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

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Message-ID: <20140131143441.478f79ee@gandalf.local.home>
Date:	Fri, 31 Jan 2014 14:34:41 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	peterz@...radead.org, 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, 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.

-- Steve
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