netdev - Re: [PATCH 3/6] sched,perf,kvm: Fix preemption condition

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Date:   Wed, 2 Jun 2021 16:10:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, bristot <bristot@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86 <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
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Subject: Re: [PATCH 3/6] sched,perf,kvm: Fix preemption condition

On Wed, Jun 02, 2021 at 09:59:07AM -0400, Mathieu Desnoyers wrote:
> ----- On Jun 2, 2021, at 9:12 AM, Peter Zijlstra peterz@...radead.org wrote:
> 
> > When ran from the sched-out path (preempt_notifier or perf_event),
> > p->state is irrelevant to determine preemption. You can get preempted
> > with !task_is_running() just fine.
> > 
> > The right indicator for preemption is if the task is still on the
> > runqueue in the sched-out path.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> > kernel/events/core.c |    7 +++----
> > virt/kvm/kvm_main.c  |    2 +-
> > 2 files changed, 4 insertions(+), 5 deletions(-)
> > 
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -8568,13 +8568,12 @@ static void perf_event_switch(struct tas
> > 		},
> > 	};
> > 
> > -	if (!sched_in && task->state == TASK_RUNNING)
> > +	if (!sched_in && current->on_rq) {
> 
> This changes from checking task->state to current->on_rq, but this change
> from "task" to "current" is not described in the commit message, which is odd.
> 
> Are we really sure that task == current here ?

Yeah, @task == @prev == current at this point, but yes, not sure why I
changed that... lemme change that back to task.