Re: Hard and soft lockups with FIO and LTP runs on a large system

[Vlastimil Babka <vbabka@suse.cz>]

On 7/9/24 6:30 AM, Bharata B Rao wrote:
> On 08-Jul-24 9:47 PM, Yu Zhao wrote:
>> On Mon, Jul 8, 2024 at 8:34 AM Bharata B Rao <bharata@amd.com> wrote:
>>>
>>> Hi Yu Zhao,
>>>
>>> Thanks for your patches. See below...
>>>
>>> On 07-Jul-24 4:12 AM, Yu Zhao wrote:
>>>> Hi Bharata,
>>>>
>>>> On Wed, Jul 3, 2024 at 9:11 AM Bharata B Rao <bharata@amd.com> wrote:
>>>>>
>>> <snip>
>>>>>
>>>>> Some experiments tried
>>>>> ======================
>>>>> 1) When MGLRU was enabled many soft lockups were observed, no hard
>>>>> lockups were seen for 48 hours run. Below is once such soft lockup.
>>>>
>>>> This is not really an MGLRU issue -- can you please try one of the
>>>> attached patches? It (truncate.patch) should help with or without
>>>> MGLRU.
>>>
>>> With truncate.patch and default LRU scheme, a few hard lockups are seen.
>> 
>> Thanks.
>> 
>> In your original report, you said:
>> 
>>    Most of the times the two contended locks are lruvec and
>>    inode->i_lock spinlocks.
>>    ...
>>    Often times, the perf output at the time of the problem shows
>>    heavy contention on lruvec spin lock. Similar contention is
>>    also observed with inode i_lock (in clear_shadow_entry path)
>> 
>> Based on this new report, does it mean the i_lock is not as contended,
>> for the same path (truncation) you tested? If so, I'll post
>> truncate.patch and add reported-by and tested-by you, unless you have
>> objections.
> 
> truncate.patch has been tested on two systems with default LRU scheme 
> and the lockup due to inode->i_lock hasn't been seen yet after 24 hours run.
> 
>> 
>> The two paths below were contended on the LRU lock, but they already
>> batch their operations. So I don't know what else we can do surgically
>> to improve them.
> 
> What has been seen with this workload is that the lruvec spinlock is 
> held for a long time from shrink_[active/inactive]_list path. In this 
> path, there is a case in isolate_lru_folios() where scanning of LRU 
> lists can become unbounded. To isolate a page from ZONE_DMA, sometimes 
> scanning/skipping of more than 150 million folios were seen. There is 

It seems weird to me to see anything that would require ZONE_DMA allocation
on a modern system. Do you know where it comes from?

> already a comment in there which explains why nr_skipped shouldn't be 
> counted, but is there any possibility of re-looking at this condition?
> 
> Regards,
> Bharata.
>