Re: [PATCH] mlx4_ib: Increase the timeout for CM cache

[Jason Gunthorpe]

On Thu, Jan 31, 2019 at 06:09:51PM +0100, HÃkon Bugge wrote:
> Using CX-3 virtual functions, either from a bare-metal machine or
> pass-through from a VM, MAD packets are proxied through the PF driver.
> 
> Since the VMs have separate name spaces for MAD Transaction Ids
> (TIDs), the PF driver has to re-map the TIDs and keep the book keeping
> in a cache.
> 
> Following the RDMA CM protocol, it is clear when an entry has to
> evicted form the cache. But life is not perfect, remote peers may die
> or be rebooted. Hence, it's a timeout to wipe out a cache entry, when
> the PF driver assumes the remote peer has gone.
> 
> We have experienced excessive amount of DREQ retries during fail-over
> testing, when running with eight VMs per database server.
> 
> The problem has been reproduced in a bare-metal system using one VM
> per physical node. In this environment, running 256 processes in each
> VM, each process uses RDMA CM to create an RC QP between himself and
> all (256) remote processes. All in all 16K QPs.
> 
> When tearing down these 16K QPs, excessive DREQ retries (and
> duplicates) are observed. With some cat/paste/awk wizardry on the
> infiniband_cm sysfs, we observe:
> 
>       dreq:       5007
> cm_rx_msgs:
>       drep:       3838
>       dreq:      13018
>        rep:       8128
>        req:       8256
>        rtu:       8256
> cm_tx_msgs:
>       drep:       8011
>       dreq:      68856
>        rep:       8256
>        req:       8128
>        rtu:       8128
> cm_tx_retries:
>       dreq:      60483
> 
> Note that the active/passive side is distributed.
> 
> Enabling pr_debug in cm.c gives tons of:
> 
> [171778.814239] <mlx4_ib> mlx4_ib_multiplex_cm_handler: id{slave:
> 1,sl_cm_id: 0xd393089f} is NULL!
> 
> By increasing the CM_CLEANUP_CACHE_TIMEOUT from 5 to 30 seconds, the
> tear-down phase of the application is reduced from 113 to 67
> seconds. Retries/duplicates are also significantly reduced:
> 
> cm_rx_duplicates:
>       dreq:       7726
> []
> cm_tx_retries:
>       drep:          1
>       dreq:       7779
> 
> Increasing the timeout further didn't help, as these duplicates and
> retries stem from a too short CMA timeout, which was 20 (~4 seconds)
> on the systems. By increasing the CMA timeout to 22 (~17 seconds), the
> numbers fell down to about one hundred for both of them.
> 
> Adjustment of the CMA timeout is _not_ part of this commit.
> 
> Signed-off-by: HÃkon Bugge <haakon.bugge@xxxxxxxxxx>
> ---
>  drivers/infiniband/hw/mlx4/cm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Jack? What do you think?

> diff --git a/drivers/infiniband/hw/mlx4/cm.c b/drivers/infiniband/hw/mlx4/cm.c
> index fedaf8260105..8c79a480f2b7 100644
> --- a/drivers/infiniband/hw/mlx4/cm.c
> +++ b/drivers/infiniband/hw/mlx4/cm.c
> @@ -39,7 +39,7 @@
>  
>  #include "mlx4_ib.h"
>  
> -#define CM_CLEANUP_CACHE_TIMEOUT  (5 * HZ)
> +#define CM_CLEANUP_CACHE_TIMEOUT  (30 * HZ)
>  
>  struct id_map_entry {
>  	struct rb_node node;
> -- 
> 2.20.1
>