linux-kernel - Re: [PATCH 2/6] mm/migrate: Use trylock while resetting rate limit

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Date:   Thu, 6 Sep 2018 13:48:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/6] mm/migrate: Use trylock while resetting rate limit

On Fri, Aug 03, 2018 at 11:43:57AM +0530, Srikar Dronamraju wrote:
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8c0af0f..dbc2cb7 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1868,16 +1868,24 @@ static struct page *alloc_misplaced_dst_page(struct page *page,
>  static bool numamigrate_update_ratelimit(pg_data_t *pgdat,
>  					unsigned long nr_pages)
>  {
> +	unsigned long next_window, interval;
> +
> +	next_window = READ_ONCE(pgdat->numabalancing_migrate_next_window);
> +	interval = msecs_to_jiffies(migrate_interval_millisecs);
> +
>  	/*
>  	 * Rate-limit the amount of data that is being migrated to a node.
>  	 * Optimal placement is no good if the memory bus is saturated and
>  	 * all the time is being spent migrating!
>  	 */
> -	if (time_after(jiffies, next_window)) {
> -		spin_lock(&pgdat->numabalancing_migrate_lock);
> +	if (time_after(jiffies, next_window) &&
> +			spin_trylock(&pgdat->numabalancing_migrate_lock)) {

This patch doesn't apply cleanly; also you introduce @next_window with
this patch, so how can it remove a user of it.

I fixed this up, still weird.