linux-kernel - Re: [RFC PATCH 2/3] softirq: implement interrupt flood detection

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Date:   Thu, 19 Dec 2019 10:23:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Jens Axboe <axboe@...nel.dk>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Long Li <longli@...rosoft.com>, Ingo Molnar <mingo@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        John Garry <john.garry@...wei.com>,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [RFC PATCH 2/3] softirq: implement interrupt flood detection

On Thu, Dec 19, 2019 at 09:59:48AM +0800, Ming Lei wrote:
> > So pray tell, why did you not integrate this with IRQ_TIME_ACCOUNTING ?
> > That already takes a timestamp and does most of what you need.
> 
> Yeah, that was the 1st approach I thought of, but IRQ_TIME_ACCOUNTING
> may be disabled, and enabling it may cause observable effect on IO
> performance.

Is that an actual concern, are people disabling it?

> > > @@ -356,6 +512,7 @@ void irq_enter(void)
> > >  	}
> > >  
> > >  	__irq_enter();
> > > +	irq_interval_update();
> > >  }
> > 
> > Arggh.. you're going to make every single interrupt take at least 2
> > extra cache misses for this gunk?!?
> 
> Could you explain it a bit why two cache misses are involved?
> 
> I understand at most one miss is caused, which should only happen in
> irq_interval_update(), and what is the other one?

The rq clock thing IIRC.