linux-kernel - Re: [PATCH] percpu_ida: Handle out-of-tags gracefully

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Date:	Wed, 12 Mar 2014 11:05:10 +0100
From:	Bart Van Assche <bvanassche@....org>
To:	Alexander Gordeev <agordeev@...hat.com>
CC:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	Shaohua Li <shli@...nel.org>, Christoph Hellwig <hch@....de>,
	Mike Christie <michaelc@...wisc.edu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] percpu_ida: Handle out-of-tags gracefully

On 03/12/14 09:41, Alexander Gordeev wrote:
> Still the hunk below (a) breaks the 'pool->percpu_max_size' threshold
> and (b) somehow suboptimal, because you wake another thread while a
> free tag was/is on this CPU. If it is still here we would better to
> grab it. If not, it was stolen by another thread and we do not need
> to wake one (not sure how could it be addressed, though).
> 
> In fact, did you try to remove this hunk at all? A following call to
> percpu_ida_free() both honors the threshold and wakes a thread, so
> your extra wake could be unnecessary.
> 
> @@ -189,6 +189,9 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
>  		spin_unlock(&pool->lock);
>  		local_irq_restore(flags);
>  
> +		if (tags->nr_free)
> +			wake_up(&pool->wait);
> +
>  		if (tag >= 0 || state == TASK_RUNNING)
>  			break;
>  
> 

Hello Alexander,

You are right, that hunk is not necessary and can be left out. That code
was added while chasing another (unrelated) bug. I will resend this
patch without that hunk.

Bart.
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