libgo/go/runtime/mpagecache.go

   1 // Copyright 2019 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package runtime
   6
   7 import (
   8         "runtime/internal/sys"
   9         "unsafe"
  10 )
  11
  12 const pageCachePages = 8 * unsafe.Sizeof(pageCache{}.cache)
  13
  14 // pageCache represents a per-p cache of pages the allocator can
  15 // allocate from without a lock. More specifically, it represents
  16 // a pageCachePages*pageSize chunk of memory with 0 or more free
  17 // pages in it.
  18 type pageCache struct {
  19         base  uintptr // base address of the chunk
  20         cache uint64  // 64-bit bitmap representing free pages (1 means free)
  21         scav  uint64  // 64-bit bitmap representing scavenged pages (1 means scavenged)
  22 }
  23
  24 // empty returns true if the pageCache has any free pages, and false
  25 // otherwise.
  26 func (c *pageCache) empty() bool {
  27         return c.cache == 0
  28 }
  29
  30 // alloc allocates npages from the page cache and is the main entry
  31 // point for allocation.
  32 //
  33 // Returns a base address and the amount of scavenged memory in the
  34 // allocated region in bytes.
  35 //
  36 // Returns a base address of zero on failure, in which case the
  37 // amount of scavenged memory should be ignored.
  38 func (c *pageCache) alloc(npages uintptr) (uintptr, uintptr) {
  39         if c.cache == 0 {
  40                 return 0, 0
  41         }
  42         if npages == 1 {
  43                 i := uintptr(sys.TrailingZeros64(c.cache))
  44                 scav := (c.scav >> i) & 1
  45                 c.cache &^= 1 << i // set bit to mark in-use
  46                 c.scav &^= 1 << i  // clear bit to mark unscavenged
  47                 return c.base + i*pageSize, uintptr(scav) * pageSize
  48         }
  49         return c.allocN(npages)
  50 }
  51
  52 // allocN is a helper which attempts to allocate npages worth of pages
  53 // from the cache. It represents the general case for allocating from
  54 // the page cache.
  55 //
  56 // Returns a base address and the amount of scavenged memory in the
  57 // allocated region in bytes.
  58 func (c *pageCache) allocN(npages uintptr) (uintptr, uintptr) {
  59         i := findBitRange64(c.cache, uint(npages))
  60         if i >= 64 {
  61                 return 0, 0
  62         }
  63         mask := ((uint64(1) << npages) - 1) << i
  64         scav := sys.OnesCount64(c.scav & mask)
  65         c.cache &^= mask // mark in-use bits
  66         c.scav &^= mask  // clear scavenged bits
  67         return c.base + uintptr(i*pageSize), uintptr(scav) * pageSize
  68 }
  69
  70 // flush empties out unallocated free pages in the given cache
  71 // into s. Then, it clears the cache, such that empty returns
  72 // true.
  73 //
  74 // p.mheapLock must be held.
  75 //
  76 // Must run on the system stack because p.mheapLock must be held.
  77 //
  78 //go:systemstack
  79 func (c *pageCache) flush(p *pageAlloc) {
  80         assertLockHeld(p.mheapLock)
  81
  82         if c.empty() {
  83                 return
  84         }
  85         ci := chunkIndex(c.base)
  86         pi := chunkPageIndex(c.base)
  87
  88         // This method is called very infrequently, so just do the
  89         // slower, safer thing by iterating over each bit individually.
  90         for i := uint(0); i < 64; i++ {
  91                 if c.cache&(1<<i) != 0 {
  92                         p.chunkOf(ci).free1(pi + i)
  93                 }
  94                 if c.scav&(1<<i) != 0 {
  95                         p.chunkOf(ci).scavenged.setRange(pi+i, 1)
  96                 }
  97         }
  98         // Since this is a lot like a free, we need to make sure
  99         // we update the searchAddr just like free does.
 100         if b := (offAddr{c.base}); b.lessThan(p.searchAddr) {
 101                 p.searchAddr = b
 102         }
 103         p.update(c.base, pageCachePages, false, false)
 104         *c = pageCache{}
 105 }
 106
 107 // allocToCache acquires a pageCachePages-aligned chunk of free pages which
 108 // may not be contiguous, and returns a pageCache structure which owns the
 109 // chunk.
 110 //
 111 // p.mheapLock must be held.
 112 //
 113 // Must run on the system stack because p.mheapLock must be held.
 114 //
 115 //go:systemstack
 116 func (p *pageAlloc) allocToCache() pageCache {
 117         assertLockHeld(p.mheapLock)
 118
 119         // If the searchAddr refers to a region which has a higher address than
 120         // any known chunk, then we know we're out of memory.
 121         if chunkIndex(p.searchAddr.addr()) >= p.end {
 122                 return pageCache{}
 123         }
 124         c := pageCache{}
 125         ci := chunkIndex(p.searchAddr.addr()) // chunk index
 126         var chunk *pallocData
 127         if p.summary[len(p.summary)-1][ci] != 0 {
 128                 // Fast path: there's free pages at or near the searchAddr address.
 129                 chunk = p.chunkOf(ci)
 130                 j, _ := chunk.find(1, chunkPageIndex(p.searchAddr.addr()))
 131                 if j == ^uint(0) {
 132                         throw("bad summary data")
 133                 }
 134                 c = pageCache{
 135                         base:  chunkBase(ci) + alignDown(uintptr(j), 64)*pageSize,
 136                         cache: ^chunk.pages64(j),
 137                         scav:  chunk.scavenged.block64(j),
 138                 }
 139         } else {
 140                 // Slow path: the searchAddr address had nothing there, so go find
 141                 // the first free page the slow way.
 142                 addr, _ := p.find(1)
 143                 if addr == 0 {
 144                         // We failed to find adequate free space, so mark the searchAddr as OoM
 145                         // and return an empty pageCache.
 146                         p.searchAddr = maxSearchAddr()
 147                         return pageCache{}
 148                 }
 149                 ci := chunkIndex(addr)
 150                 chunk = p.chunkOf(ci)
 151                 c = pageCache{
 152                         base:  alignDown(addr, 64*pageSize),
 153                         cache: ^chunk.pages64(chunkPageIndex(addr)),
 154                         scav:  chunk.scavenged.block64(chunkPageIndex(addr)),
 155                 }
 156         }
 157
 158         // Set the page bits as allocated and clear the scavenged bits, but
 159         // be careful to only set and clear the relevant bits.
 160         cpi := chunkPageIndex(c.base)
 161         chunk.allocPages64(cpi, c.cache)
 162         chunk.scavenged.clearBlock64(cpi, c.cache&c.scav /* free and scavenged */)
 163
 164         // Update as an allocation, but note that it's not contiguous.
 165         p.update(c.base, pageCachePages, false, true)
 166
 167         // Set the search address to the last page represented by the cache.
 168         // Since all of the pages in this block are going to the cache, and we
 169         // searched for the first free page, we can confidently start at the
 170         // next page.
 171         //
 172         // However, p.searchAddr is not allowed to point into unmapped heap memory
 173         // unless it is maxSearchAddr, so make it the last page as opposed to
 174         // the page after.
 175         p.searchAddr = offAddr{c.base + pageSize*(pageCachePages-1)}
 176         return c
 177 }