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

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Message-ID: <ef03ceac-e7dc-f17a-8d4d-28adf90f6ded@redhat.com>
Date:   Fri, 23 Oct 2020 11:33:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Josh Don <joshdon@...gle.com>,
        g@...ez.programming.kicks-ass.net
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        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 23/10/20 09:19, Peter Zijlstra wrote:
>> +	/*
>> +	 * preemption needs to be kept disabled between prepare_to_busy_poll()
>> +	 * and end_busy_poll().
>> +	 */
>> +	BUG_ON(preemptible());
>> +	if (allow_resched)
>> +		preempt_enable();
>> +	else
>> +		preempt_enable_no_resched();
> NAK on @allow_resched
> 

Since KVM is the one passing false, indeed I see no reason for the
argument; you can just use preempt_enable().  There is no impact for
example on the tracking of how much time was spent polling; that
ktime_get() for the end of the polling period is done before calling
end_busy_poll().

Paolo