linux-kernel - Re: BUG: ib_mad ftrace event unsupported migration

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Message-ID: <Y2JqX3vC1mG/JDex@ziepe.ca>
Date:   Wed, 2 Nov 2022 10:02:23 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Leonid Ravich <leonid.ravich@...anetworks.com>,
        "linux-trace-kernel@...r.kernel.org" 
        <IMCEAMAILTO-linux-trace-kernel+40vger+2Ekernel+2Eorg@...prd02.prod.outlook.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yigal Korman <yigal.korman@...anetworks.com>
Subject: Re: BUG:  ib_mad ftrace event unsupported migration

On Wed, Nov 02, 2022 at 07:44:57AM -0400, Steven Rostedt wrote:
> > before starting throwing some patch into the the air  I would like to align with you the approach we should take here. 
> > 
> > my suggestion here : 
> > - ftrace infra should verify no migration happen  (end and start happens on same CPU)  in case not we will  throw warning for the issue  .
> 
> The scheduler should have. On entering the ring buffer code
> ring_buffer_lock_reserver() it disables preemption and does not
> re-enable it until ring_buffer_unlock_commit().
> 
> The only way to migrate is if you re-enable preemption. WHICH IS A
> BUG!

So what on earth did that?

I'm guessing some driver's query_pkey op, but AFAIK we don't have any
explicit pre-emption reenablements in the code - unless it is sneaky..

Leonid what driver are you testing?

Jason