linux-kernel - Re: [PATCH 1/3] sched: define a function to report the number of context switches on a CPU

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Message-ID: <20190821103456.GY2349@hirez.programming.kicks-ass.net>
Date:   Wed, 21 Aug 2019 12:34:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Long Li <longli@...rosoft.com>
Cc:     "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>,
        Ingo Molnar <mingo@...hat.com>,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] sched: define a function to report the number of
 context switches on a CPU

On Wed, Aug 21, 2019 at 08:20:48AM +0000, Long Li wrote:
> >>>Subject: Re: [PATCH 1/3] sched: define a function to report the number of
> >>>context switches on a CPU
> >>>
> >>>On Mon, Aug 19, 2019 at 11:14:27PM -0700, longli@...uxonhyperv.com
> >>>wrote:
> >>>> From: Long Li <longli@...rosoft.com>
> >>>>
> >>>> The number of context switches on a CPU is useful to determine how
> >>>> busy this CPU is on processing IRQs. Export this information so it can
> >>>> be used by device drivers.
> >>>
> >>>Please do explain that; because I'm not seeing how number of switches
> >>>relates to processing IRQs _at_all_!
> 
> Some kernel components rely on context switch to progress, for example
> watchdog and RCU. On a CPU with reasonable interrupt load, it
> continues to make context switches, normally a number of switches per
> seconds. 

That isn't true; RCU is perfectly fine with a single task always running
and not making context switches, and so is the watchdog.