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 | Sun, 17 Mar 2019 19:27:49 +0100 |
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Hi Waiman,
On 2/28/19 7:47 PM, Waiman Long wrote: > 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, the id allocation will be done > cyclically to cycle through all the 24-bit id space before wrapping > around when in ipcmni_extend mode. This may cause the use of more memory > in term of the number of xa_nodes allocated as well as potentially more > cachelines used as the xa_nodes may be spread more sparsely in this case. > > There is probably a slight memory and performance cost in doing cyclic > id allocation. For applications that really need more than 32k unique IPC > identifiers, this is a small price to pay to avoid the id reuse problem.
Have you measured it?
I have observed -3% for semop() for a 4 level radix tree compared to a 1-level radix tree, and I'm a bit reluctant to accept that. Especially as the percentage will increase if the syscall overhead goes down again (-> less spectre impact).
[...]
> --- a/ipc/util.c > +++ b/ipc/util.c > @@ -221,7 +221,12 @@ 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 */ > - idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT); > + if (ipc_mni_extended) > + idx = idr_alloc_cyclic(&ids->ipcs_idr, new, 0, ipc_mni, > + GFP_NOWAIT); > + else > + idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT); > + > if ((idx <= ids->last_idx) && (++ids->seq > IPCID_SEQ_MAX)) > ids->seq = 0;
I don't like it that there are two different codepaths.
Attached is a different proposal:
Always use cyclic allocation, with some logic to minimize the additional radix tree levels.
What do you think?
--
Manfred
From 491ea87cc3022e50c02caae009f9aeba2b6ddcb4 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 objects.
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. I consider this as a good compromise.
Notes: 1) Normal "x=semget();y=semget();" is unaffected: Then the idx is e.g. a and a+1, regardless of 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?).
Signed-off-by: Manfred Spraul <manfred@colorfullife.com> --- ipc/ipc_sysctl.c | 2 ++ ipc/util.c | 10 +++++++++- ipc/util.h | 3 +++ 3 files changed, 14 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..90764b67af51 100644 --- 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; /* 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) #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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