Re: [PATCH v2 6/6] mm, hugetlb: improve page-fault scalability

[Andrew Morton]

On Fri, 31 Jan 2014 09:36:46 -0800 Davidlohr Bueso <davidlohr@xxxxxx> wrote:

> From: Davidlohr Bueso <davidlohr@xxxxxx>
> 
> The kernel can currently only handle a single hugetlb page fault at a time.
> This is due to a single mutex that serializes the entire path. This lock
> protects from spurious OOM errors under conditions of low of low availability
> of free hugepages. This problem is specific to hugepages, because it is
> normal to want to use every single hugepage in the system - with normal pages
> we simply assume there will always be a few spare pages which can be used
> temporarily until the race is resolved.
> 
> Address this problem by using a table of mutexes, allowing a better chance of
> parallelization, where each hugepage is individually serialized. The hash key
> is selected depending on the mapping type. For shared ones it consists of the
> address space and file offset being faulted; while for private ones the mm and
> virtual address are used. The size of the table is selected based on a compromise
> of collisions and memory footprint of a series of database workloads.
> 
> Large database workloads that make heavy use of hugepages can be particularly
> exposed to this issue, causing start-up times to be painfully slow. This patch
> reduces the startup time of a 10 Gb Oracle DB (with ~5000 faults) from 37.5 secs
> to 25.7 secs. Larger workloads will naturally benefit even more.

hm, no magic bullet.  Where's the rest of the time being spent?

> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
> ---
> 
> NOTE:
> The only downside to this patch, detected by Joonsoo Kim, is that a small race
> is possible in private mappings: A child process (with its own mm, after cow)
> can instantiate a page that is already being handled by the parent in a cow
> fault. When low on pages, can trigger spurious OOMs. I have not been able to
> think of a efficient way of handling this... but do we really care about such
> a tiny window? We already maintain another theoretical race with normal pages.
> If not, one possible way to is to maintain the single hash for private mappings
> -- any workloads that *really* suffer from this scaling problem should already
> use shared mappings.
> 
>  mm/hugetlb.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 72 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 7ab913c..9b77686 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -22,6 +22,7 @@
>  #include <linux/swap.h>
>  #include <linux/swapops.h>
>  #include <linux/page-isolation.h>
> +#include <linux/jhash.h>
>  
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
> @@ -53,6 +54,13 @@ static unsigned long __initdata default_hstate_size;
>   */
>  DEFINE_SPINLOCK(hugetlb_lock);
>  
> +/*
> ++ * Serializes faults on the same logical page.  This is used to
> ++ * prevent spurious OOMs when the hugepage pool is fully utilized.
> ++ */

Strangeness.  I'll clean it up.

> +static int num_fault_mutexes;
> +static struct mutex *htlb_fault_mutex_table ____cacheline_aligned_in_smp;
> +
>  static inline void unlock_or_release_subpool(struct hugepage_subpool *spool)
>  {
>  	bool free = (spool->count == 0) && (spool->used_hpages == 0);
> @@ -1961,11 +1969,14 @@ static void __exit hugetlb_exit(void)
>  	}
>  
>  	kobject_put(hugepages_kobj);
> +	kfree(htlb_fault_mutex_table);
>  }
>  module_exit(hugetlb_exit);
>  
>  static int __init hugetlb_init(void)
>  {
> +	int i;
> +
>  	/* Some platform decide whether they support huge pages at boot
>  	 * time. On these, such as powerpc, HPAGE_SHIFT is set to 0 when
>  	 * there is no such support
> @@ -1990,6 +2001,18 @@ static int __init hugetlb_init(void)
>  	hugetlb_register_all_nodes();
>  	hugetlb_cgroup_file_init();
>  
> +#ifdef CONFIG_SMP
> +	num_fault_mutexes = roundup_pow_of_two(8 * num_possible_cpus());
> +#else
> +	num_fault_mutexes = 1;
> +#endif
> +	htlb_fault_mutex_table =
> +		kmalloc(sizeof(struct mutex) * num_fault_mutexes, GFP_KERNEL);
> +	if (!htlb_fault_mutex_table)
> +		return -ENOMEM;

If htlb_fault_mutex_table==NULL, the kernel will later oops.  Let's
just go BUG here.

> +	for (i = 0; i < num_fault_mutexes; i++)
> +		mutex_init(&htlb_fault_mutex_table[i]);
>  	return 0;
>  }
>  module_init(hugetlb_init);
> @@ -2767,15 +2790,14 @@ static bool hugetlbfs_pagecache_present(struct hstate *h,
>  }
>  
>  static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
> -			unsigned long address, pte_t *ptep, unsigned int flags)
> +			   struct address_space *mapping, pgoff_t idx,
> +			   unsigned long address, pte_t *ptep, unsigned int flags)
>  {
>  	struct hstate *h = hstate_vma(vma);
>  	int ret = VM_FAULT_SIGBUS;
>  	int anon_rmap = 0;
> -	pgoff_t idx;
>  	unsigned long size;
>  	struct page *page;
> -	struct address_space *mapping;
>  	pte_t new_pte;
>  	spinlock_t *ptl;
>  
> @@ -2790,9 +2812,6 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		return ret;
>  	}
>  
> -	mapping = vma->vm_file->f_mapping;
> -	idx = vma_hugecache_offset(h, vma, address);
> -
>  	/*
>  	 * Use page lock to guard against racing truncation
>  	 * before we get page_table_lock.
> @@ -2902,17 +2921,53 @@ backout_unlocked:
>  	goto out;
>  }
>  
> +#ifdef CONFIG_SMP
> +static u32 fault_mutex_hash(struct hstate *h, struct mm_struct *mm,
> +			    struct vm_area_struct *vma,
> +			    struct address_space *mapping,
> +			    pgoff_t idx, unsigned long address)
> +{
> +	unsigned long key[2];
> +	u32 hash;
> +
> +	if (vma->vm_flags & VM_SHARED) {
> +		key[0] = (unsigned long) mapping;
> +		key[1] = idx;
> +	} else {
> +		key[0] = (unsigned long) mm;
> +		key[1] = address >> huge_page_shift(h);
> +	}
> +
> +	hash = jhash2((u32 *)&key, sizeof(key)/sizeof(u32), 0);

This looks a bit overengineered to me.  What happens if we just do

	hash = jhash2((u32 *)vma, sizeof(vma)/sizeof(u32), 0);

?

> +	return hash & (num_fault_mutexes - 1);
> +}
> +#else
> +/*
> + * For uniprocesor systems we always use a single mutex, so just
> + * return 0 and avoid the hashing overhead.
> + */
> +static u32 fault_mutex_hash(struct hstate *h, struct mm_struct *mm,
> +			    struct vm_area_struct *vma,
> +			    struct address_space *mapping,
> +			    pgoff_t idx, unsigned long address)
> +{
> +	return 0;
> +}
> +#endif
> +
>  int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  			unsigned long address, unsigned int flags)
>  {
> -	pte_t *ptep;
> -	pte_t entry;
> +	pte_t *ptep, entry;
>  	spinlock_t *ptl;
>  	int ret;
> +	u32 hash;
> +	pgoff_t idx;
>  	struct page *page = NULL;
>  	struct page *pagecache_page = NULL;
> -	static DEFINE_MUTEX(hugetlb_instantiation_mutex);
>  	struct hstate *h = hstate_vma(vma);
> +	struct address_space *mapping;
>  
>  	address &= huge_page_mask(h);
>  
> @@ -2931,15 +2986,20 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	if (!ptep)
>  		return VM_FAULT_OOM;
>  
> +	mapping = vma->vm_file->f_mapping;
> +	idx = vma_hugecache_offset(h, vma, address);
> +
>  	/*
>  	 * Serialize hugepage allocation and instantiation, so that we don't
>  	 * get spurious allocation failures if two CPUs race to instantiate
>  	 * the same page in the page cache.
>  	 */
> -	mutex_lock(&hugetlb_instantiation_mutex);
> +	hash = fault_mutex_hash(h, mm, vma, mapping, idx, address);
> +	mutex_lock(&htlb_fault_mutex_table[hash]);
> +
>  	entry = huge_ptep_get(ptep);
>  	if (huge_pte_none(entry)) {
> -		ret = hugetlb_no_page(mm, vma, address, ptep, flags);
> +		ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep, flags);
>  		goto out_mutex;
>  	}
>  
> @@ -3008,8 +3068,7 @@ out_ptl:
>  	put_page(page);
>  
>  out_mutex:
> -	mutex_unlock(&hugetlb_instantiation_mutex);
> -
> +	mutex_unlock(&htlb_fault_mutex_table[hash]);
>  	return ret;

That's nice and simple.



From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Subject: mm-hugetlb-improve-page-fault-scalability-fix

remove stray + characters, go BUG if hugetlb_init() kmalloc fails

Cc: Davidlohr Bueso <davidlohr@xxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/hugetlb.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff -puN mm/hugetlb.c~mm-hugetlb-improve-page-fault-scalability-fix mm/hugetlb.c
--- a/mm/hugetlb.c~mm-hugetlb-improve-page-fault-scalability-fix
+++ a/mm/hugetlb.c
@@ -55,9 +55,9 @@ static unsigned long __initdata default_
 DEFINE_SPINLOCK(hugetlb_lock);
 
 /*
-+ * Serializes faults on the same logical page.  This is used to
-+ * prevent spurious OOMs when the hugepage pool is fully utilized.
-+ */
+ * Serializes faults on the same logical page.  This is used to
+ * prevent spurious OOMs when the hugepage pool is fully utilized.
+ */
 static int num_fault_mutexes;
 static struct mutex *htlb_fault_mutex_table ____cacheline_aligned_in_smp;
 
@@ -2008,8 +2008,7 @@ static int __init hugetlb_init(void)
 #endif
 	htlb_fault_mutex_table =
 		kmalloc(sizeof(struct mutex) * num_fault_mutexes, GFP_KERNEL);
-	if (!htlb_fault_mutex_table)
-		return -ENOMEM;
+	BUG_ON(!htlb_fault_mutex_table);
 
 	for (i = 0; i < num_fault_mutexes; i++)
 		mutex_init(&htlb_fault_mutex_table[i]);
_

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