Re: [Linux-cachefs] [PATCH] cachefiles: make on-demand request distribution fairer

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Hi Yin,

On Wed, Aug 17, 2022 at 02:52:00PM +0800, Xin Yin wrote:
> For now, enqueuing and dequeuing on-demand requests all start from
> idx 0, this makes request distribution unfair. In the weighty
> concurrent I/O scenario, the request stored in higher idx will starve.
> 
> Searching requests cyclically in cachefiles_ondemand_daemon_read,
> makes distribution fairer.

Yeah, thanks for the catch.  The previous approach could cause somewhat
unfairness and make some requests starving... But we don't need strict
FIFO here.

> 
> Reported-by: Yongqing Li <liyongqing@xxxxxxxxxxxxx>
> Signed-off-by: Xin Yin <yinxin.x@xxxxxxxxxxxxx>
> ---
>  fs/cachefiles/internal.h |  1 +
>  fs/cachefiles/ondemand.c | 12 +++++++++---
>  2 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
> index 6cba2c6de2f9..2ad58c465208 100644
> --- a/fs/cachefiles/internal.h
> +++ b/fs/cachefiles/internal.h
> @@ -111,6 +111,7 @@ struct cachefiles_cache {
>  	char				*tag;		/* cache binding tag */
>  	refcount_t			unbind_pincount;/* refcount to do daemon unbind */
>  	struct xarray			reqs;		/* xarray of pending on-demand requests */
> +	unsigned long			req_id_next;

	unsigned long			ondemand_req_id_next; ?

Otherwise it looks good to me,

Thanks,
Gao Xiang

>  	struct xarray			ondemand_ids;	/* xarray for ondemand_id allocation */
>  	u32				ondemand_id_next;
>  };
> diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
> index 1fee702d5529..247961d65369 100644
> --- a/fs/cachefiles/ondemand.c
> +++ b/fs/cachefiles/ondemand.c
> @@ -238,14 +238,19 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
>  	unsigned long id = 0;
>  	size_t n;
>  	int ret = 0;
> -	XA_STATE(xas, &cache->reqs, 0);
> +	XA_STATE(xas, &cache->reqs, cache->req_id_next);
>  
>  	/*
> -	 * Search for a request that has not ever been processed, to prevent
> -	 * requests from being processed repeatedly.
> +	 * Cyclically search for a request that has not ever been processed,
> +	 * to prevent requests from being processed repeatedly, and make
> +	 * request distribution fair.
>  	 */
>  	xa_lock(&cache->reqs);
>  	req = xas_find_marked(&xas, UINT_MAX, CACHEFILES_REQ_NEW);
> +	if (!req && cache->req_id_next > 0) {
> +		xas_set(&xas, 0);
> +		req = xas_find_marked(&xas, cache->req_id_next - 1, CACHEFILES_REQ_NEW);
> +	}
>  	if (!req) {
>  		xa_unlock(&cache->reqs);
>  		return 0;
> @@ -260,6 +265,7 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
>  	}
>  
>  	xas_clear_mark(&xas, CACHEFILES_REQ_NEW);
> +	cache->req_id_next = xas.xa_index + 1;
>  	xa_unlock(&cache->reqs);
>  
>  	id = xas.xa_index;
> -- 
> 2.25.1
> 
> --
> Linux-cachefs mailing list
> Linux-cachefs@xxxxxxxxxx
> https://listman.redhat.com/mailman/listinfo/linux-cachefs



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