Re: 5.7-rc0: kswapd eats cpu during a disk test?!

Hillf Danton
2020-06-13T04:48:02Z

On Sat, 13 Jun 2020 01:05:52 +0200 Pavel Machek wrote:
> > > +CC linux-mm
> > >
> > > On 5/31/20 12:34 PM, Pavel Machek wrote:
> > > > Hi!
> > > >
> > > > This is simple cat /dev/sda > /dev/zero... on thinkpad x60 (x86-32),
> > > > with spinning rust.
> > > >
> > > > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> > > > 1000 root 20 0 0 0 0 R 53.3 0.0 57:34.93 kswapd0
> > > > 27897 root 20 0 6976 580 536 R 44.5 0.0 1:44.53 cat
> > > >
> > > > It keeps both CPUs busy... and I don't think that's right.
> > >
> > > Does an older kernel behave differently here?
> >
> > Let me try on x220 (x86-64, first):
> >
> > 737 root 20 0 5404 744 680 R 31.2 0.0 0:09.98 cat
> > 1024 root 20 0 0 0 0 S 21.4 0.0 165:22.68 kswapd0
> >
> > That was with ssd, result with spinning rust is similar:
> >
> > 859 root 20 0 5404 740 672 D 21.1 0.0 0:03.33 cat
> > 1024 root 20 0 0 0 0 R 11.8 0.0 165:33.07 kswapd0
> >
> > 5.7-rc1+ kernel.
> >
> > Performance of spinning rust is down, too, on x60:
> >
> > pavel@amd:~/misc/hw/hdd1t$ sudo ddrescue --force /dev/sda1 /dev/null
> > GNU ddrescue 1.19
> > Press Ctrl-C to interrupt
> > rescued: 2147 MB, errsize: 0 B, current rate: 3080 kB/s
> > ipos: 2147 MB, errors: 0, average rate: 5382 kB/s
> > opos: 2147 MB, run time: 6.65 m, successful read:
> > 0 s ago
> > Finished
> > pavel@amd:~/misc/hw/hdd1t$ uname -a
> > Linux amd 5.7.0-next-20200611+ #123 SMP PREEMPT Thu Jun 11
> > 15:41:22 CEST 2020 i686 GNU/Linux
> >
> > And there's something clearly wrong here:
> >
> > 966 root 20 0 0 0 0 R 94.4 0.0 8:18.82 kswapd0
> > 23933 root 20 0 4612 1112 1028 D 80.6 0.0 0:26.40 ddrescue
> >
>
> Same x60 under older kernel:
>
> pavel@amd:/data/fast/pavel$ sudo ddrescue --force /dev/sda4 /dev/null
> GNU ddrescue 1.19
> Press Ctrl-C to interrupt
> rescued: 6593 MB, errsize: 0 B, current rate: 60424 kB/s
> ipos: 6593 MB, errors: 0, average rate: 95563 kB/s
>
> 3539 root 20 0 4616 1136 1048 D 21.4 0.0 0:15.63 ddrescue
> 865 root 20 0 0 0 0 S 6.9 0.0 0:04.91 kswapd0
>
> Linux amd 4.6.0+ #172 SMP Sun Aug 14 11:25:34 CEST 2016 i686 GNU/Linux
>
> These are more reasonable numbers.

Treat referenced & active pages as reclaim cost.

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2020,6 +2020,7 @@ static void shrink_active_list(unsigned
struct page *page;
unsigned nr_deactivate, nr_activate;
unsigned nr_rotated = 0;
+ unsigned nr_refered = 0;
int file = is_file_lru(lru);
struct pglist_data *pgdat = lruvec_pgdat(lruvec);

@@ -2070,7 +2071,8 @@ static void shrink_active_list(unsigned
nr_rotated += hpage_nr_pages(page);
list_add(&page->lru, &l_active);
continue;
- }
+ } else if (!file)
+ nr_refered++;
}

ClearPageActive(page); /* we are de-activating */
@@ -2098,6 +2100,14 @@ static void shrink_active_list(unsigned
free_unref_page_list(&l_active);
trace_mm_vmscan_lru_shrink_active(pgdat->node_id, nr_taken, nr_activate,
nr_deactivate, nr_rotated, sc->priority, file);
+ if (file)
+ sc->file_cost += nr_rotated;
+ else
+ /*
+ * add cost to avoid swapin in the near future which incurs IO
+ * on top of reclaim
+ */
+ sc->anon_cost += nr_refered;
}

unsigned long reclaim_pages(struct list_head *page_list)
@@ -2311,11 +2321,13 @@ static void get_scan_count(struct lruvec
file_cost = total_cost + sc->file_cost;
total_cost = anon_cost + file_cost;

- ap = swappiness * (total_cost + 1);
- ap /= anon_cost + 1;
-
- fp = (200 - swappiness) * (total_cost + 1);
- fp /= file_cost + 1;
+ ap = swappiness * total_cost;
+ if (anon_cost)
+ ap /= anon_cost;
+
+ fp = (200 - swappiness) * total_cost;
+ if (file_cost)
+ fp /= file_cost;

fraction[0] = ap;
fraction[1] = fp;