netdev - Re: [PATCH net] net: avoid indirect memory pressure calls

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Message-ID: <0650079e-2cc1-626b-ac04-2230b41fd842@intel.com>
Date:   Tue, 28 Feb 2023 17:28:36 +0100
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>, Florian Westphal <fw@...len.de>
CC:     <edumazet@...gle.com>, <netdev@...r.kernel.org>,
        <davem@...emloft.net>, <pabeni@...hat.com>, <shakeelb@...gle.com>,
        <soheil@...gle.com>
Subject: Re: [PATCH net] net: avoid indirect memory pressure calls

From: Jakub Kicinski <kuba@...nel.org>
Date: Mon, 27 Feb 2023 15:27:41 -0800

> On Fri, 24 Feb 2023 19:46:06 +0100 Florian Westphal wrote:
>> There is a noticeable tcp performance regression (loopback or cross-netns),
>> seen with iperf3 -Z (sendfile mode) when generic retpolines are needed.
>>
>> With SK_RECLAIM_THRESHOLD checks gone number of calls to enter/leave
>> memory pressure happen much more often. For TCP indirect calls are
>> used.
>>
>> We can't remove the if-set-return short-circuit check in
>> tcp_enter_memory_pressure because there are callers other than
>> sk_enter_memory_pressure.  Doing a check in the sk wrapper too
>> reduces the indirect calls enough to recover some performance.
>>
>> Before,
>> 0.00-60.00  sec   322 GBytes  46.1 Gbits/sec                  receiver
>>
>> After:
>> 0.00-60.04  sec   359 GBytes  51.4 Gbits/sec                  receiver
>>
>> "iperf3 -c $peer -t 60 -Z -f g", connected via veth in another netns.
>>
>> Fixes: 4890b686f408 ("net: keep sk->sk_forward_alloc as small as possible")
>> Signed-off-by: Florian Westphal <fw@...len.de>
> 
> Looks acceptable, Eric?
> 
I'm no Eric, but I'd only change this:

+	if (!memory_pressure || READ_ONCE(*memory_pressure) == 0)

to

+	if (!memory_pressure || !READ_ONCE(*memory_pressure))

:p

The perf boost looks gross, love that *_*

Thanks,
Olek