Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Dec 11, 2019 at 06:10:15PM +0000, SeongJae Park wrote:
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index fd1e19f1a49f..98823d150905 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -142,6 +142,21 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
>  		HZ * xen_blkif_pgrant_timeout);
>  }
>  
> +/* Once a memory pressure is detected, squeeze free page pools for a while. */
> +static unsigned int buffer_squeeze_duration_ms = 10;
> +module_param_named(buffer_squeeze_duration_ms,
> +		buffer_squeeze_duration_ms, int, 0644);
> +MODULE_PARM_DESC(buffer_squeeze_duration_ms,
> +"Duration in ms to squeeze pages buffer when a memory pressure is detected");
> +
> +static unsigned long buffer_squeeze_end;
> +
> +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
> +{
> +	buffer_squeeze_end = jiffies +
> +		msecs_to_jiffies(buffer_squeeze_duration_ms);

I'm not sure this is fully correct. This function will be called for
each blkback instance, but the timeout is stored in a global variable
that's shared between all blkback instances. Shouldn't this timeout be
stored in xen_blkif so each instance has it's own local variable?

Or else in the case you have 1k blkback instances the timeout is
certainly going to be longer than expected, because each call to
xen_blkbk_reclaim_memory will move it forward.

Thanks, Roger.