Re: [PATCH v12 3/3] ipc: Do cyclic id allocation with ipcmni_extend mode

[Waiman Long]

On 03/19/2019 02:18 PM, Manfred Spraul wrote:
> From 844c9d78cea41983a89c820bd5265ceded59883b Mon Sep 17 00:00:00 2001
> From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxxxxx>
> ---
>  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

Acked-by: Waiman Long <longman@xxxxxxxxxx>