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

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Date:   Thu, 15 Oct 2020 16:37:52 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Roger Pau Monne <roger.pau@...rix.com>,
        linux-kernel@...r.kernel.org
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        SeongJae Park <sjpark@...zon.de>,
        xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org,
        "J . Roeleveld" <joost@...arean.org>
Subject: Re: [PATCH 2/2] xen/blkback: turn the cache purge percent into a
 parameter

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@...rix.com>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: SeongJae Park <sjpark@...zon.de>
> Cc: xen-devel@...ts.xenproject.org
> Cc: linux-block@...r.kernel.org
> Cc: J. Roeleveld <joost@...arean.org>
> Cc: Jürgen Groß <jgross@...e.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@...rix.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