EPT と TLB でしくじった話

•

4 recomendaciones•2,418 vistas

Takaaki Fukai

第12回カーネル／VM探検隊発表資料 http://kernelvm12.peatix.com/

Tecnología

EPT と TLB でしくじった話
@deep_tkkn
2016.10.08 Kernel/VM 探検隊

自己紹介
• 氏名: 深井貴明
• 所属: TKB大学
• Twitter: @deep_tkkn
• BitVisor であれこれしてる人
2

今日のお話
• EPT と TLB 周りでハマって時間溶けて悲しかったお話
• 新規性はないと思う
• 理屈としては教科書にも載ってそう
• 同じような経験したことある人いると思う
• しかし，実際起きるとどれくらい悲しいか伝えたい
3
EPT, TLB, マルチコア，ｳｯ頭が

事の始まり
• BitVisor でライブマイグレーションを実装
• シングルコアでは動いた  マルチコア対応
4
ライブマイグレーション!
移動元の物理マシン移動先の物理マシン

マイグレーションってどうやるの?
VMを構成するハードウェアの状態をすべて転送
• CPU
• メモリ
• デバイス
5

マイグレーションってどうやるの?
VMを構成するハードウェアの状態をすべて転送
• CPU
• メモリ
• デバイス
メモリ転送には時間がかかる
“ライブ” にするには，OSのバックグラウンドでメモリ転送
6

バックグラウンドでメモリ転送
7
Src RAM Dst RAM転送
OS が
メモリ更新
Dirty page を再送
Dirty page を再送
OS が
メモリ更新
・・・

Dirty pages をどうやって追うか
• VT-x のメモリ管理機能 (EPT) の機能の一部を使う
• EPT: ゲスト物理アドレス  マシン物理アドレスへ変換する
ページング機構
• 挙動は普通のページングとほぼ同じ
• EPT エントリ (EPTE) にも Dirty bit がある
• 該当ページが更新されると立つビット
8

9
Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3C:
System Programming Guide, Part 3 Order Number: 326019-048US

何も考えずに実装してみた
• メモリ転送は1コアでやる (並列にやるとロックがうざい)
• Dirty bit の操作も 1 コアでやる
• Dirty page 送りきったら，すべてのコアの EPTE を走査
• Dirty bit が立ってるページを記録
10

何が起きたか
11
20% の確率でエラー && エラーログが毎回違う
2~3カ月くらい

TLB と EPT の落とし穴
• TLB(Table Lookaside Buffer): アドレス変換のキャッシュ
• 毎回ページテーブルを引くと遅いから
• EPT による変換も TLB に載る
• Dirty Bit の情報も TLB に載る
• たぶん EPTE がキャッシュに載るイメージ
• TLB はHW がコア間でコヒーレンシー取られない
• CPU のデータキャッシュとは違う
12

何が起きていたのか?
• あるコアでほかのコアの EPT エントリを読む
• Dirty bit を読む
• 一部の情報はTLBに載ってて古い
• 該当ページを書き換えたのに dirty bit が立ってないように見
える
• Dirty ページを見逃す
•  一部のページだけ古い状態でマイグレーション
13

修正版の実装
• EPTE 読む前に全コアで TLB shootdown する
• ちゃんと動く
• コードとしては数行の変更
• これに2,3カ月取られたと思うと…
14

大事なこと
• 負けないこと投げ出さないこと逃げ出さないこと
• メモリ周りの挙動が怪しければ TLB も疑おう
• 思い当たれば気付けるし直せる
• 知らない & 周りに知ってる人がいないと祈るしかない
• 知る者と知らざる者の差
15

まとめ
• EPTE は TLB に載る
• TLB はハードウェアがコヒーレンシ―取らない
• データキャッシュみたいな感覚だと痛い目を見る
• TLB 絡みのバグはつらい
• メモリが微妙に壊れる
• メモリ周りの挙動が怪しいときは TLB を疑う
16

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EPT と TLB でしくじった話

1. EPT と TLB でしくじった話 @deep_tkkn 2016.10.08 Kernel/VM 探検隊

2. 自己紹介 • 氏名: 深井貴明 • 所属: TKB大学 • Twitter: @deep_tkkn • BitVisor であれこれしてる人 2

3. 今日のお話 • EPT と TLB 周りでハマって時間溶けて悲しかったお話 • 新規性はないと思う • 理屈としては教科書にも載ってそう • 同じような経験したことある人いると思う • しかし，実際起きるとどれくらい悲しいか伝えたい 3 EPT, TLB, マルチコア，ｳｯ頭が

4. 事の始まり • BitVisor でライブマイグレーションを実装 • シングルコアでは動いた  マルチコア対応 4 ライブマイグレーション! 移動元の物理マシン移動先の物理マシン

5. マイグレーションってどうやるの? VMを構成するハードウェアの状態をすべて転送 • CPU • メモリ • デバイス 5

6. マイグレーションってどうやるの? VMを構成するハードウェアの状態をすべて転送 • CPU • メモリ • デバイスメモリ転送には時間がかかる “ライブ” にするには，OSのバックグラウンドでメモリ転送 6

7. バックグラウンドでメモリ転送 7 Src RAM Dst RAM転送 OS がメモリ更新 Dirty page を再送 Dirty page を再送 OS がメモリ更新・・・

8. Dirty pages をどうやって追うか • VT-x のメモリ管理機能 (EPT) の機能の一部を使う • EPT: ゲスト物理アドレス  マシン物理アドレスへ変換するページング機構 • 挙動は普通のページングとほぼ同じ • EPT エントリ (EPTE) にも Dirty bit がある • 該当ページが更新されると立つビット 8

9. 9 Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3C: System Programming Guide, Part 3 Order Number: 326019-048US

10. 何も考えずに実装してみた • メモリ転送は1コアでやる (並列にやるとロックがうざい) • Dirty bit の操作も 1 コアでやる • Dirty page 送りきったら，すべてのコアの EPTE を走査 • Dirty bit が立ってるページを記録 10

11. 何が起きたか 11 20% の確率でエラー && エラーログが毎回違う 2~3カ月くらい

12. TLB と EPT の落とし穴 • TLB(Table Lookaside Buffer): アドレス変換のキャッシュ • 毎回ページテーブルを引くと遅いから • EPT による変換も TLB に載る • Dirty Bit の情報も TLB に載る • たぶん EPTE がキャッシュに載るイメージ • TLB はHW がコア間でコヒーレンシー取られない • CPU のデータキャッシュとは違う 12

13. 何が起きていたのか? • あるコアでほかのコアの EPT エントリを読む • Dirty bit を読む • 一部の情報はTLBに載ってて古い • 該当ページを書き換えたのに dirty bit が立ってないように見える • Dirty ページを見逃す •  一部のページだけ古い状態でマイグレーション 13

14. 修正版の実装 • EPTE 読む前に全コアで TLB shootdown する • ちゃんと動く • コードとしては数行の変更 • これに2,3カ月取られたと思うと… 14

15. 大事なこと • 負けないこと投げ出さないこと逃げ出さないこと • メモリ周りの挙動が怪しければ TLB も疑おう • 思い当たれば気付けるし直せる • 知らない & 周りに知ってる人がいないと祈るしかない • 知る者と知らざる者の差 15

16. まとめ • EPTE は TLB に載る • TLB はハードウェアがコヒーレンシ―取らない • データキャッシュみたいな感覚だと痛い目を見る • TLB 絡みのバグはつらい • メモリが微妙に壊れる • メモリ周りの挙動が怪しいときは TLB を疑う 16

EPT と TLB でしくじった話

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EPT と TLB でしくじった話