[PATCH 0/2] Miscellaneous updates for v6.12'

[PATCH 0/2] Miscellaneous updates for v6.12'

[Paul E. McKenney]

Hello!

This series contains a couple of miscellaneous updates:

/home/paulmck/bin/extractsubject.sh: 5: cannot open /tmp/rcu/*: No such file
						Thanx, Paul

------------------------------------------------------------------------

1.	Add check for clocks going backwards to wq_worker_tick().

2.	Sleep at TASK_IDLE when waiting for application core dump.

						Thanx, Paul

------------------------------------------------------------------------

 exit.c      |    2 +-
 workqueue.c |    2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

[PATCH misc 1/2] workqueue: Add check for clocks going backwards to wq_worker_tick()

[Paul E. McKenney]

Experimental, might never go to mainline.

There has been some evidence of clocks going backwards, producing
"workqueue: kfree_rcu_monitor hogged CPU" diagnostics on idle systems
just after a change in clocksource.  This diagnostic commit checks for
this, ignoring differences that would be negative if interpreted as a
signed 64-bit integer.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Rik van Riel <riel@surriel.com>
---
 kernel/workqueue.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1745ca788ede3..4f7b4b32e6b4e 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1482,6 +1482,7 @@ void wq_worker_tick(struct task_struct *task)
 	 * If the current worker is concurrency managed and hogged the CPU for
 	 * longer than wq_cpu_intensive_thresh_us, it's automatically marked
 	 * CPU_INTENSIVE to avoid stalling other concurrency-managed work items.
+	 * If the time is negative, ignore, assuming a backwards clock.
 	 *
 	 * Set @worker->sleeping means that @worker is in the process of
 	 * switching out voluntarily and won't be contributing to
@@ -1491,6 +1492,7 @@ void wq_worker_tick(struct task_struct *task)
 	 * We probably want to make this prettier in the future.
 	 */
 	if ((worker->flags & WORKER_NOT_RUNNING) || READ_ONCE(worker->sleeping) ||
+	    WARN_ON_ONCE((s64)(worker->task->se.sum_exec_runtime - worker->current_at) < 0) ||
 	    worker->task->se.sum_exec_runtime - worker->current_at <
 	    wq_cpu_intensive_thresh_us * NSEC_PER_USEC)
 		return;
-- 
2.40.1

Re: [PATCH misc 1/2] workqueue: Add check for clocks going backwards to wq_worker_tick()

[Rik van Riel]

On Thu, 2024-08-01 at 17:30 -0700, Paul E. McKenney wrote:
> 
> +++ b/kernel/workqueue.c
> @@ -1482,6 +1482,7 @@ void wq_worker_tick(struct task_struct *task)
>  	 * If the current worker is concurrency managed and hogged
> the CPU for
>  	 * longer than wq_cpu_intensive_thresh_us, it's
> automatically marked
>  	 * CPU_INTENSIVE to avoid stalling other concurrency-managed
> work items.
> +	 * If the time is negative, ignore, assuming a backwards
> clock.
>  	 *
>  	 * Set @worker->sleeping means that @worker is in the
> process of
>  	 * switching out voluntarily and won't be contributing to
> @@ -1491,6 +1492,7 @@ void wq_worker_tick(struct task_struct *task)
>  	 * We probably want to make this prettier in the future.
>  	 */
>  	if ((worker->flags & WORKER_NOT_RUNNING) ||
> READ_ONCE(worker->sleeping) ||
> +	    WARN_ON_ONCE((s64)(worker->task->se.sum_exec_runtime -
> worker->current_at) < 0) ||
>  	    worker->task->se.sum_exec_runtime - worker->current_at <
>  	    wq_cpu_intensive_thresh_us * NSEC_PER_USEC)
>  		return;

What is the code path by which sum_exec_runtime could go backward
in time, if the TSC and sched_clock() jump backward?

Might it make sense to check in the place where sum_exec_runtime is
updated, instead, and catch a wider net?

On the flip side, the run time increments are "fairly large" in
number of TSC cycles, while most of the negative TSC jumps we 
have seen are quite small, so even that wider net might not catch
much because of how coarse these updates typically are...

-- 
All Rights Reversed.

Re: [PATCH misc 1/2] workqueue: Add check for clocks going backwards to wq_worker_tick()

[Paul E. McKenney]

On Fri, Aug 02, 2024 at 12:06:06PM -0400, Rik van Riel wrote:
> On Thu, 2024-08-01 at 17:30 -0700, Paul E. McKenney wrote:
> > 
> > +++ b/kernel/workqueue.c
> > @@ -1482,6 +1482,7 @@ void wq_worker_tick(struct task_struct *task)
> >  	 * If the current worker is concurrency managed and hogged
> > the CPU for
> >  	 * longer than wq_cpu_intensive_thresh_us, it's
> > automatically marked
> >  	 * CPU_INTENSIVE to avoid stalling other concurrency-managed
> > work items.
> > +	 * If the time is negative, ignore, assuming a backwards
> > clock.
> >  	 *
> >  	 * Set @worker->sleeping means that @worker is in the
> > process of
> >  	 * switching out voluntarily and won't be contributing to
> > @@ -1491,6 +1492,7 @@ void wq_worker_tick(struct task_struct *task)
> >  	 * We probably want to make this prettier in the future.
> >  	 */
> >  	if ((worker->flags & WORKER_NOT_RUNNING) ||
> > READ_ONCE(worker->sleeping) ||
> > +	    WARN_ON_ONCE((s64)(worker->task->se.sum_exec_runtime -
> > worker->current_at) < 0) ||
> >  	    worker->task->se.sum_exec_runtime - worker->current_at <
> >  	    wq_cpu_intensive_thresh_us * NSEC_PER_USEC)
> >  		return;
> 
> What is the code path by which sum_exec_runtime could go backward
> in time, if the TSC and sched_clock() jump backward?
> 
> Might it make sense to check in the place where sum_exec_runtime is
> updated, instead, and catch a wider net?
> 
> On the flip side, the run time increments are "fairly large" in
> number of TSC cycles, while most of the negative TSC jumps we 
> have seen are quite small, so even that wider net might not catch
> much because of how coarse these updates typically are...

Good points!  Even more telling, this patch didn't catch anything during
Breno's tests.  I will drop it with a workqueue.2024.08.01 branch in
-rcu in case someone needs it later.

							Thanx, Paul

> All Rights Reversed.

[PATCH misc 2/2] exit: Sleep at TASK_IDLE when waiting for application core dump

[Paul E. McKenney]

Currently, the coredump_task_exit() function sets the task state
to TASK_UNINTERRUPTIBLE|TASK_FREEZABLE, which usually works well.
But a combination of large memory and slow (and/or highly contended)
mass storage can cause application core dumps to take more than
two minutes, which can cause check_hung_task(), which is invoked by
check_hung_uninterruptible_tasks(), to produce task-blocked splats.
There does not seem to be any reasonable benefit to getting these splats.

Furthermore, as Oleg Nesterov points out, TASK_UNINTERRUPTIBLE could
be misleading because the task sleeping in coredump_task_exit() really
is killable, albeit indirectly.  See the check of signal->core_state
in prepare_signal() and the check of fatal_signal_pending()
in dump_interrupted(), which bypass the normal unkillability of
TASK_UNINTERRUPTIBLE, resulting in coredump_finish() invoking
wake_up_process() on any threads sleeping in coredump_task_exit().

Therefore, change that TASK_UNINTERRUPTIBLE to TASK_IDLE.

Reported-by: Anhad Jai Singh <ffledgling@meta.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: Rik van Riel <riel@surriel.com>
---
 kernel/exit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 7430852a85712..0d62a53605dfc 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -428,7 +428,7 @@ static void coredump_task_exit(struct task_struct *tsk)
 			complete(&core_state->startup);
 
 		for (;;) {
-			set_current_state(TASK_UNINTERRUPTIBLE|TASK_FREEZABLE);
+			set_current_state(TASK_IDLE|TASK_FREEZABLE);
 			if (!self.task) /* see coredump_finish() */
 				break;
 			schedule();
-- 
2.40.1