linux-kernel - Re: [PATCH] kernel/fork: put some fork variables into read-mostly section

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Date:   Tue, 14 Jan 2020 14:16:51 +0800
From:   chenqiwu <qiwuchen55@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     peterz@...radead.org, mingo@...nel.org, tglx@...utronix.de,
        oleg@...hat.com, elena.reshetova@...el.com, jgg@...pe.ca,
        christian@...lner.me, aarcange@...hat.com, viro@...iv.linux.org.uk,
        cyphar@...har.com, ldv@...linux.org, linux-kernel@...r.kernel.org,
        chenqiwu <chenqiwu@...omi.com>
Subject: Re: [PATCH] kernel/fork: put some fork variables into read-mostly
 section

On Mon, Jan 13, 2020 at 10:43:43AM +0100, Christian Brauner wrote:
> On Mon, Jan 13, 2020 at 11:23:13AM +0800, qiwuchen55@...il.com wrote:
> > From: chenqiwu <chenqiwu@...omi.com>
> > 
> > Since total_forks/nr_threads/max_threads global variables are
> > frequently used for process fork, putting these variables into
> > read_mostly section can avoid unnecessary cache line bouncing.
> > 
> > Signed-off-by: chenqiwu <chenqiwu@...omi.com>
> > ---
> >  kernel/fork.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 0808095..163e152 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -120,10 +120,10 @@
> >  /*
> >   * Protected counters by write_lock_irq(&tasklist_lock)
> >   */
> > -unsigned long total_forks;	/* Handle normal Linux uptimes. */
> > -int nr_threads;			/* The idle threads do not count.. */
> > +unsigned long total_forks __read_mostly; /* Handle normal Linux uptimes. */
> > +int nr_threads __read_mostly;  /* The idle threads do not count.. */
> 
> total_forks is incremented at every ~CLONE_THREAD and nr_threads at
> CLONE_THREAD I wouldn't exactly say that this qualifies as mostly
> reading.
>

Hi Christian,
I'm holding different views on this matter.
1) total_forks is incremented when any process does sys_fork() reasonablely,
   it counts for every process fork.
2) nr_threads counts for any parent process (except for idle thread) does
   fork successfully, it never exceeds than max_threads.

For an example of arm64 kdump, the system has been running at 119057s,
we can see total_forks is very large, so total_forks is very qualified
as mostly reading because it is referenced frequntly.

crash> p max_threads
max_threads = $2 = 39150
crash> p nr_threads
nr_threads = $1 = 2676
crash> p total_forks
total_forks = $3 = 2413880

nr_threads and max_threads is also qualified as mostly reading because
they are frequntly referenced in the following code of copy_process():
	if (nr_threads >= max_threads)
		goto bad_fork_cleanup_count;
   
> >  
> > -static int max_threads;		/* tunable limit on nr_threads */
> > +static int max_threads __read_mostly; /* tunable limit on nr_threads */
> 
> That make sense.
> 
> Christian

Thanks for your review!
Qiwu