linux-kernel - Re: [PATCH 2/2] timer: really raise softirq if there is irq

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Message-ID: <20140131195407.GQ5002@laptop.programming.kicks-ass.net>
Date:	Fri, 31 Jan 2014 20:54:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	paulmck@...ux.vnet.ibm.com, linux-rt-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 2/2] timer: really raise softirq if there is irq_work to
 do

On Fri, Jan 31, 2014 at 02:34:41PM -0500, Steven Rostedt wrote:
> On Fri, 31 Jan 2014 20:26:54 +0100
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > On 01/31/2014 06:57 PM, Steven Rostedt wrote:
> > 
> > > In vanilla Linux, irq_work_run() is called from update_process_times()
> > > when it is called from the timer interrupt. In -rt, there's reasons we
> > 
> > and in vanilla Linux some architectures (like x86 or sparc to name just
> > a few) overwrite arch_irq_work_raise() which means they provide
> > their "own" interrupt like callback. That means on those architectures
> > irq_work_run() gets invoked twice: once via update_process_times() and
> > via and once the custom interface.
> > So my question to the original inventor of this code: Peter, do we
> > really need that arch specific callback? Wouldn't one be enough? Is it
> > that critical that it can't wait to the next timer tick?
> 
> There's flags that determine when the next call should be invoked. The
> irq_work_run() should return immediately if it was already done by the
> arch specific call. The work wont be called twice.
> 
> As I have worked on code that uses irq_work() I can say that we want
> the arch specific interrupts. For those architectures that don't have
> it will experience larger latencies for the work required. It's
> basically, a "too bad" for them.
> 
> But to answer your question, no we want the immediate response.

Yah, what Steve said. That fallback is really a you suck to have to use
this.
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