Re: [PATCH] mm: vmscan: split khugepaged stats from direct reclaim stats

From: Yang Shi
Date: Tue Oct 25 2022 - 15:40:38 EST

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On Tue, Oct 25, 2022 at 10:05 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> Direct reclaim stats are useful for identifying a potential source for
> application latency, as well as spotting issues with kswapd. However,
> khugepaged currently distorts the picture: as a kernel thread it
> doesn't impose allocation latencies on userspace, and it explicitly
> opts out of kswapd reclaim. Its activity showing up in the direct
> reclaim stats is misleading. Counting it as kswapd reclaim could also
> cause confusion when trying to understand actual kswapd behavior.
>
> Break out khugepaged from the direct reclaim counters into new
> pgsteal_khugepaged, pgdemote_khugepaged, pgscan_khugepaged counters.
>
> Test with a huge executable (CONFIG_READ_ONLY_THP_FOR_FS):
>
> pgsteal_kswapd 1342185
> pgsteal_direct 0
> pgsteal_khugepaged 3623
> pgscan_kswapd 1345025
> pgscan_direct 0
> pgscan_khugepaged 3623

There are other kernel threads or works may allocate memory then
trigger memory reclaim, there may be similar problems for them and
someone may try to add a new stat. So how's about we make the stats
more general, for example, call it "pg{steal|scan}_kthread"?

>
> Reported-by: Eric Bergen <ebergen@xxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
> Documentation/admin-guide/cgroup-v2.rst | 6 +++++
> include/linux/khugepaged.h | 6 +++++
> include/linux/vm_event_item.h | 3 +++
> mm/khugepaged.c | 5 +++++
> mm/memcontrol.c | 8 +++++--
> mm/vmscan.c | 30 ++++++++++++++++++-------
> mm/vmstat.c | 3 +++
> 7 files changed, 51 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index dc254a3cb956..74cec76be9f2 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1488,12 +1488,18 @@ PAGE_SIZE multiple when read back.
> pgscan_direct (npn)
> Amount of scanned pages directly (in an inactive LRU list)
>
> + pgscan_khugepaged (npn)
> + Amount of scanned pages by khugepaged (in an inactive LRU list)
> +
> pgsteal_kswapd (npn)
> Amount of reclaimed pages by kswapd
>
> pgsteal_direct (npn)
> Amount of reclaimed pages directly
>
> + pgsteal_khugepaged (npn)
> + Amount of reclaimed pages by khugepaged
> +
> pgfault (npn)
> Total number of page faults incurred
>
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index 70162d707caf..f68865e19b0b 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -15,6 +15,7 @@ extern void __khugepaged_exit(struct mm_struct *mm);
> extern void khugepaged_enter_vma(struct vm_area_struct *vma,
> unsigned long vm_flags);
> extern void khugepaged_min_free_kbytes_update(void);
> +extern bool current_is_khugepaged(void);
> #ifdef CONFIG_SHMEM
> extern int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> bool install_pmd);
> @@ -57,6 +58,11 @@ static inline int collapse_pte_mapped_thp(struct mm_struct *mm,
> static inline void khugepaged_min_free_kbytes_update(void)
> {
> }
> +
> +static inline bool current_is_khugepaged(void)
> +{
> + return false;
> +}
> #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
> #endif /* _LINUX_KHUGEPAGED_H */
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 3518dba1e02f..7f5d1caf5890 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -40,10 +40,13 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
> PGREUSE,
> PGSTEAL_KSWAPD,
> PGSTEAL_DIRECT,
> + PGSTEAL_KHUGEPAGED,
> PGDEMOTE_KSWAPD,
> PGDEMOTE_DIRECT,
> + PGDEMOTE_KHUGEPAGED,
> PGSCAN_KSWAPD,
> PGSCAN_DIRECT,
> + PGSCAN_KHUGEPAGED,
> PGSCAN_DIRECT_THROTTLE,
> PGSCAN_ANON,
> PGSCAN_FILE,
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4734315f7940..36318ebbf50d 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2528,6 +2528,11 @@ void khugepaged_min_free_kbytes_update(void)
> mutex_unlock(&khugepaged_mutex);
> }
>
> +bool current_is_khugepaged(void)
> +{
> + return kthread_func(current) == khugepaged;
> +}
> +
> static int madvise_collapse_errno(enum scan_result r)
> {
> /*
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2d8549ae1b30..a17a5cfa6a55 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -661,8 +661,10 @@ static const unsigned int memcg_vm_event_stat[] = {
> PGPGOUT,
> PGSCAN_KSWAPD,
> PGSCAN_DIRECT,
> + PGSCAN_KHUGEPAGED,
> PGSTEAL_KSWAPD,
> PGSTEAL_DIRECT,
> + PGSTEAL_KHUGEPAGED,
> PGFAULT,
> PGMAJFAULT,
> PGREFILL,
> @@ -1574,10 +1576,12 @@ static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize)
> /* Accumulated memory events */
> seq_buf_printf(&s, "pgscan %lu\n",
> memcg_events(memcg, PGSCAN_KSWAPD) +
> - memcg_events(memcg, PGSCAN_DIRECT));
> + memcg_events(memcg, PGSCAN_DIRECT) +
> + memcg_events(memcg, PGSCAN_KHUGEPAGED));
> seq_buf_printf(&s, "pgsteal %lu\n",
> memcg_events(memcg, PGSTEAL_KSWAPD) +
> - memcg_events(memcg, PGSTEAL_DIRECT));
> + memcg_events(memcg, PGSTEAL_DIRECT) +
> + memcg_events(memcg, PGSTEAL_KHUGEPAGED));
>
> for (i = 0; i < ARRAY_SIZE(memcg_vm_event_stat); i++) {
> if (memcg_vm_event_stat[i] == PGPGIN ||
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 04d8b88e5216..8ceae125bbf7 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -54,6 +54,7 @@
> #include <linux/shmem_fs.h>
> #include <linux/ctype.h>
> #include <linux/debugfs.h>
> +#include <linux/khugepaged.h>
>
> #include <asm/tlbflush.h>
> #include <asm/div64.h>
> @@ -1047,6 +1048,22 @@ void drop_slab(void)
> drop_slab_node(nid);
> }
>
> +static int reclaimer_offset(void)
> +{
> + BUILD_BUG_ON(PGSTEAL_DIRECT - PGSTEAL_KSWAPD != 1);
> + BUILD_BUG_ON(PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD != 2);
> + BUILD_BUG_ON(PGDEMOTE_DIRECT - PGDEMOTE_KSWAPD != 1);
> + BUILD_BUG_ON(PGDEMOTE_KHUGEPAGED - PGDEMOTE_KSWAPD != 2);
> + BUILD_BUG_ON(PGSCAN_DIRECT - PGSCAN_KSWAPD != 1);
> + BUILD_BUG_ON(PGSCAN_KHUGEPAGED - PGSCAN_KSWAPD != 2);
> +
> + if (current_is_kswapd())
> + return 0;
> + if (current_is_khugepaged())
> + return 2;
> + return 1;
> +}
> +
> static inline int is_page_cache_freeable(struct folio *folio)
> {
> /*
> @@ -1599,10 +1616,7 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
> (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
> &nr_succeeded);
>
> - if (current_is_kswapd())
> - __count_vm_events(PGDEMOTE_KSWAPD, nr_succeeded);
> - else
> - __count_vm_events(PGDEMOTE_DIRECT, nr_succeeded);
> + __count_vm_events(PGDEMOTE_KSWAPD + reclaimer_offset(), nr_succeeded);
>
> return nr_succeeded;
> }
> @@ -2475,7 +2489,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> &nr_scanned, sc, lru);
>
> __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
> - item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;
> + item = PGSCAN_KSWAPD + reclaimer_offset();
> if (!cgroup_reclaim(sc))
> __count_vm_events(item, nr_scanned);
> __count_memcg_events(lruvec_memcg(lruvec), item, nr_scanned);
> @@ -2492,7 +2506,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> move_folios_to_lru(lruvec, &folio_list);
>
> __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
> - item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;
> + item = PGSTEAL_KSWAPD + reclaimer_offset();
> if (!cgroup_reclaim(sc))
> __count_vm_events(item, nr_reclaimed);
> __count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
> @@ -4857,7 +4871,7 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
> break;
> }
>
> - item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;
> + item = PGSCAN_KSWAPD + reclaimer_offset();
> if (!cgroup_reclaim(sc)) {
> __count_vm_events(item, isolated);
> __count_vm_events(PGREFILL, sorted);
> @@ -5015,7 +5029,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> if (walk && walk->batched)
> reset_batch_size(lruvec, walk);
>
> - item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;
> + item = PGSTEAL_KSWAPD + reclaimer_offset();
> if (!cgroup_reclaim(sc))
> __count_vm_events(item, reclaimed);
> __count_memcg_events(memcg, item, reclaimed);
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index b2371d745e00..1ea6a5ce1c41 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1271,10 +1271,13 @@ const char * const vmstat_text[] = {
> "pgreuse",
> "pgsteal_kswapd",
> "pgsteal_direct",
> + "pgsteal_khugepaged",
> "pgdemote_kswapd",
> "pgdemote_direct",
> + "pgdemote_khugepaged",
> "pgscan_kswapd",
> "pgscan_direct",
> + "pgscan_khugepaged",
> "pgscan_direct_throttle",
> "pgscan_anon",
> "pgscan_file",
> --
> 2.38.1
>
>

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In reply to: Johannes Weiner: "Re: [PATCH] mm: vmscan: split khugepaged stats from direct reclaim stats"
Next in thread: Johannes Weiner: "Re: [PATCH] mm: vmscan: split khugepaged stats from direct reclaim stats"
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