linux-kernel - Re: [PATCH] swap: Add percpu cluster_next to reduce lock contention on swap cache

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Date:   Fri, 15 May 2020 19:51:40 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Huang Ying <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        Minchan Kim <minchan@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] swap: Add percpu cluster_next to reduce lock contention
 on swap cache

On Thu, May 14, 2020 at 03:04:24PM +0800, Huang Ying wrote:
> And the pmbench score increases 15.9%.

What metric is that, and how long did you run the benchmark for?

Given that this thing is probabilistic, did you notice much variance from run
to run?

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 35be7a7271f4..9f1343b066c1 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -746,7 +746,16 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>  	 */
>  
>  	si->flags += SWP_SCANNING;
> -	scan_base = offset = si->cluster_next;
> +	/*
> +	 * Use percpu scan base for SSD to reduce lock contention on
> +	 * cluster and swap cache.  For HDD, sequential access is more
> +	 * important.
> +	 */
> +	if (si->flags & SWP_SOLIDSTATE)
> +		scan_base = this_cpu_read(*si->cluster_next_cpu);
> +	else
> +		scan_base = si->cluster_next;
> +	offset = scan_base;
>  
>  	/* SSD algorithm */
>  	if (si->cluster_info) {

It's just a nit but SWP_SOLIDSTATE and 'if (si->cluster_info)' are two ways to
check the same thing and I'd stick with the one that's already there.

> @@ -2962,6 +2979,8 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
>  
>  	p->lowest_bit  = 1;
>  	p->cluster_next = 1;
> +	for_each_possible_cpu(i)
> +		per_cpu(*p->cluster_next_cpu, i) = 1;

These are later overwritten if the device is an SSD which seems to be the only
case where these are used, so why have this?

>  	p->cluster_nr = 0;
>  
>  	maxpages = max_swapfile_size();
> @@ -3204,6 +3223,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  		 * SSD
>  		 */
>  		p->cluster_next = 1 + prandom_u32_max(p->highest_bit);
> +		for_each_possible_cpu(cpu) {
> +			per_cpu(*p->cluster_next_cpu, cpu) =
> +				1 + prandom_u32_max(p->highest_bit);
> +		}

Is there a reason for adding one?  The history didn't enlighten me about why
cluster_next does it.