Re: [PATCH 2/3] block: align nr_active_requests_shared_tags to avoid cache line contention

[Bart Van Assche <bvanassche@acm.org>]

On 4/2/26 12:46 AM, Sumit Saxena wrote:
> From: James Rizzo <james.rizzo@broadcom.com>
> 
> Place nr_active_requests_shared_tags on its own cache line so it does not
> share a cache line with nr_requests and other hot fields, avoiding
> significant performance hits from false sharing on some CPU architectures.
> 
> Signed-off-by: James Rizzo <james.rizzo@broadcom.com>
> Signed-off-by: Sumit Saxena <sumit.saxena@broadcom.com>
> ---
>   include/linux/blkdev.h | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index d463b9b5a0a5..7ed566c81c1b 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -561,7 +561,9 @@ struct request_queue {
>   	struct timer_list	timeout;
>   	struct work_struct	timeout_work;
>   
> -	atomic_t		nr_active_requests_shared_tags;
> +	/* ensure nr_active_requests_shared_tags and nr_requests are on different cache lines
> +	   to avoid significant performance hits on cache line contention on some CPU architectures */
> +	atomic_t		nr_active_requests_shared_tags ____cacheline_aligned_in_smp;
>   
>   	struct blk_mq_tags	*sched_shared_tags;
>   

A possible alternative is this patch that removes
nr_active_requests_shared_tags:

https://lore.kernel.org/linux-block/20240529213921.3166462-1-bvanassche@acm.org/

Thanks,

Bart.