linux-kernel - Re: [PATCH rdma-next v1 10/15] RDMA/cm: Use an attribute_group on the ib_port

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Date:   Mon, 7 Jun 2021 14:39:45 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Adit Ranadive <aditr@...are.com>,
        Ariel Elior <aelior@...vell.com>,
        Christian Benvenuti <benve@...co.com>,
        clang-built-linux@...glegroups.com,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Devesh Sharma <devesh.sharma@...adcom.com>,
        Gal Pressman <galpress@...zon.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        Michal Kalderon <mkalderon@...vell.com>,
        Mike Marciniszyn <mike.marciniszyn@...nelisnetworks.com>,
        Mustafa Ismail <mustafa.ismail@...el.com>,
        Naresh Kumar PBS <nareshkumar.pbs@...adcom.com>,
        Nelson Escobar <neescoba@...co.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Potnuri Bharat Teja <bharat@...lsio.com>,
        Selvin Xavier <selvin.xavier@...adcom.com>,
        Shiraz Saleem <shiraz.saleem@...el.com>,
        VMware PV-Drivers <pv-drivers@...are.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Zhu Yanjun <zyjzyj2000@...il.com>
Subject: Re: [PATCH rdma-next v1 10/15] RDMA/cm: Use an attribute_group on
 the ib_port_attribute intead of kobj's

On Mon, Jun 07, 2021 at 09:14:11AM -0300, Jason Gunthorpe 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@...dia.com>
> > > 
> > > 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?  
> 
> The write side of the counter is being updated from concurrent kernel
> threads without locking, so this is an atomic because the write side
> needs atomic_add().

So the atomic write forces a lock :(

> Making them a naked u64 will cause significant corruption on the write
> side, and packet counters that are not accurate after quiescence are
> not very useful things.

How "accurate" do these have to be?

And have you all tried them?

I'm pushing back here as I see a lot of atomics used for debugging
statistics for no good reason all over the place.  Especially when
userspace just does not care.

thanks,

greg k-h