Re: [PATCH] mm/swapfile: add cond_resched() in get_swap_pages()

[Huang, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Sat, 28 Jan 2023 09:47:57 +0000 Longlong Xia <xialonglong1@xxxxxxxxxx> 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?

Sounds reasonable to me.  And if we want to debug, we can use bpf too.

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

Best Regards,
Huang, Ying