path: root/ipc/util.h
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authorManfred Spraul <>2019-05-14 15:46:36 -0700
committerLinus Torvalds <>2019-05-14 19:52:52 -0700
commit99db46ea292780cd978d56932d9445b1e8bdafe8 (patch)
treea61a2121c48635478afb64875f8d8170c36bdfc0 /ipc/util.h
parent3278a2c20cb302d27e6f6ee45a3f57361176e426 (diff)
ipc: do cyclic id allocation for the ipc object.
For ipcmni_extend mode, the sequence number space is only 7 bits. So the chance of id reuse is relatively high compared with the non-extended mode. To alleviate this id reuse problem, this patch enables cyclic allocation for the index to the radix tree (idx). The disadvantage is that this can cause a slight slow-down of the fast path, as the radix tree could be higher than necessary. To limit the radix tree height, I have chosen the following limits: 1) The cycling is done over in_use*1.5. 2) At least, the cycling is done over "normal" ipcnmi mode: RADIX_TREE_MAP_SIZE elements "ipcmni_extended": 4096 elements Result: - for normal mode: No change for <= 42 active ipc elements. With more than 42 active ipc elements, a 2nd level would be added to the radix tree. Without cyclic allocation, a 2nd level would be added only with more than 63 active elements. - for extended mode: Cycling creates always at least a 2-level radix tree. With more than 2730 active objects, a 3rd level would be added, instead of > 4095 active objects until the 3rd level is added without cyclic allocation. For a 2-level radix tree compared to a 1-level radix tree, I have observed < 1% performance impact. Notes: 1) Normal "x=semget();y=semget();" is unaffected: Then the idx is e.g. a and a+1, regardless if idr_alloc() or idr_alloc_cyclic() is used. 2) The -1% happens in a microbenchmark after this situation: x=semget(); for(i=0;i<4000;i++) {t=semget();semctl(t,0,IPC_RMID);} y=semget(); Now perform semget calls on x and y that do not sleep. 3) The worst-case reuse cycle time is unfortunately unaffected: If you have 2^24-1 ipc objects allocated, and get/remove the last possible element in a loop, then the id is reused after 128 get/remove pairs. Performance check: A microbenchmark that performes no-op semop() randomly on two IDs, with only these two IDs allocated. The IDs were set using /proc/sys/kernel/sem_next_id. The test was run 5 times, averages are shown. 1 & 2: Base (6.22 seconds for 10.000.000 semops) 1 & 40: -0.2% 1 & 3348: - 0.8% 1 & 27348: - 1.6% 1 & 15777204: - 3.2% Or: ~12.6 cpu cycles per additional radix tree level. The cpu is an Intel I3-5010U. ~1300 cpu cycles/syscall is slower than what I remember (spectre impact?). V2 of the patch: - use "min" and "max" - use RADIX_TREE_MAP_SIZE * RADIX_TREE_MAP_SIZE instead of (2<<12). [ fix max() warning] Link: Signed-off-by: Manfred Spraul <> Acked-by: Waiman Long <> Cc: "Luis R. Rodriguez" <> Cc: Kees Cook <> Cc: Jonathan Corbet <> Cc: Al Viro <> Cc: Matthew Wilcox <> Cc: "Eric W . Biederman" <> Cc: Takashi Iwai <> Cc: Davidlohr Bueso <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'ipc/util.h')
1 files changed, 3 insertions, 0 deletions
diff --git a/ipc/util.h b/ipc/util.h
index 8c834ed39012..0fcf8e719b76 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -27,12 +27,14 @@
#define IPCMNI_SHIFT 15
#define IPCMNI (1 << IPCMNI_SHIFT)
extern int ipc_mni;
extern int ipc_mni_shift;
+extern int ipc_min_cycle;
#define ipcmni_seq_shift() ipc_mni_shift
#define IPCMNI_IDX_MASK ((1 << ipc_mni_shift) - 1)
@@ -40,6 +42,7 @@ extern int ipc_mni_shift;
#define ipc_mni IPCMNI
+#define ipc_min_cycle ((int)RADIX_TREE_MAP_SIZE)
#define ipcmni_seq_shift() IPCMNI_SHIFT
#define IPCMNI_IDX_MASK ((1 << IPCMNI_SHIFT) - 1)