Re: [PATCH 1/2] xen/blkback: turn the cache purge LRU interval into a parameter

[Jürgen Groß <jgross@xxxxxxxx>]

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 LRU intervals is not calculated
> properly if a partially written value is read, but that would only be
> a transient issue.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: SeongJae Park <sjpark@xxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: linux-block@xxxxxxxxxxxxxxx
> Cc: J. Roeleveld <joost@xxxxxxxxxxxx>
> Cc: Jürgen Groß <jgross@xxxxxxxx>
> ---
>   Documentation/ABI/testing/sysfs-driver-xen-blkback | 10 ++++++++++
>   drivers/block/xen-blkback/blkback.c                |  9 ++++++---
>   2 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback 
> b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> index ecb7942ff146..776f25d335ca 100644
> --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
> +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> @@ -35,3 +35,13 @@ Description:
>                   controls the duration in milliseconds that blkback will not
>                   cache any page not backed by a grant mapping.
>                   The default is 10ms.
> +
> +What:           /sys/module/xen_blkback/parameters/lru_internval
s/lru_internval/lru_interval/

> +Date:           October 2020
> +KernelVersion:  5.10
> +Contact:        Roger Pau Monné <roger.pau@xxxxxxxxxx>
> +Description:
> +                The LRU mechanism to clean the lists of persistent grants needs
> +                to be executed periodically. This parameter controls the time
> +                interval between consecutive executions of the purge mechanism
> +                is set in ms.
> diff --git a/drivers/block/xen-blkback/blkback.c 
> b/drivers/block/xen-blkback/blkback.c
> index adfc9352351d..6ad9b76fdb2b 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -117,7 +117,10 @@ MODULE_PARM_DESC(max_ring_page_order, "Maximum order of 
> pages to be used for the
>    * be executed periodically. The time interval between consecutive executions
>    * of the purge mechanism is set in ms.
>    */
> -#define LRU_INTERVAL 100
> +static unsigned int lru_interval = 100;
> +module_param_named(lru_interval, lru_interval, uint, 0644);
> +MODULE_PARM_DESC(lru_internval,
s/lru_internval/lru_interval/

> +                "Time interval between consecutive executions of the cache purge 
> mechanism (in ms)");
>   
>   /*
>    * When the persistent grants list is full we will remove unused grants
> @@ -620,7 +623,7 @@ int xen_blkif_schedule(void *arg)
>                 if (unlikely(vbd->size != vbd_sz(vbd)))
>                         xen_vbd_resize(blkif);
>   
> -		timeout = msecs_to_jiffies(LRU_INTERVAL);
> +               timeout = msecs_to_jiffies(lru_interval);
>   
>   		timeout = wait_event_interruptible_timeout(
>                         ring->wq,
> @@ -650,7 +653,7 @@ int xen_blkif_schedule(void *arg)
>                 if (blkif->vbd.feature_gnt_persistent &&
>                     time_after(jiffies, ring->next_lru)) {
>                         purge_persistent_gnt(ring);
> -                       ring->next_lru = jiffies + 
> msecs_to_jiffies(LRU_INTERVAL);
> +                       ring->next_lru = jiffies + 
> msecs_to_jiffies(lru_interval);
>                 }
>   
>   		/* Shrink the free pages pool if it is too large. */

Juergen