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

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Message-ID: <YL4MOyxQi1O3dog5@kroah.com>
Date:   Mon, 7 Jun 2021 14:08:27 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...dia.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 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@...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?  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