Re: [PATCH rdma-next v1 10/15] RDMA/cm: Use an attribute_group on the ib_port_attribute intead of kobj's

[Greg KH]

On Mon, Jun 07, 2021 at 02:37:16PM +0300, Leon Romanovsky wrote:
> On Mon, Jun 07, 2021 at 01:22:26PM +0200, Greg KH wrote:
> > On Mon, Jun 07, 2021 at 01:51:36PM +0300, Leon Romanovsky wrote:
> > > On Mon, Jun 07, 2021 at 12:25:03PM +0200, Greg KH wrote:
> > > > On Mon, Jun 07, 2021 at 11:17:35AM +0300, Leon Romanovsky wrote:
> > > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > > > 
> > > > > This code is trying to attach a list of counters grouped into 4 groups to
> > > > > the ib_port sysfs. Instead of creating a bunch of kobjects simply express
> > > > > everything naturally as an ib_port_attribute and add a single
> > > > > attribute_groups list.
> > > > > 
> > > > > Remove all the naked kobject manipulations.
> > > > 
> > > > Much nicer.
> > > > 
> > > > But why do you need your counters to be atomic in the first place?  What
> > > > are they counting that requires this?  Given that they are just a
> > > > statistic for userspace, making them be a u64 should work just the same,
> > > > right?
> > > 
> > > The statistic counters are per-port, while the cm.c flows run in
> > > asynchronically in parallel for every CM connection.
> > > 
> > > We need atomic variable to ensure that "write to u64" is not
> > > interrupted.
> > 
> > On what system is "write to u64" interruptable? 
> 
> On 32 bits, and yes, we have a customer who still uses such system.

So you will see what, a "tear"?  Or a stale value?

> > As these are per-port, do multiple threads try to increment these at
> > the same time?  
> 
> Yes, CM connection can be seen as thread. Bottom line everything in parallel.
> 
> > And even if they do, what happens if one is 'dropped' somehow because of this?
> 
> Probably nothing, we increment the statistics only.

So you are hitting cache lines for no good reason, probably not a good
idea, you are wasting cpu cycles for nothing :(

thanks,

greg k-h