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Subject: Re: [PATCH] inetpeer: use else if instead of if to reduce judgment
To:     Yejune Deng <yejune.deng@gmail.com>, davem@davemloft.net,
        yoshfuji@linux-ipv6.org, dsahern@kernel.org, kuba@kernel.org
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20210226105746.29240-1-yejune.deng@gmail.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <d16327f3-33c0-61c1-3adf-78f7edcbec6a@gmail.com>
Date:   Fri, 26 Feb 2021 15:38:06 +0100
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On 2/26/21 11:57 AM, Yejune Deng wrote:
> In inet_initpeers(), if si.totalram <= (8192*1024)/PAGE_SIZE, it will
> be judged three times. Use else if instead of if, it only needs to be
> judged once.
> 
> Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
> ---
>  net/ipv4/inetpeer.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
> index ff327a62c9ce..07cd1f8204b3 100644
> --- a/net/ipv4/inetpeer.c
> +++ b/net/ipv4/inetpeer.c
> @@ -81,12 +81,12 @@ void __init inet_initpeers(void)
>  	 * <kuznet@ms2.inr.ac.ru>.  I don't have any opinion about the values
>  	 * myself.  --SAW
>  	 */
> -	if (si.totalram <= (32768*1024)/PAGE_SIZE)
> +	if (si.totalram <= (8192 * 1024) / PAGE_SIZE)
> +		inet_peer_threshold >>= 4; /* about 128KB */
> +	else if (si.totalram <= (16384 * 1024) / PAGE_SIZE)
> +		inet_peer_threshold >>= 2; /* about 512KB */
> +	else if (si.totalram <= (32768 * 1024) / PAGE_SIZE)
>  		inet_peer_threshold >>= 1; /* max pool size about 1MB on IA32 */


If you really want to change this stuff, I would suggest updating comments,
because nowadays, struct inet_peer on IA32 uses 128 bytes.

So 32768 entries would consume 4 MB,
   16384 entries would consume 2 MB

and 4096 entries would consume 512KB

Another idea would be to get rid of the cascade and use something that
will not need to be adjusted in the future.

diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
index ff327a62c9ce9b1794104c3c924f5f2b9820ac8b..d5f486bd8c35234f99b22842e756a10531e070d6 100644
--- a/net/ipv4/inetpeer.c
+++ b/net/ipv4/inetpeer.c
@@ -65,7 +65,7 @@ EXPORT_SYMBOL_GPL(inet_peer_base_init);
 #define PEER_MAX_GC 32
 
 /* Exported for sysctl_net_ipv4.  */
-int inet_peer_threshold __read_mostly = 65536 + 128;   /* start to throw entries more
+int inet_peer_threshold __read_mostly; /* start to throw entries more
                                         * aggressively at this stage */
 int inet_peer_minttl __read_mostly = 120 * HZ; /* TTL under high load: 120 sec */
 int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;     /* usual time to live: 10 min */
@@ -73,20 +73,13 @@ int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;  /* usual time to live: 10 min
 /* Called from ip_output.c:ip_init  */
 void __init inet_initpeers(void)
 {
-       struct sysinfo si;
+       u64 nr_entries;
 
-       /* Use the straight interface to information about memory. */
-       si_meminfo(&si);
-       /* The values below were suggested by Alexey Kuznetsov
-        * <kuznet@ms2.inr.ac.ru>.  I don't have any opinion about the values
-        * myself.  --SAW
-        */
-       if (si.totalram <= (32768*1024)/PAGE_SIZE)
-               inet_peer_threshold >>= 1; /* max pool size about 1MB on IA32 */
-       if (si.totalram <= (16384*1024)/PAGE_SIZE)
-               inet_peer_threshold >>= 1; /* about 512KB */
-       if (si.totalram <= (8192*1024)/PAGE_SIZE)
-               inet_peer_threshold >>= 2; /* about 128KB */
+       /* 1% of physical memory */
+       nr_entries = div64_ul((u64)totalram_pages() << PAGE_SHIFT,
+                             100 * L1_CACHE_ALIGN(sizeof(struct inet_peer)));
+
+       inet_peer_threshold = clamp_val(nr_entries, 4096, 65536 + 128);
 
        peer_cachep = kmem_cache_create("inet_peer_cache",
                        sizeof(struct inet_peer),