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From:   SeongJae Park <sj@kernel.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     SeongJae Park <sj@kernel.org>, shakeelb@google.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH] mm/damon/core: Avoid fake load reports due to uninterruptible sleeps
Date:   Thu, 25 Nov 2021 07:49:08 +0000
Message-Id: <20211125074908.14412-1-sj@kernel.org>
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On Wed, 24 Nov 2021 13:42:10 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 24 Nov 2021 14:52:19 +0000 SeongJae Park <sj@kernel.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)?

Makes sense, agreed.  I will post next version soon.


Thanks,
SJ