linux-kernel - Re: [PATCH misc 1/2] workqueue: Add check for clocks going backwards to wq_worker

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Message-ID: <bc6cf2940cee5bf3414616051c821c0b17dd61b1.camel@surriel.com>
Date: Fri, 02 Aug 2024 12:06:06 -0400
From: Rik van Riel <riel@...riel.com>
To: "Paul E. McKenney" <paulmck@...nel.org>, linux-kernel@...r.kernel.org
Cc: Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, Breno
 Leitao <leitao@...ian.org>, Anhad Jai Singh <ffledgling@...a.com>, Oleg
 Nesterov <oleg@...hat.com>,  Jens Axboe <axboe@...nel.dk>, Christian
 Brauner <brauner@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,  Chris Mason <clm@...com>
Subject: Re: [PATCH misc 1/2] workqueue: Add check for clocks going
 backwards to wq_worker_tick()

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...

-- 
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