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). [akpm@linux-foundation.org: fix max() warning] Link: http://lkml.kernel.org/r/20190329204930.21620-3-longman@redhat.com Signed-off-by: Manfred Spraul <manfred@colorfullife.com> Acked-by: Waiman Long <longman@redhat.com> Cc: "Luis R. Rodriguez" <mcgrof@kernel.org> Cc: Kees Cook <keescook@chromium.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Matthew Wilcox <willy@infradead.org> Cc: "Eric W . Biederman" <ebiederm@xmission.com> Cc: Takashi Iwai <tiwai@suse.de> Cc: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -122,6 +122,7 @@ static int one = 1;
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static int int_max = INT_MAX;
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int ipc_mni = IPCMNI;
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int ipc_mni_shift = IPCMNI_SHIFT;
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int ipc_min_cycle = RADIX_TREE_MAP_SIZE;
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static struct ctl_table ipc_kern_table[] = {
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{
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@ -252,6 +253,7 @@ static int __init ipc_mni_extend(char *str)
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{
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ipc_mni = IPCMNI_EXTEND;
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ipc_mni_shift = IPCMNI_EXTEND_SHIFT;
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ipc_min_cycle = IPCMNI_EXTEND_MIN_CYCLE;
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pr_info("IPCMNI extended to %d.\n", ipc_mni);
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return 0;
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}
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@ -220,9 +220,14 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
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*/
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if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
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int max_idx;
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max_idx = max(ids->in_use*3/2, ipc_min_cycle);
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max_idx = min(max_idx, ipc_mni);
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/* allocate the idx, with a NULL struct kern_ipc_perm */
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idx = idr_alloc(&ids->ipcs_idr, NULL, 0, 0, GFP_NOWAIT);
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idx = idr_alloc_cyclic(&ids->ipcs_idr, NULL, 0, max_idx,
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GFP_NOWAIT);
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if (idx >= 0) {
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/*
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@ -27,12 +27,14 @@
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*/
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#define IPCMNI_SHIFT 15
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#define IPCMNI_EXTEND_SHIFT 24
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#define IPCMNI_EXTEND_MIN_CYCLE (RADIX_TREE_MAP_SIZE * RADIX_TREE_MAP_SIZE)
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#define IPCMNI (1 << IPCMNI_SHIFT)
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#define IPCMNI_EXTEND (1 << IPCMNI_EXTEND_SHIFT)
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#ifdef CONFIG_SYSVIPC_SYSCTL
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extern int ipc_mni;
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extern int ipc_mni_shift;
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extern int ipc_min_cycle;
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#define ipcmni_seq_shift() ipc_mni_shift
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#define IPCMNI_IDX_MASK ((1 << ipc_mni_shift) - 1)
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@ -40,6 +42,7 @@ extern int ipc_mni_shift;
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#else /* CONFIG_SYSVIPC_SYSCTL */
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#define ipc_mni IPCMNI
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#define ipc_min_cycle ((int)RADIX_TREE_MAP_SIZE)
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#define ipcmni_seq_shift() IPCMNI_SHIFT
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#define IPCMNI_IDX_MASK ((1 << IPCMNI_SHIFT) - 1)
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#endif /* CONFIG_SYSVIPC_SYSCTL */
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