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

[Yu Zhao]

On Mon, Jul 8, 2024 at 10:31 PM Bharata B Rao <bharata@xxxxxxx> wrote:
>
> 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.

Thanks.

> >
> > 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?

For this specific case, probably this can help:

@@ -1659,8 +1659,15 @@ static unsigned long
isolate_lru_folios(unsigned long nr_to_scan,
                if (folio_zonenum(folio) > sc->reclaim_idx ||
                                skip_cma(folio, sc)) {
                        nr_skipped[folio_zonenum(folio)] += nr_pages;
-                       move_to = &folios_skipped;
-                       goto move;
+                       list_move(&folio->lru, &folios_skipped);
+                       if (spin_is_contended(&lruvec->lru_lock)) {
+                               if (!list_empty(dst))
+                                       break;
+                               spin_unlock_irq(&lruvec->lru_lock);
+                               cond_resched();
+                               spin_lock_irq(&lruvec->lru_lock);
+                       }
+                       continue;
                }