CONFIG_GENERIC_CLOCKEVENTS anwyay.

So how does this look?  If "OK" then would you be able to test it please?


[ Sigh.  The field tick_sched.cpu shouldn't even exist on
  uniprocessor builds.  Ifdeffing it away is trivial and a bit messy,
  but it's still only a partial solution.  Passing the `cpu' argument
  to nr_iowait_cpu() will generate additional code, and it's unneeded
  on uniprocessor builds.]


 include/linux/tick.h      |    1 +
 kernel/time/tick-common.c |    1 +
 kernel/time/tick-sched.c  |   11 ++++++++---
 3 files changed, 10 insertions(+), 3 deletions(-)

diff -puN include/linux/tick.h~cpuidle-avoid-using-smp_processor_id-in-preemptible-code-nr_iowait_cpu-v4-fix include/linux/tick.h
--- a/include/linux/tick.h~cpuidle-avoid-using-smp_processor_id-in-preemptible-code-nr_iowait_cpu-v4-fix
+++ a/include/linux/tick.h
@@ -71,6 +71,7 @@ struct tick_sched {
 };
 
 extern void __init tick_init(void);
+extern void __init tick_sched_init(void);
 extern int tick_is_oneshot_available(void);
 extern struct tick_device *tick_get_device(int cpu);
 
diff -puN kernel/time/tick-sched.c~cpuidle-avoid-using-smp_processor_id-in-preemptible-code-nr_iowait_cpu-v4-fix kernel/time/tick-sched.c
--- a/kernel/time/tick-sched.c~cpuidle-avoid-using-smp_processor_id-in-preemptible-code-nr_iowait_cpu-v4-fix
+++ a/kernel/time/tick-sched.c
@@ -38,9 +38,6 @@ static ktime_t last_jiffies_update;
 
 struct tick_sched *tick_get_tick_sched(int cpu)
 {
-	/*FIXME: Arjan van de Ven:
-	 can we do this bit once, when the ts structure gets initialized?*/
-	per_cpu(tick_cpu_sched, cpu).cpu = cpu;
 	return &per_cpu(tick_cpu_sched, cpu);
 }
 
@@ -880,3 +877,11 @@ int tick_check_oneshot_change(int allow_
 	tick_nohz_switch_to_nohz();
 	return 0;
 }
+
+void __init tick_sched_init(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		per_cpu(tick_cpu_sched, cpu).cpu = cpu;
+}
diff -puN kernel/time/tick-common.c~cpuidle-avoid-using-smp_processor_id-in-preemptible-code-nr_iowait_cpu-v4-fix kernel/time/tick-common.c
--- a/kernel/time/tick-common.c~cpuidle-avoid-using-smp_processor_id-in-preemptible-code-nr_iowait_cpu-v4-fix
+++ a/kernel/time/tick-common.c
@@ -413,4 +413,5 @@ static struct notifier_block tick_notifi
 void __init tick_init(void)
 {
 	clockevents_register_notifier(&tick_notifier);
+	tick_sched_init();
 }
_

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