Re: [PATCH 2/2] xen/blkback: turn the cache purge percent into a parameter

["Jürgen Groß" <jgross@suse.com>]

On 15.10.20 16:24, Roger Pau Monne wrote:
> Assume that reads and writes to the variable will be atomic. The worse
> that could happen is that one of the purges removes a partially
> written percentage of grants, but the cache itself will recover.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: SeongJae Park <sjpark@amazon.de>
> Cc: xen-devel@lists.xenproject.org
> Cc: linux-block@vger.kernel.org
> Cc: J. Roeleveld <joost@antarean.org>
> Cc: Jürgen Groß <jgross@suse.com>
> ---
>   Documentation/ABI/testing/sysfs-driver-xen-blkback | 9 +++++++++
>   drivers/block/xen-blkback/blkback.c                | 7 +++++--
>   2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> index 776f25d335ca..7de791ad61f9 100644
> --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
> +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> @@ -45,3 +45,12 @@ Description:
>                   to be executed periodically. This parameter controls the time
>                   interval between consecutive executions of the purge mechanism
>                   is set in ms.
> +
> +What:           /sys/module/xen_blkback/parameters/lru_percent_clean
> +Date:           October 2020
> +KernelVersion:  5.10
> +Contact:        Roger Pau Monné <roger.pau@citrix.com>
> +Description:
> +                When the persistent grants list is full we will remove unused
> +                grants from the list. The percent number of grants to be
> +                removed at each LRU execution.
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index 6ad9b76fdb2b..772852d45a5a 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -127,7 +127,10 @@ MODULE_PARM_DESC(lru_internval,
>    * from the list. The percent number of grants to be removed at each LRU
>    * execution.
>    */
> -#define LRU_PERCENT_CLEAN 5
> +static unsigned int lru_percent_clean = 5;
> +module_param_named(lru_percent_clean, lru_percent_clean, uint, 0644);
> +MODULE_PARM_DESC(lru_percent_clean,
> +		 "Percentage of persistent grants to remove from the cache when full");
>   
>   /* Run-time switchable: /sys/module/blkback/parameters/ */
>   static unsigned int log_stats;
> @@ -404,7 +407,7 @@ static void purge_persistent_gnt(struct xen_blkif_ring *ring)
>   	    !ring->blkif->vbd.overflow_max_grants)) {
>   		num_clean = 0;
>   	} else {
> -		num_clean = (max_pgrants / 100) * LRU_PERCENT_CLEAN;
> +		num_clean = (max_pgrants / 100) * lru_percent_clean;

Hmm, wouldn't it be better to use (max_grants * lru_percent_clean) / 100
here in order to support max_grants values less than 100?


Juergen