Re: [PATCH] RDMA/iw_cxgb4: disable delayed ack by default

[Potnuri Bharat Teja <bharat@chelsio.com>]

On Tuesday, September 09/15/20, 2020 at 12:17:06 +0530, Leon Romanovsky wrote:
> On Mon, Sep 14, 2020 at 10:31:17PM +0530, Potnuri Bharat Teja wrote:
> > On Sunday, September 09/13/20, 2020 at 12:43:25 +0530, Leon Romanovsky wrote:
> > > On Fri, Sep 11, 2020 at 09:39:57PM +0530, Potnuri Bharat Teja wrote:
> > > > On Friday, September 09/11/20, 2020 at 18:41:52 +0530, Leon Romanovsky wrote:
> > > > > On Thu, Sep 10, 2020 at 07:14:50PM +0000, Potnuri Bharat Teja wrote:
> > > > > > >> Subject: [PATCH] RDMA/iw_cxgb4: disable delayed ack by default
> > > > > > >>
> > > > > > >> Receive side delayed ack mode is needed only for certain area networks/ connections. Therefore disable it by default.
> > > > > > >>
> > > > > > >> Signed-off-by: Potnuri Bharat Teja <bharat@chelsio.com>
> > > > > > >> ---
> > > > > > >>  drivers/infiniband/hw/cxgb4/cm.c | 4 ++--
> > > > > > >>  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > >>
> > > > > > >> diff --git a/drivers/infiniband/hw/cxgb4/cm.c b/drivers/infiniband/hw/cxgb4/cm.c
> > > > > > >> index 1f288c73ccfc..8769e7aa097f 100644
> > > > > > >> --- a/drivers/infiniband/hw/cxgb4/cm.c
> > > > > > >> +++ b/drivers/infiniband/hw/cxgb4/cm.c
> > > > > > >> @@ -77,9 +77,9 @@ static int enable_ecn;  module_param(enable_ecn, int, 0644);  MODULE_PARM_DESC(enable_ecn, "Enable ECN (default=0/disabled)");
> > > > > > >>
> > > > > > >> -static int dack_mode = 1;
> > > > > > >> +static int dack_mode;
> > > > > > >>  module_param(dack_mode, int, 0644);
> > > > > > >> -MODULE_PARM_DESC(dack_mode, "Delayed ack mode (default=1)");
> > > > > > >> +MODULE_PARM_DESC(dack_mode, "Delayed ack mode (default=0)");
> > > > > > >
> > > > > > >Are you sure that this doesn't break user scripts?
> > > > > > Yes, I am sure. This does not interfere with user/kernel RDMA functionalities.
> > > > >
> > > > > How is it possible?
> > > > > Before this change user that did "modprobe iw_cxgb4" had delayed mode
> > > > > enabled, after this he will need to issue "modprobe iw_cxgb4 dack_mode=1"
> > > > >
> > > > Sorry I didnt get it right earlier. Yes now if user wants delayed ack mode,
> > > > user can issue "modprobe iw_cxgb4 dack_mode=1" or change it dynamically via
> > > > /sys/module/<>/paramters.
> > > > Chelsio adapters for better performance in most cases needs delayed ack mode to
> > > > be disabled. This change somehow got missed upstream all this while.
> > > > In fewer explicit cases, user can enable delayed ack mode where it is needed.
> > >
> > > So why doesn't rdma-core have dack_mode=0 in kernel-boot scripts?
> > >
> > Do you mean why dont I simply change rdma-hw-modules.rules instead?
> > I could do that if this is a user mode only setting. But this is needed for kernel
> > mode too where in some cases rdma-core may not be installed.
> 
> No, I'm asking why if "dack_mode=0" is better and preferable way to
> operate, it was never set in rdma-core.
> 
Found this only recently that it was wrongly enabled by default which affects 
severely the performance of some of our adapters with low HW resources for larger 
IO sizes. I could have mentioned this in commit description but I thought it's 
a trivial change code wise.
> I'm not saying that your kernel patch is wrong, just trying to get sense
> on implications for the rdma-core.
>
ok. yes rdma-hw-modules.rules provides a cleaner way. I am still used to setting module
params via /sys/module/<>/parameters/*.

Thanks for the review.