Re: [PATCH 3/3] scsi: align scsi_device iodone_cnt 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 iodone_cnt on its own cache line so it does not share a cache line
> with iorequest_cnt, avoiding significant performance hits from false
> sharing when request and completion paths update these counters on some
> CPU architectures.
> 
> Signed-off-by: James Rizzo <james.rizzo@broadcom.com>
> Signed-off-by: Sumit Saxena <sumit.saxena@broadcom.com>
> ---
>   include/scsi/scsi_device.h | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
> index 9c2a7bbe5891..86c2a3a6b206 100644
> --- a/include/scsi/scsi_device.h
> +++ b/include/scsi/scsi_device.h
> @@ -272,7 +272,9 @@ struct scsi_device {
>   #define SCSI_DEFAULT_DEVICE_BLOCKED	3
>   
>   	atomic_t iorequest_cnt;
> -	atomic_t iodone_cnt;
> +	/* ensure iorequest_cnt and iodone_cnt are on different cache lines to avoid significant
> +	   performance hits on cache line contention on some CPU architectures */
> +	atomic_t iodone_cnt ____cacheline_aligned_in_smp;
>   	atomic_t ioerr_cnt;
>   	atomic_t iotmo_cnt;

Has it been considered to change both iorequest_cnt and iodone_cnt into
per-cpu counters?

Thanks,

Bart.