linux-kernel - Re: [PATCH] mm/swapfile: add cond_resched() in get_swap

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Date:   Sun, 29 Jan 2023 13:03:20 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Longlong Xia <xialonglong1@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <ying.huang@...el.com>, <chenwandun@...wei.com>,
        <wangkefeng.wang@...wei.com>, <sunnanyong@...wei.com>
Subject: Re: [PATCH] mm/swapfile: add cond_resched() in get_swap_pages()

On Sat, 28 Jan 2023 09:47:57 +0000 Longlong Xia <xialonglong1@...wei.com> wrote:

> The softlockup still occurs in get_swap_pages() under memory pressure.
> 64 CPU cores, 64GB memory, and 28 zram devices, the disksize of each
> zram device is 50MB with same priority as si. Use the stress-ng tool
> to increase memory pressure, causing the system to oom frequently.
> 
> The plist_for_each_entry_safe() loops in get_swap_pages() could reach
> tens of thousands of times to find available space (extreme case:
> cond_resched() is not called in scan_swap_map_slots()). Let's add
> cond_resched() into get_swap_pages() when failed to find available
> space to avoid softlockup.
> 
> ...
>
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1100,6 +1100,7 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
>  			goto check_out;
>  		pr_debug("scan_swap_map of si %d failed to find offset\n",
>  			si->type);
> +		cond_resched();
>  
>  		spin_lock(&swap_avail_lock);
>  nextsi:

This must be pretty rare?  My googling for "scan_swap_map of si %d
failed to find offset" turns up zero reports, but I guess few people
enable pr_debug.

I wonder if we should remove that pr_debug().  I mean, it's known that
this happens, what value does the printk add?

I'm thinking this fix should be backported into -stable kernels.