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Date: Sat, 7 Feb 2009 17:34:41 +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 05:23:28PM +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 :-)
>
> Other than these comments, that looks good!
> Thanks.
>
> Frederic.
>
>
> >
> > out:
> > @@ -211,20 +187,14 @@ int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
> > static int watchdog(void *dummy)
> > {
> > set_user_nice(current, 0);
> > - update_poll_jiffies();
> >
> > for ( ; ; ) {
> > - unsigned long timeout;
> > + unsigned long timeout = sysctl_hung_task_timeout_secs;
> >
> > - while (schedule_timeout_interruptible(hung_task_poll_jiffies));
> > + while (schedule_timeout_interruptible(timeout_jiffies(timeout)))
> > + timeout = sysctl_hung_task_timeout_secs;
> >
> > - /*
> > - * Need to cache timeout here to avoid timeout being set
> > - * to 0 via sysctl while inside check_hung_*_tasks().
> > - */
> > - timeout = sysctl_hung_task_timeout_secs;
> > - if (timeout)
> > - check_hung_uninterruptible_tasks(timeout);
> > + check_hung_uninterruptible_tasks(timeout);
> > }
> >
> > return 0;
> > --
> > 1.5.4.5
> >
--
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