[net-next] net/smc: increase SMC_WR_BUF_CNT

Commit Message

Wenjia Zhang Oct. 25, 2024, 7:46 a.m. UTC

From: Halil Pasic <[e-mail]>

The current value of SMC_WR_BUF_CNT is 16 which leads to heavy
contention on the wr_tx_wait workqueue of the SMC-R linkgroup and its
spinlock when many connections are  competing for the buffer. Currently
up to 256 connections per linkgroup are supported.

To make things worse when finally a buffer becomes available and
smc_wr_tx_put_slot() signals the linkgroup's wr_tx_wait wq, because
WQ_FLAG_EXCLUSIVE is not used all the waiters get woken up, most of the
time a single one can proceed, and the rest is contending on the
spinlock of the wq to go to sleep again.

For some reason include/linux/wait.h does not offer a top level wrapper
macro for wait_event with interruptible, exclusive and timeout. I did
not spend too many cycles on thinking if that is even a combination that
makes sense (on the quick I don't see why not) and conversely I
refrained from making an attempt to accomplish the interruptible,
exclusive and timeout combo by using the abstraction-wise lower
level __wait_event interface.

To alleviate the tx performance bottleneck and the CPU overhead due to
the spinlock contention, let us increase SMC_WR_BUF_CNT to 256.

Signed-off-by: Halil Pasic <[e-mail]>
Reported-by: Nils Hoppmann <[e-mail]>
Reviewed-by: Wenjia Zhang <[e-mail]>
Signed-off-by: Wenjia Zhang <[e-mail]>
---
 net/smc/smc_wr.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Simon Horman Oct. 25, 2024, 1:56 p.m. UTC | #1

On Fri, Oct 25, 2024 at 09:46:19AM +0200, Wenjia Zhang wrote:
> From: Halil Pasic <[e-mail]>
> 
> The current value of SMC_WR_BUF_CNT is 16 which leads to heavy
> contention on the wr_tx_wait workqueue of the SMC-R linkgroup and its
> spinlock when many connections are  competing for the buffer. Currently
> up to 256 connections per linkgroup are supported.
> 
> To make things worse when finally a buffer becomes available and
> smc_wr_tx_put_slot() signals the linkgroup's wr_tx_wait wq, because
> WQ_FLAG_EXCLUSIVE is not used all the waiters get woken up, most of the
> time a single one can proceed, and the rest is contending on the
> spinlock of the wq to go to sleep again.
> 
> For some reason include/linux/wait.h does not offer a top level wrapper
> macro for wait_event with interruptible, exclusive and timeout. I did
> not spend too many cycles on thinking if that is even a combination that
> makes sense (on the quick I don't see why not) and conversely I
> refrained from making an attempt to accomplish the interruptible,
> exclusive and timeout combo by using the abstraction-wise lower
> level __wait_event interface.
> 
> To alleviate the tx performance bottleneck and the CPU overhead due to
> the spinlock contention, let us increase SMC_WR_BUF_CNT to 256.
> 
> Signed-off-by: Halil Pasic <[e-mail]>
> Reported-by: Nils Hoppmann <[e-mail]>
> Reviewed-by: Wenjia Zhang <[e-mail]>
> Signed-off-by: Wenjia Zhang <[e-mail]>

Reviewed-by: Simon Horman <[e-mail]>

Dust Li Oct. 25, 2024, 11:58 p.m. UTC | #2

On 2024-10-25 09:46:19, Wenjia Zhang wrote:
>From: Halil Pasic <[e-mail]>
>
>The current value of SMC_WR_BUF_CNT is 16 which leads to heavy
>contention on the wr_tx_wait workqueue of the SMC-R linkgroup and its
>spinlock when many connections are  competing for the buffer. Currently
>up to 256 connections per linkgroup are supported.
>
>To make things worse when finally a buffer becomes available and
>smc_wr_tx_put_slot() signals the linkgroup's wr_tx_wait wq, because
>WQ_FLAG_EXCLUSIVE is not used all the waiters get woken up, most of the
>time a single one can proceed, and the rest is contending on the
>spinlock of the wq to go to sleep again.
>
>For some reason include/linux/wait.h does not offer a top level wrapper
>macro for wait_event with interruptible, exclusive and timeout. I did
>not spend too many cycles on thinking if that is even a combination that
>makes sense (on the quick I don't see why not) and conversely I
>refrained from making an attempt to accomplish the interruptible,
>exclusive and timeout combo by using the abstraction-wise lower
>level __wait_event interface.
>
>To alleviate the tx performance bottleneck and the CPU overhead due to
>the spinlock contention, let us increase SMC_WR_BUF_CNT to 256.

Hi,

Have you tested other values, such as 64? In our internal version, we
have used 64 for some time.

Increasing this to 256 will require a 36K continuous physical memory
allocation in smc_wr_alloc_link_mem(). Based on my experience, this may
fail on servers that have been running for a long time and have
fragmented memory.

    link->wr_rx_bufs = kcalloc(SMC_WR_BUF_CNT * 3, SMC_WR_BUF_SIZE,
                               GFP_KERNEL);

As we can see, the link->wr_rx_bufs will increase from 16*3*48 = 2,304
to 256*3*48=36,864 (1 page to 9 pages).

Best regards,
Dust

>
>Signed-off-by: Halil Pasic <[e-mail]>
>Reported-by: Nils Hoppmann <[e-mail]>
>Reviewed-by: Wenjia Zhang <[e-mail]>
>Signed-off-by: Wenjia Zhang <[e-mail]>
>---
> net/smc/smc_wr.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/net/smc/smc_wr.h b/net/smc/smc_wr.h
>index f3008dda222a..81e772e241f3 100644
>--- a/net/smc/smc_wr.h
>+++ b/net/smc/smc_wr.h
>@@ -19,7 +19,7 @@
> #include "smc.h"
> #include "smc_core.h"
> 
>-#define SMC_WR_BUF_CNT 16	/* # of ctrl buffers per link */
>+#define SMC_WR_BUF_CNT 256	/* # of ctrl buffers per link */
> 
> #define SMC_WR_TX_WAIT_FREE_SLOT_TIME	(10 * HZ)
> 
>-- 
>2.43.0
>

Patch

diff --git a/net/smc/smc_wr.h b/net/smc/smc_wr.h
index f3008dda222a..81e772e241f3 100644
--- a/net/smc/smc_wr.h
+++ b/net/smc/smc_wr.h
@@ -19,7 +19,7 @@ 
 #include "smc.h"
 #include "smc_core.h"
 
-#define SMC_WR_BUF_CNT 16	/* # of ctrl buffers per link */
+#define SMC_WR_BUF_CNT 256	/* # of ctrl buffers per link */
 
 #define SMC_WR_TX_WAIT_FREE_SLOT_TIME	(10 * HZ)