Re: [v2 1/2] mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged

Michal Hocko
2020-09-16T07:47:30Z

On Tue 15-09-20 14:10:57, Vijay Balakrishna wrote:
> When memory is hotplug added or removed the min_free_kbytes must be
> recalculated based on what is expected by khugepaged. Currently
> after hotplug, min_free_kbytes will be set to a lower default and higher
> default set when THP enabled is lost. This leaves the system with small
> min_free_kbytes which isn't suitable for systems especially with network
> intensive loads. Typical failure symptoms include HW WATCHDOG reset,
> soft lockup hang notices, NETDEVICE WATCHDOG timeouts, and OOM process
> kills.

This changelog still doesn't explain the underlying problem IMO. It
sounds it is papering over some other problem. I do agree that the
current behavior is non intuitive and surprising. Justifying the change
by that inconsistency is likely much better than arguing with soft
lockups and OOMs which shouldn't be a result of this inconsistency.

> Fixes: f000565adb77 ("thp: set recommended min free kbytes")
>
> Signed-off-by: Vijay Balakrishna <vijayb@linux.microsoft.com>
> Cc: stable@vger.kernel.org
> Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> ---
> include/linux/khugepaged.h | 5 +++++
> mm/khugepaged.c | 13 +++++++++++--
> mm/memory_hotplug.c | 3 +++
> 3 files changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index bc45ea1efbf7..c941b7377321 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -15,6 +15,7 @@ extern int __khugepaged_enter(struct mm_struct *mm);
> extern void __khugepaged_exit(struct mm_struct *mm);
> extern int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
> unsigned long vm_flags);
> +extern void khugepaged_min_free_kbytes_update(void);
> #ifdef CONFIG_SHMEM
> extern void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr);
> #else
> @@ -85,6 +86,10 @@ static inline void collapse_pte_mapped_thp(struct mm_struct *mm,
> unsigned long addr)
> {
> }
> +
> +static inline void khugepaged_min_free_kbytes_update(void)
> +{
> +}
> #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
> #endif /* _LINUX_KHUGEPAGED_H */
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index cfa0dba5fd3b..4f7107476a6f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -56,6 +56,9 @@ enum scan_result {
> #define CREATE_TRACE_POINTS
> #include <trace/events/huge_memory.h>
>
> +static struct task_struct *khugepaged_thread __read_mostly;
> +static DEFINE_MUTEX(khugepaged_mutex);
> +
> /* default scan 8*512 pte (or vmas) every 30 second */
> static unsigned int khugepaged_pages_to_scan __read_mostly;
> static unsigned int khugepaged_pages_collapsed;
> @@ -2292,8 +2295,6 @@ static void set_recommended_min_free_kbytes(void)
>
> int start_stop_khugepaged(void)
> {
> - static struct task_struct *khugepaged_thread __read_mostly;
> - static DEFINE_MUTEX(khugepaged_mutex);
> int err = 0;
>
> mutex_lock(&khugepaged_mutex);
> @@ -2320,3 +2321,11 @@ int start_stop_khugepaged(void)
> mutex_unlock(&khugepaged_mutex);
> return err;
> }
> +
> +void khugepaged_min_free_kbytes_update(void)
> +{
> + mutex_lock(&khugepaged_mutex);
> + if (khugepaged_enabled() && khugepaged_thread)
> + set_recommended_min_free_kbytes();
> + mutex_unlock(&khugepaged_mutex);
> +}
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index e9d5ab5d3ca0..3e19272c1fad 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -36,6 +36,7 @@
> #include <linux/memblock.h>
> #include <linux/compaction.h>
> #include <linux/rmap.h>
> +#include <linux/khugepaged.h>
>
> #include <asm/tlbflush.h>
>
> @@ -857,6 +858,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
> zone_pcp_update(zone);
>
> init_per_zone_wmark_min();
> + khugepaged_min_free_kbytes_update();
>
> kswapd_run(nid);
> kcompactd_run(nid);
> @@ -1600,6 +1602,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
> pgdat_resize_unlock(zone->zone_pgdat, &flags);
>
> init_per_zone_wmark_min();
> + khugepaged_min_free_kbytes_update();
>
> if (!populated_zone(zone)) {
> zone_pcp_reset(zone);
> --
> 2.28.0

--
Michal Hocko
SUSE Labs