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Subject[PATCH] mm: Add new vma flag VM_LOCAL_CPU
Date

On a call to mmap an mmap provider (like an FS) can put
this flag on vma->vm_flags.

The VM_LOCAL_CPU flag tells the Kernel that the vma will be used
from a single-core only, and therefore invalidation (flush_tlb) of
PTE(s) need not be a wide CPU scheduling.

The motivation of this flag is the ZUFS project where we want
to optimally map user-application buffers into a user-mode-server
execute the operation and efficiently unmap.

In this project we utilize a per-core server thread so everything
is kept local. If we use the regular zap_ptes() API All CPU's
are scheduled for the unmap, though in our case we know that we
have only used a single core. The regular zap_ptes adds a very big
latency on every operation and mostly kills the concurrency of the
over all system. Because it imposes a serialization between all cores

Some preliminary measurements on a 40 core machines:

unpatched patched
Threads Op/s Lat [us] Op/s Lat [us]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 185391 4.9 200799 4.6
2 197993 9.6 314321 5.9
4 310597 12.1 565574 6.6
8 546702 13.8 1113138 6.6
12 641728 17.2 1598451 6.8
18 744750 22.2 1648689 7.8
24 790805 28.3 1702285 8
36 849763 38.9 1783346 13.4
48 792000 44.6 1741873 17.4

We can clearly see that on an unpatched Kernel we do not scale
and the threads are interfering with each other. This is because
flush-tlb is scheduled on all (other) CPUs.

NOTE: This vma (VM_LOCAL_CPU) is never used during a page_fault. It is
always used in a synchronous way from a thread pinned to a single core.

Signed-off-by: Boaz Harrosh <boazh@netapp.com>
---
arch/x86/mm/tlb.c | 3 ++-
fs/proc/task_mmu.c | 3 +++
include/linux/mm.h | 3 +++
mm/memory.c | 13 +++++++++++--
4 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index e055d1a..1d398a0 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -640,7 +640,8 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
local_irq_enable();
}

- if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
+ if (!(vmflag & VM_LOCAL_CPU) &&
+ cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
flush_tlb_others(mm_cpumask(mm), &info);

put_cpu();
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index c486ad4..305d6e4 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -680,6 +680,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
[ilog2(VM_PKEY_BIT2)] = "",
[ilog2(VM_PKEY_BIT3)] = "",
#endif
+#ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS
+ [ilog2(VM_LOCAL_CPU)] = "lc",
+#endif
};
size_t i;

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ac1f06..3d14107 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -226,6 +226,9 @@ extern unsigned int kobjsize(const void *objp);
#define VM_HIGH_ARCH_2 BIT(VM_HIGH_ARCH_BIT_2)
#define VM_HIGH_ARCH_3 BIT(VM_HIGH_ARCH_BIT_3)
#define VM_HIGH_ARCH_4 BIT(VM_HIGH_ARCH_BIT_4)
+#define VM_LOCAL_CPU BIT(37) /* FIXME: Needs to move from here */
+#else /* ! CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
+#define VM_LOCAL_CPU 0 /* FIXME: Needs to move from here */
#endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */

#if defined(CONFIG_X86)
diff --git a/mm/memory.c b/mm/memory.c
index 01f5464..6236f5e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1788,6 +1788,7 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
int retval;
pte_t *pte, entry;
spinlock_t *ptl;
+ bool need_flush = false;

retval = -ENOMEM;
pte = get_locked_pte(mm, addr, &ptl);
@@ -1795,7 +1796,12 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
goto out;
retval = -EBUSY;
if (!pte_none(*pte)) {
- if (mkwrite) {
+ if ((vma->vm_flags & VM_LOCAL_CPU)) {
+ /* VM_LOCAL_CPU is set, A single CPU is allowed to not
+ * go through zap_vma_ptes before changing a pte
+ */
+ need_flush = true;
+ } else if (mkwrite) {
/*
* For read faults on private mappings the PFN passed
* in may not match the PFN we have mapped if the
@@ -1807,8 +1813,9 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
goto out_unlock;
entry = *pte;
goto out_mkwrite;
- } else
+ } else {
goto out_unlock;
+ }
}

/* Ok, finally just insert the thing.. */
@@ -1824,6 +1831,8 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
}

set_pte_at(mm, addr, pte, entry);
+ if (need_flush)
+ flush_tlb_range(vma, addr, addr + PAGE_SIZE);
update_mmu_cache(vma, addr, pte); /* XXX: why not for insert_page? */

retval = 0;
--
2.5.5

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