linux-kernel - Re: [PATCH] xen: release lock occasionally during ballooning

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Date:	Fri, 10 Jul 2015 11:58:24 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, konrad.wilk@...cle.com,
	david.vrabel@...rix.com
Subject: Re: [PATCH] xen: release lock occasionally during ballooning

On 07/10/2015 10:42 AM, Juergen Gross wrote:
> When dom0 is being ballooned balloon_process() will hold the balloon
> mutex until it is finished. This will block e.g. creation of new
> domains as the device backends for the new domain need some
> autoballooned pages for the ring buffers.
>
> Avoid this by releasing the balloon mutex from time to time during
> ballooning. Add a state variable to indicate one balloon_process()
> is active to avoid multiple balloon processes fighting for the mutex.
>
> Instead of open coding it, just use cond_resched().
>
> Signed-off-by: Juergen Gross <jgross@...e.com>


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>


> ---
>   drivers/xen/balloon.c | 19 +++++++++++++++----
>   1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index fd93369..e6d9eee 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -481,9 +481,16 @@ static void balloon_process(struct work_struct *work)
>   {
>   	enum bp_state state = BP_DONE;
>   	long credit;
> +	static bool active;
>   
>   	mutex_lock(&balloon_mutex);
>   
> +	if (active) {
> +		mutex_unlock(&balloon_mutex);
> +		return;
> +	}
> +	active = true;
> +
>   	do {
>   		credit = current_credit();
>   
> @@ -499,12 +506,16 @@ static void balloon_process(struct work_struct *work)
>   
>   		state = update_schedule(state);
>   
> -#ifndef CONFIG_PREEMPT
> -		if (need_resched())
> -			schedule();
> -#endif
> +		mutex_unlock(&balloon_mutex);
> +
> +		cond_resched();
> +
> +		mutex_lock(&balloon_mutex);
> +
>   	} while (credit && state == BP_DONE);
>   
> +	active = false;
> +
>   	/* Schedule more work if there is some still to be done. */
>   	if (state == BP_EAGAIN)
>   		schedule_delayed_work(&balloon_worker, balloon_stats.schedule_delay * HZ);

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