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Shutemov" , Mel Gorman , Vlastimil Babka , Andrew Morton Subject: Re: [PATCH] mm: always consider THP when adjusting min_free_kbytes In-Reply-To: <20200204194156.61672-1-mike.kravetz@oracle.com> Message-ID: References: <20200204194156.61672-1-mike.kravetz@oracle.com> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 4 Feb 2020, Mike Kravetz wrote: > At system initialization time, min_free_kbytes is calculated based > on the amount of memory in the system. If THP is enabled, then > khugepaged is started and min_free_kbytes may be adjusted in an > attempt to reserve some pageblocks for THP allocations. > > When memory is offlined or onlined, min_free_kbytes is recalculated > and adjusted based on the amount of memory. However, the adjustment > for THP is not considered. Here is an example from a 2 node system > with 8GB of memory. > > # cat /proc/sys/vm/min_free_kbytes > 90112 > # echo 0 > /sys/devices/system/node/node1/memory56/online > # cat /proc/sys/vm/min_free_kbytes > 11243 > # echo 1 > /sys/devices/system/node/node1/memory56/online > # cat /proc/sys/vm/min_free_kbytes > 11412 > Ah, that doesn't look good. > One would expect that min_free_kbytes would return to it's original > value after the offline/online operations. > > Create a simple interface for THP/khugepaged based adjustment and > call this whenever min_free_kbytes is adjusted. > > Signed-off-by: Mike Kravetz > --- > include/linux/khugepaged.h | 5 +++++ > mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++----- > mm/page_alloc.c | 4 +++- > 3 files changed, 38 insertions(+), 6 deletions(-) > > diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h > index bc45ea1efbf7..8f02d3575829 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 bool khugepaged_adjust_min_free_kbytes(void); > #ifdef CONFIG_SHMEM > extern void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr); > #else > @@ -81,6 +82,10 @@ static inline int khugepaged_enter_vma_merge(struct vm_area_struct *vma, > { > return 0; > } > +static bool khugepaged_adjust_min_free_kbytes(void) > +{ > + return false; > +} > static inline void collapse_pte_mapped_thp(struct mm_struct *mm, > unsigned long addr) > { > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index b679908743cb..d8040cf19e98 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2138,7 +2138,7 @@ static int khugepaged(void *none) > return 0; > } > > -static void set_recommended_min_free_kbytes(void) > +bool __khugepaged_adjust_min_free_kbytes(void) > { > struct zone *zone; > int nr_zones = 0; > @@ -2174,17 +2174,26 @@ static void set_recommended_min_free_kbytes(void) > > if (recommended_min > min_free_kbytes) { > if (user_min_free_kbytes >= 0) > - pr_info("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations\n", > + pr_info_once("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations\n", > min_free_kbytes, recommended_min); > > min_free_kbytes = recommended_min; > + return true; > } > - setup_per_zone_wmarks(); > + > + return false; > +} > + > +static void set_recommended_min_free_kbytes(void) > +{ > + if (__khugepaged_adjust_min_free_kbytes()) > + setup_per_zone_wmarks(); > } > > -int start_stop_khugepaged(void) > +static struct task_struct *khugepaged_thread __read_mostly; > + > +int __ref start_stop_khugepaged(void) > { > - static struct task_struct *khugepaged_thread __read_mostly; > static DEFINE_MUTEX(khugepaged_mutex); > int err = 0; > > @@ -2207,8 +2216,24 @@ int start_stop_khugepaged(void) > } else if (khugepaged_thread) { > kthread_stop(khugepaged_thread); > khugepaged_thread = NULL; > + init_per_zone_wmark_min(); > } > fail: > mutex_unlock(&khugepaged_mutex); > return err; > } > + > +bool khugepaged_adjust_min_free_kbytes(void) > +{ > + bool ret = false; > + > + /* > + * This is a bit racy, and we could miss transitions. However, > + * start/stop code above will make additional adjustments at the > + * end of transitions. > + */ > + if (khugepaged_enabled() && khugepaged_thread) > + ret = __khugepaged_adjust_min_free_kbytes(); > + > + return ret; > +} > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index d047bf7d8fd4..a7b3a6663ba6 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -68,6 +68,7 @@ > #include > #include > #include > +#include > > #include > #include > @@ -7827,9 +7828,10 @@ int __meminit init_per_zone_wmark_min(void) > if (min_free_kbytes > 65536) > min_free_kbytes = 65536; > } else { > - pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n", > + pr_warn_once("min_free_kbytes is not updated to %d because user defined value %d is preferred\n", > new_min_free_kbytes, user_min_free_kbytes); > } > + (void)khugepaged_adjust_min_free_kbytes(); > setup_per_zone_wmarks(); > refresh_zone_stat_thresholds(); > setup_per_zone_lowmem_reserve(); Hmm, if khugepaged_adjust_min_free_kbytes() increases min_free_kbytes for thp, then the user has no ability to override this increase by using vm.min_free_kbytes? IIUC, with this change, it looks like memory hotplug events properly increase min_free_kbytes for thp optimization but also doesn't respect a previous user-defined value? So it looks like this is fixing an obvious correctness issue but also now requires users to rewrite the sysctl if they want to decrease the min watermark.