linux-kernel - Re: [PATCH 1/5] irq_work: Split raised and lazy lists

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Message-ID: <20140526155944.GL30445@twins.programming.kicks-ass.net>
Date:	Mon, 26 May 2014 17:59:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 1/5] irq_work: Split raised and lazy lists

On Sun, May 25, 2014 at 04:29:47PM +0200, Frederic Weisbecker wrote:
> An irq work can be handled from two places: from the tick if the work
> carries the "lazy" flag and the tick is periodic, or from a self IPI.
> 
> We merge all these works in a single list and we use some per cpu latch
> to avoid raising a self-IPI when one is already pending.
> 
> Now we could do away with this ugly latch if only the list was only made of
> non-lazy works. Just enqueueing a work on the empty list would be enough
> to know if we need to raise an IPI or not.
> 
> Also we are going to implement remote irq work queuing. Then the per CPU
> latch will need to become atomic in the global scope. That's too bad
> because, here as well, just enqueueing a work on an empty list of
> non-lazy works would be enough to know if we need to raise an IPI or not.
> 
> So lets take a way out of this: split the works in two distinct lists,
> one for the works that can be handled by the next tick and another
> one for those handled by the IPI. Just checking if the latter is empty
> when we queue a new work is enough to know if we need to raise an IPI.

That ^

>  bool irq_work_queue(struct irq_work *work)
>  {
> +	unsigned long flags;
> +
>  	/* Only queue if not already pending */
>  	if (!irq_work_claim(work))
>  		return false;
>  
> -	/* Queue the entry and raise the IPI if needed. */
> -	preempt_disable();
> +	/* Check dynticks safely */
> +	local_irq_save(flags);

Does not mention this ^

'sup?

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