netdev - Re: [PATCH net-next 3/4] net: dev: Makes sure netif

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Message-ID: <YfwUskgPuOREd9hs@linutronix.de>
Date:   Thu, 3 Feb 2022 18:45:22 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH net-next 3/4] net: dev: Makes sure netif_rx() can be
 invoked in any context.

On 2022-02-03 17:44:33 [+0100], To Eric Dumazet wrote:
> > I guess the cost of the  local_bh_enable()/local_bh_disable() pair
> > will be roughly the same, please measure it :)
> 
> We would avoid that branch maybe that helps. Will measure.

|  BH OFF/ON     : 722922586
|  BH OFF/ON     : 722931661
|  BH OFF/ON     : 725341486
|  BH OFF/ON     : 725909591
|  BH OFF/ON     : 741705606
|  BH OFF/ON-OPT : 536683873
|  BH OFF/ON-OPT : 536933779
|  BH OFF/ON-OPT : 536967581
|  BH OFF/ON-OPT : 537109700
|  BH OFF/ON-OPT : 537148631

in a tight loop of 100000000 iterations:
BH OFF/ON = local_bh_disable(); local_bh_enable()
BH OFF/ON-OPT = local_bh_disable(); local_bh_enable_opt()
where local_bh_enable_opt() is the proposed function.

725341486 = ~7.3ns for one iteration.
536967581 = ~5.4ns for one iteration.

This is without tracing+lockdep. So I don't need to sell this to peterz
and focus one the previously suggested version.

Sebastian