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

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Message-ID: <20200113094342.5ghlgttmhuxfqv2v@wittgenstein>
Date:   Mon, 13 Jan 2020 10:43:43 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     qiwuchen55@...il.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 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.

>  
> -static int max_threads;		/* tunable limit on nr_threads */
> +static int max_threads __read_mostly; /* tunable limit on nr_threads */

That make sense.

Christian