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Date: Sat, 7 Feb 2009 18:02:22 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: Mandeep Singh Baines <msb@...gle.com>
Cc: mingo@...e.hu, linux-kernel@...r.kernel.org, rientjes@...gle.com,
mbligh@...gle.com, thockin@...gle.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] softlockup: remove timestamp checking from hung_task
On Sat, Feb 07, 2009 at 05:51:55PM +0100, Frederic Weisbecker wrote:
> On Sat, Feb 07, 2009 at 05:34:40PM +0100, Frederic Weisbecker wrote:
> > On Sat, Feb 07, 2009 at 05:23:28PM +0100, Frederic Weisbecker wrote:
> > > Hi Mandeep,
> > >
> > > On Fri, Feb 06, 2009 at 03:37:47PM -0800, Mandeep Singh Baines wrote:
> > > > Patch against tip/core/softlockup
> > > >
> > > > ---
> > > > Impact: saves sizeof(long) bytes per task_struct
> > > >
> > > > By guaranteeing that sysctl_hung_task_timeout_secs have elapsed between
> > > > tasklist scans we can avoid using timestamps.
> > > >
> > > > Signed-off-by: Mandeep Singh Baines <msb@...gle.com>
> > >
> > >
> > > Good idea.
> > > BTW, why haven't you put your name on top of this file?
> > > That would help those who will send patches knowing to whom they have to
> > > route their mails.
> > >
> > > I made some comments below about small things...
> > >
> > > > ---
> > > > include/linux/sched.h | 1 -
> > > > kernel/fork.c | 8 +++-----
> > > > kernel/hung_task.c | 48 +++++++++---------------------------------------
> > > > 3 files changed, 12 insertions(+), 45 deletions(-)
> > > >
> > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > > index 2a2811c..e0d723f 100644
> > > > --- a/include/linux/sched.h
> > > > +++ b/include/linux/sched.h
> > > > @@ -1241,7 +1241,6 @@ struct task_struct {
> > > > #endif
> > > > #ifdef CONFIG_DETECT_HUNG_TASK
> > > > /* hung task detection */
> > > > - unsigned long last_switch_timestamp;
> > > > unsigned long last_switch_count;
> > > > #endif
> > > > /* CPU-specific state of this task */
> > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > index fb94442..bf582f7 100644
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -639,6 +639,9 @@ static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)
> > > >
> > > > tsk->min_flt = tsk->maj_flt = 0;
> > > > tsk->nvcsw = tsk->nivcsw = 0;
> > > > +#ifdef CONFIG_DETECT_HUNG_TASK
> > > > + tsk->last_switch_count = tsk->nvcsw + tsk->nivcsw;
> > > > +#endif
> > >
> > >
> > > I think you can directly assign a zero here :-)
> > > Or you want to let it as is to give some sense and explanation
> > > about the role of this field?
> > > Why not, I guess gcc will optimize it anyway.
> > >
> > >
> > > > tsk->mm = NULL;
> > > > tsk->active_mm = NULL;
> > > > @@ -1041,11 +1044,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> > > >
> > > > p->default_timer_slack_ns = current->timer_slack_ns;
> > > >
> > > > -#ifdef CONFIG_DETECT_HUNG_TASK
> > > > - p->last_switch_count = 0;
> > > > - p->last_switch_timestamp = 0;
> > > > -#endif
> > > > -
> > > > task_io_accounting_init(&p->ioac);
> > > > acct_clear_integrals(p);
> > > >
> > > > diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> > > > index 3951a80..4a10756 100644
> > > > --- a/kernel/hung_task.c
> > > > +++ b/kernel/hung_task.c
> > > > @@ -34,7 +34,6 @@ unsigned long __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
> > > > * Zero means infinite timeout - no checking done:
> > > > */
> > > > unsigned long __read_mostly sysctl_hung_task_timeout_secs = 120;
> > > > -static unsigned long __read_mostly hung_task_poll_jiffies;
> > > >
> > > > unsigned long __read_mostly sysctl_hung_task_warnings = 10;
> > > >
> > > > @@ -69,33 +68,17 @@ static struct notifier_block panic_block = {
> > > > .notifier_call = hung_task_panic,
> > > > };
> > > >
> > > > -/*
> > > > - * Returns seconds, approximately. We don't need nanosecond
> > > > - * resolution, and we don't need to waste time with a big divide when
> > > > - * 2^30ns == 1.074s.
> > > > - */
> > > > -static unsigned long get_timestamp(void)
> > > > -{
> > > > - int this_cpu = raw_smp_processor_id();
> > > > -
> > > > - return cpu_clock(this_cpu) >> 30LL; /* 2^30 ~= 10^9 */
> > > > -}
> > > > -
> > > > -static void check_hung_task(struct task_struct *t, unsigned long now,
> > > > - unsigned long timeout)
> > > > +static void check_hung_task(struct task_struct *t, unsigned long timeout)
> > > > {
> > > > unsigned long switch_count = t->nvcsw + t->nivcsw;
> > > >
> > > > if (t->flags & PF_FROZEN)
> > > > return;
> > > >
> > > > - if (switch_count != t->last_switch_count || !t->last_switch_timestamp) {
> > > > + if (switch_count != t->last_switch_count) {
> > > > t->last_switch_count = switch_count;
> > > > - t->last_switch_timestamp = now;
> > > > return;
> > > > }
> > >
> > >
> > >
> > > What happens here if khungtaskd is scheduled in after tsk is inserted on the task_list
> > > in copy_process() but before tsk has been scheduled once?
> > >
> > > tsk->last_switch_count and tsk->nvcsw + tsk->nivcsw will still be equal to zero right?
> > >
> > > Perhaps you could add another check such as
> > >
> > > if (!switch_count)
> > > return;
> > >
> > >
> > > > - if ((long)(now - t->last_switch_timestamp) < timeout)
> > > > - return;
> > > > if (!sysctl_hung_task_warnings)
> > > > return;
> > > > sysctl_hung_task_warnings--;
> > > > @@ -111,7 +94,6 @@ static void check_hung_task(struct task_struct *t, unsigned long now,
> > > > sched_show_task(t);
> > > > __debug_show_held_locks(t);
> > > >
> > > > - t->last_switch_timestamp = now;
> > > > touch_nmi_watchdog();
> > > >
> > > > if (sysctl_hung_task_panic)
> > > > @@ -145,7 +127,6 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> > > > {
> > > > int max_count = sysctl_hung_task_check_count;
> > > > int batch_count = HUNG_TASK_BATCHING;
> > > > - unsigned long now = get_timestamp();
> > > > struct task_struct *g, *t;
> > > >
> > > > /*
> > > > @@ -168,19 +149,16 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> > > > }
> > > > /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
> > > > if (t->state == TASK_UNINTERRUPTIBLE)
> > > > - check_hung_task(t, now, timeout);
> > > > + check_hung_task(t, timeout);
> > > > } while_each_thread(g, t);
> > > > unlock:
> > > > rcu_read_unlock();
> > > > }
> > > >
> > > > -static void update_poll_jiffies(void)
> > > > +static unsigned long timeout_jiffies(unsigned long timeout)
> > > > {
> > > > /* timeout of 0 will disable the watchdog */
> > > > - if (sysctl_hung_task_timeout_secs == 0)
> > > > - hung_task_poll_jiffies = MAX_SCHEDULE_TIMEOUT;
> > > > - else
> > > > - hung_task_poll_jiffies = sysctl_hung_task_timeout_secs * HZ / 2;
> > > > + return (timeout ? timeout * HZ : MAX_SCHEDULE_TIMEOUT);
> > > > }
> > > >
> > > > /*
> > > > @@ -197,8 +175,6 @@ int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
> > > > if (ret || !write)
> > > > goto out;
> > > >
> > > > - update_poll_jiffies();
> > > > -
> > > > wake_up_process(watchdog_task);
> > >
> > >
> > > I'm not sure what does this function now that you dropped update_poll_jiffies()
> > > So if the user sets up a new timeout value, the only effect will be that khungtaskd will
> > > be awakened?
> > >
> > > But actually the /sys file doesn't seem to be set up.
> >
> >
> > Oops, I should have grep on proc_dohung_task_timeout_secs which is set on kernel/sysctl.
> > Sorry.
> > But still, sysctl_hung_task_timeout_secs doesn't seem to be set :-)
>
>
> And once again I'm wrong, I shoud read sysctl.c or at least read the sysfs documentation.
> Sorry for the noise.
>
> BTW, here is a small fixlet on top of your patch about what I commented concerning
> the tasks than weren't yet scheduled once:
But it is not supposed to be TASK_UNINTERRUPTIBLE.
Forget this fixlet.
So, again sorry for the noise, I'm going to hide somewhere...
> --
> From e7120e424b031978e482b5fe311d90916ffb8b7e Mon Sep 17 00:00:00 2001
> From: Frederic Weisbecker <fweisbec@...il.com>
> Date: Sat, 7 Feb 2009 17:45:12 +0100
> Subject: [PATCH] softlockup: ensure the task has been scheduled once
>
> When we check if the task has been scheduled since the last scan, we might
> have a race condition if the task has been inserted on the task list but not
> yet scheduled once. So we just add a small check to ensure it has been switched
> in at least one time to avoid false positive.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
> kernel/hung_task.c | 6 +++++-
> 1 files changed, 5 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 4a10756..7f57a71 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -72,7 +72,11 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
> {
> unsigned long switch_count = t->nvcsw + t->nivcsw;
>
> - if (t->flags & PF_FROZEN)
> + /*
> + * Ensure the task is not frozen and that it has been scheduled
> + * at least once.
> + */
> + if (t->flags & PF_FROZEN || !switch_count)
> return;
>
> if (switch_count != t->last_switch_count) {
> --
> 1.6.1
>
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