netdev - Re: [PATCH 1/3] sched: better handling for busy polling loops

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Date:   Fri, 23 Oct 2020 10:48:53 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "David S. Miller" <davem@...emloft.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, Xi Wang <xii@...gle.com>
Subject: Re: [PATCH 1/3] sched: better handling for busy polling loops

On Thu, 22 Oct 2020 20:29:42 -0700 Josh Don wrote:
> Busy polling loops in the kernel such as network socket poll and kvm
> halt polling have performance problems related to process scheduler load
> accounting.
> 
> Both of the busy polling examples are opportunistic - they relinquish
> the cpu if another thread is ready to run.

That makes it sound like the busy poll code is trying to behave like an
idle task. I thought need_resched() meant we leave when we run out of
slice, or kernel needs to go through a resched for internal reasons. No?

> This design, however, doesn't
> extend to multiprocessor load balancing very well. The scheduler still
> sees the busy polling cpu as 100% busy and will be less likely to put
> another thread on that cpu. In other words, if all cores are 100%
> utilized and some of them are running real workloads and some others are
> running busy polling loops, newly woken up threads will not prefer the
> busy polling cpus. System wide throughput and latency may suffer.

IDK how well this extends to networking. Busy polling in networking is
a conscious trade-off of CPU for latency, if application chooses to
busy poll (which isn't the default) we should respect that.

Is your use case primarily kvm?