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Date:   Mon, 9 Aug 2021 21:48:59 -0400
From:   Waiman Long <llong@...hat.com>
To:     Rafael Aquini <aquini@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Manfred Spraul <manfred@...orfullife.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        Waiman Long <llong@...hat.com>
Subject: Re: [PATCH] ipc: replace costly bailout check in sysvipc_find_ipc()

On 8/9/21 4:35 PM, Rafael Aquini wrote:
> sysvipc_find_ipc() was left with a costly way to check if the offset
> position fed to it is bigger than the total number of IPC IDs in use.
> So much so that the time it takes to iterate over /proc/sysvipc/* files
> grows exponentially for a custom benchmark that creates "N" SYSV shm
> segments and then times the read of /proc/sysvipc/shm (milliseconds):
>
>      12 msecs to read   1024 segs from /proc/sysvipc/shm
>      18 msecs to read   2048 segs from /proc/sysvipc/shm
>      65 msecs to read   4096 segs from /proc/sysvipc/shm
>     325 msecs to read   8192 segs from /proc/sysvipc/shm
>    1303 msecs to read  16384 segs from /proc/sysvipc/shm
>    5182 msecs to read  32768 segs from /proc/sysvipc/shm
>
> The root problem lies with the loop that computes the total amount of ids
> in use to check if the "pos" feeded to sysvipc_find_ipc() grew bigger than
> "ids->in_use". That is a quite inneficient way to get to the maximum index
> in the id lookup table, specially when that value is already provided by
> struct ipc_ids.max_idx.
>
> This patch follows up on the optimization introduced via commit 15df03c879836
> ("sysvipc: make get_maxid O(1) again") and gets rid of the aforementioned
> costly loop replacing it by a simpler checkpoint based on ipc_get_maxidx()
> returned value, which allows for a smooth linear increase in time complexity
> for the same custom benchmark:
>
>       2 msecs to read   1024 segs from /proc/sysvipc/shm
>       2 msecs to read   2048 segs from /proc/sysvipc/shm
>       4 msecs to read   4096 segs from /proc/sysvipc/shm
>       9 msecs to read   8192 segs from /proc/sysvipc/shm
>      19 msecs to read  16384 segs from /proc/sysvipc/shm
>      39 msecs to read  32768 segs from /proc/sysvipc/shm
>
> Signed-off-by: Rafael Aquini <aquini@...hat.com>
> ---
>   ipc/util.c | 16 ++++------------
>   1 file changed, 4 insertions(+), 12 deletions(-)
>
> diff --git a/ipc/util.c b/ipc/util.c
> index 0027e47626b7..d48d8cfa1f3f 100644
> --- a/ipc/util.c
> +++ b/ipc/util.c
> @@ -788,21 +788,13 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *s)
>   static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
>   					      loff_t *new_pos)
>   {
> -	struct kern_ipc_perm *ipc;
> -	int total, id;
> -
> -	total = 0;
> -	for (id = 0; id < pos && total < ids->in_use; id++) {
> -		ipc = idr_find(&ids->ipcs_idr, id);
> -		if (ipc != NULL)
> -			total++;
> -	}
> +	struct kern_ipc_perm *ipc = NULL;
> +	int max_idx = ipc_get_maxidx(ids);
>   
> -	ipc = NULL;
> -	if (total >= ids->in_use)
> +	if (max_idx == -1 || pos > max_idx)
>   		goto out;
>   
> -	for (; pos < ipc_mni; pos++) {
> +	for (; pos <= max_idx; pos++) {
>   		ipc = idr_find(&ids->ipcs_idr, pos);
>   		if (ipc != NULL) {
>   			rcu_read_lock();

The "pos > max_idx" check is redundant and so is not really necessary. 
Other than that, the patch looks good to me.

Reviewed-by: Waiman Long <longman@...hat.com>

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