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

[Bharata B Rao]

On 08-Jul-24 9:47 PM, Yu Zhao wrote:
> On Mon, Jul 8, 2024 at 8:34 AM Bharata B Rao <bharata@xxxxxxx> 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@xxxxxxx> 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 
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.