Re: [PATCH net-next v2] net/smc: Use percpu ref for wr tx reference

[Wenjia Zhang <wenjia@linux.ibm.com>]

On 27.02.23 13:16, Kai wrote:
> The refcount wr_tx_refcnt may cause cache thrashing problems among
> cores and we can use percpu ref to mitigate this issue here. We
> gain some performance improvement with percpu ref here on our
> customized smc-r verion. Applying cache alignment may also mitigate
> this problem but it seem more reasonable to use percpu ref here.
> 
> redis-benchmark on smc-r with atomic wr_tx_refcnt:
> SET: 525817.62 requests per second, p50=0.087 msec
> GET: 570841.44 requests per second, p50=0.087 msec
> 
> redis-benchmark on the percpu_ref version:
> SET: 539956.81 requests per second, p50=0.087 msec
> GET: 587613.12 requests per second, p50=0.079 msec
> 
> Signed-off-by: Kai <KaiShen@linux.alibaba.com>
> ---
>   net/smc/smc_core.h |  5 ++++-
>   net/smc/smc_wr.c   | 18 ++++++++++++++++--
>   net/smc/smc_wr.h   |  5 ++---
>   3 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/net/smc/smc_core.h b/net/smc/smc_core.h
> index 08b457c2d294..0705e33e2d68 100644
> --- a/net/smc/smc_core.h
> +++ b/net/smc/smc_core.h
> @@ -106,7 +106,10 @@ struct smc_link {
>   	unsigned long		*wr_tx_mask;	/* bit mask of used indexes */
>   	u32			wr_tx_cnt;	/* number of WR send buffers */
>   	wait_queue_head_t	wr_tx_wait;	/* wait for free WR send buf */
> -	atomic_t		wr_tx_refcnt;	/* tx refs to link */
> +	struct {
> +		struct percpu_ref	wr_tx_refs;
> +	} ____cacheline_aligned_in_smp;
> +	struct completion	ref_comp;
>   
>   	struct smc_wr_buf	*wr_rx_bufs;	/* WR recv payload buffers */
>   	struct ib_recv_wr	*wr_rx_ibs;	/* WR recv meta data */
> diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c
> index b0678a417e09..dd923e76139f 100644
> --- a/net/smc/smc_wr.c
> +++ b/net/smc/smc_wr.c
> @@ -648,7 +648,8 @@ void smc_wr_free_link(struct smc_link *lnk)
>   
>   	smc_wr_tx_wait_no_pending_sends(lnk);
>   	wait_event(lnk->wr_reg_wait, (!atomic_read(&lnk->wr_reg_refcnt)));
> -	wait_event(lnk->wr_tx_wait, (!atomic_read(&lnk->wr_tx_refcnt)));
> +	percpu_ref_kill(&lnk->wr_tx_refs);
> +	wait_for_completion(&lnk->ref_comp);
>   
>   	if (lnk->wr_rx_dma_addr) {
>   		ib_dma_unmap_single(ibdev, lnk->wr_rx_dma_addr,
> @@ -847,6 +848,13 @@ void smc_wr_add_dev(struct smc_ib_device *smcibdev)
>   	tasklet_setup(&smcibdev->send_tasklet, smc_wr_tx_tasklet_fn);
>   }
>   
> +static void smcr_wr_tx_refs_free(struct percpu_ref *ref)
> +{
> +	struct smc_link *lnk = container_of(ref, struct smc_link, wr_tx_refs);
> +
> +	complete(&lnk->ref_comp);
> +}
> +
>   int smc_wr_create_link(struct smc_link *lnk)
>   {
>   	struct ib_device *ibdev = lnk->smcibdev->ibdev;
> @@ -890,7 +898,13 @@ int smc_wr_create_link(struct smc_link *lnk)
>   	smc_wr_init_sge(lnk);
>   	bitmap_zero(lnk->wr_tx_mask, SMC_WR_BUF_CNT);
>   	init_waitqueue_head(&lnk->wr_tx_wait);
> -	atomic_set(&lnk->wr_tx_refcnt, 0);
> +
> +	rc = percpu_ref_init(&lnk->wr_tx_refs, smcr_wr_tx_refs_free,
> +			     PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
> +	if (rc)
> +		goto dma_unmap;
> +	init_completion(&lnk->ref_comp);
> +
>   	init_waitqueue_head(&lnk->wr_reg_wait);
>   	atomic_set(&lnk->wr_reg_refcnt, 0);
>   	init_waitqueue_head(&lnk->wr_rx_empty_wait);
> diff --git a/net/smc/smc_wr.h b/net/smc/smc_wr.h
> index 45e9b894d3f8..f3008dda222a 100644
> --- a/net/smc/smc_wr.h
> +++ b/net/smc/smc_wr.h
> @@ -63,14 +63,13 @@ static inline bool smc_wr_tx_link_hold(struct smc_link *link)
>   {
>   	if (!smc_link_sendable(link))
>   		return false;
> -	atomic_inc(&link->wr_tx_refcnt);
> +	percpu_ref_get(&link->wr_tx_refs);
>   	return true;
>   }
>   
>   static inline void smc_wr_tx_link_put(struct smc_link *link)
>   {
> -	if (atomic_dec_and_test(&link->wr_tx_refcnt))
> -		wake_up_all(&link->wr_tx_wait);
> +	percpu_ref_put(&link->wr_tx_refs);
>   }
>   
>   static inline void smc_wr_drain_cq(struct smc_link *lnk)

@Tony, thank you for the sugguestion! The decription now looks much 
better to me.

@Kai, the performance improvement seems not so giant, but the method 
looks good, indeed. However, to keep the consistency of the code, I'm 
wondering why you only use the perf_ref for wr_tx_wait, but not for 
wr_reg_refcnt?