Messages in this thread Patch in this message | | | Subject | Re: [PATCH v12 3/3] ipc: Do cyclic id allocation with ipcmni_extend mode | From | Manfred Spraul <> | Date | Tue, 19 Mar 2019 19:18:39 +0100 |
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Hi Waiman,
On 3/18/19 7:46 PM, Waiman Long wrote: > --- a/ipc/util.c >> +++ b/ipc/util.c >> @@ -221,9 +221,17 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new) >> */ >> >> if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */ >> + int max_idx; >> + >> + max_idx = ids->in_use*3/2; >> + if (max_idx > ipc_mni) >> + max_idx = ipc_mni; >> + if (max_idx < ipc_min_cycle) >> + max_idx = ipc_min_cycle; > > Why don't you use the min() and max() macros which will make it easier > to read? > Changed. >> >> /* allocate the idx, with a NULL struct kern_ipc_perm */ >> - idx = idr_alloc(&ids->ipcs_idr, NULL, 0, 0, GFP_NOWAIT); >> + idx = idr_alloc_cyclic(&ids->ipcs_idr, NULL, 0, max_idx, >> + GFP_NOWAIT); >> >> if (idx >= 0) { >> /* >> diff --git a/ipc/util.h b/ipc/util.h >> index 8c834ed39012..ef4e86bb2db8 100644 >> --- a/ipc/util.h >> +++ b/ipc/util.h >> @@ -27,12 +27,14 @@ >> */ >> #define IPCMNI_SHIFT 15 >> #define IPCMNI_EXTEND_SHIFT 24 >> +#define IPCMNI_EXTEND_MIN_CYCLE (2 << 12) > > How about > > #define IPCMNI_EXTEND_MIN_CYCLE (RADIX_TREE_MAP_SIZE * > RADIX_TREE_MAP_SIZE) > Good idea. Actually, "2<<12" was the initial guess.
And then I noticed that this ends up as a two level radix tree during testing :-)
Updated patch attached.
--
Manfred
From 844c9d78cea41983a89c820bd5265ceded59883b Mon Sep 17 00:00:00 2001 From: Manfred Spraul <manfred@colorfullife.com> Date: Sun, 17 Mar 2019 06:29:00 +0100 Subject: [PATCH 2/2] 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).
Signed-off-by: Manfred Spraul <manfred@colorfullife.com> --- ipc/ipc_sysctl.c | 2 ++ ipc/util.c | 7 ++++++- ipc/util.h | 3 +++ 3 files changed, 11 insertions(+), 1 deletion(-)
diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c index 73b7782eccf4..bfaae457810c 100644 --- a/ipc/ipc_sysctl.c +++ b/ipc/ipc_sysctl.c @@ -122,6 +122,7 @@ static int one = 1; static int int_max = INT_MAX; int ipc_mni = IPCMNI; int ipc_mni_shift = IPCMNI_SHIFT; +int ipc_min_cycle = RADIX_TREE_MAP_SIZE; static struct ctl_table ipc_kern_table[] = { { @@ -252,6 +253,7 @@ static int __init ipc_mni_extend(char *str) { ipc_mni = IPCMNI_EXTEND; ipc_mni_shift = IPCMNI_EXTEND_SHIFT; + ipc_min_cycle = IPCMNI_EXTEND_MIN_CYCLE; pr_info("IPCMNI extended to %d.\n", ipc_mni); return 0; } diff --git a/ipc/util.c b/ipc/util.c index 6e0fe3410423..1a492afb1d8b 100644 --- a/ipc/util.c +++ b/ipc/util.c @@ -221,9 +221,14 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new) */ if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */ + int max_idx; + + max_idx = max(ids->in_use*3/2, ipc_min_cycle); + max_idx = min(max_idx, ipc_mni); /* allocate the idx, with a NULL struct kern_ipc_perm */ - idx = idr_alloc(&ids->ipcs_idr, NULL, 0, 0, GFP_NOWAIT); + idx = idr_alloc_cyclic(&ids->ipcs_idr, NULL, 0, max_idx, + GFP_NOWAIT); if (idx >= 0) { /* diff --git a/ipc/util.h b/ipc/util.h index 8c834ed39012..d316399f0c32 100644 --- a/ipc/util.h +++ b/ipc/util.h @@ -27,12 +27,14 @@ */ #define IPCMNI_SHIFT 15 #define IPCMNI_EXTEND_SHIFT 24 +#define IPCMNI_EXTEND_MIN_CYCLE (RADIX_TREE_MAP_SIZE * RADIX_TREE_MAP_SIZE) #define IPCMNI (1 << IPCMNI_SHIFT) #define IPCMNI_EXTEND (1 << IPCMNI_EXTEND_SHIFT) #ifdef CONFIG_SYSVIPC_SYSCTL 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; #else /* CONFIG_SYSVIPC_SYSCTL */ #define ipc_mni IPCMNI +#define ipc_min_cycle RADIX_TREE_MAP_SIZE #define ipcmni_seq_shift() IPCMNI_SHIFT #define IPCMNI_IDX_MASK ((1 << IPCMNI_SHIFT) - 1) #endif /* CONFIG_SYSVIPC_SYSCTL */ -- 2.17.2
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