linux-kernel - Re: [PATCH] mm/damon/core: Avoid fake load reports due to uninterruptible sleeps

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Date:   Wed, 24 Nov 2021 13:42:10 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     SeongJae Park <sj@...nel.org>
Cc:     shakeelb@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm/damon/core: Avoid fake load reports due to
 uninterruptible sleeps

On Wed, 24 Nov 2021 14:52:19 +0000 SeongJae Park <sj@...nel.org> wrote:

> Because DAMON sleeps in uninterruptible mode, /proc/loadavg reports fake
> load while DAMON is turned on, though it is doing nothing.  This can
> confuse users[1].  To avoid the case, this commit makes DAMON sleeps in
> idle mode.
> 
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -12,6 +12,8 @@
>  #include <linux/kthread.h>
>  #include <linux/mm.h>
>  #include <linux/random.h>
> +#include <linux/sched.h>
> +#include <linux/sched/debug.h>
>  #include <linux/slab.h>
>  #include <linux/string.h>
>  
> @@ -976,12 +978,25 @@ static unsigned long damos_wmark_wait_us(struct damos *scheme)
>  	return 0;
>  }
>  
> +/* sleep for @usecs in idle mode */
> +static void __sched damon_usleep_idle(unsigned long usecs)
> +{
> +	ktime_t exp = ktime_add_us(ktime_get(), usecs);
> +	u64 delta = usecs * NSEC_PER_USEC / 100;	/* allow 1% error */
> +
> +	for (;;) {
> +		__set_current_state(TASK_IDLE);
> +		if (!schedule_hrtimeout_range(&exp, delta, HRTIMER_MODE_ABS))
> +			break;
> +	}
> +}

Let's not copy-n-paste usleep_range() into damon code?

A new usleep_idle_range() in kernel/time/timer.c seems like a
worthwhile addition.  Perhaps usleep_idle_range() and usleep_range()
can be static inline wrappers against a new, more general function in
kernel/time/timer.c - usleep_range_state(min, max, state)?