linux-kernel - Re: [patch V3 14/18] posix-timers: Avoid false cacheline sharing

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Message-ID: <od57wrws3tmcwuyi3rkbgoysg73vdtq6czstwbq4zjr6qrd6ns@zpwvqdiyfvfp>
Date: Mon, 17 Mar 2025 11:50:19 +0530
From: "Nysal Jan K.A." <nysal@...ux.ibm.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>, Andrey Vagin <avagin@...nvz.org>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Cyrill Gorcunov <gorcunov@...il.com>
Subject: Re: [patch V3 14/18] posix-timers: Avoid false cacheline sharing

On Sat, Mar 08, 2025 at 05:48:42PM +0100, Thomas Gleixner wrote:
> ---
> V2: New patch
> ---
>  include/linux/posix-timers.h |   21 ++++++++++++---------
>  kernel/time/posix-timers.c   |    4 ++--
>  2 files changed, 14 insertions(+), 11 deletions(-)
> 
> --- a/include/linux/posix-timers.h
> +++ b/include/linux/posix-timers.h
> @@ -177,23 +177,26 @@ static inline void posix_cputimers_init_
>   * @rcu:		RCU head for freeing the timer.
>   */
>  struct k_itimer {
> -	struct hlist_node	list;
> -	struct hlist_node	ignored_list;
> +	/* 1st cacheline contains read-mostly fields */
>  	struct hlist_node	t_hash;
> -	spinlock_t		it_lock;
> -	const struct k_clock	*kclock;
> -	clockid_t		it_clock;
> +	struct hlist_node	list;
>  	timer_t			it_id;
> +	clockid_t		it_clock;
> +	int			it_sigev_notify;
> +	enum pid_type		it_pid_type;
> +	struct signal_struct	*it_signal;
> +	const struct k_clock	*kclock;
> +
> +	/* 2nd cacheline and above contain fields which are modified regularly */

On architectures like powerpc where cache line size is 128 bytes, we might still
run into false sharing. Perhaps rearranging it towards the end of the struct might
help avoid it? Is the benchmark code public? I can collect perf c2c data on powerpc.

> +	spinlock_t		it_lock;
>  	int			it_status;
>  	bool			it_sig_periodic;
>  	s64			it_overrun;
>  	s64			it_overrun_last;
>  	unsigned int		it_signal_seq;
>  	unsigned int		it_sigqueue_seq;
> -	int			it_sigev_notify;
> -	enum pid_type		it_pid_type;
>  	ktime_t			it_interval;
> -	struct signal_struct	*it_signal;
> +	struct hlist_node	ignored_list;
>  	union {
>  		struct pid		*it_pid;
>  		struct task_struct	*it_process;
> @@ -210,7 +213,7 @@ struct k_itimer {
>  		} alarm;
>  	} it;
>  	struct rcu_head		rcu;
> -};
> +} ____cacheline_aligned_in_smp;
>  

--Nysal