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

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Date:   Wed, 2 Nov 2022 11:04:44 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Leonid Ravich <leonid.ravich@...anetworks.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "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>,
        "linux-trace-kernel@...r.kernel.org" 
        <linux-trace-kernel@...r.kernel.org>,
        Leon Ravich <lravich@...il.com>
Subject: Re: BUG:  ib_mad ftrace event unsupported migration

On Wed, Nov 02, 2022 at 02:02:49PM +0000, Leonid Ravich 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..
> trace infra uses preempt_disable_notrace/preempt_enable_notrace  to disable/enable preemtion but  my kernel compiled without CONFIG_PREEMPTION so this functions are only barriers - looks like the idea behind was to avoid involuntary preemtion  but in our case it is a voluntary (there is a wait_for_completion in the query_pkey  rabbit hole).

So this tracepoint is just wrong, you can't call a sleepable function
from a tracepoint like that?

Presumably lockdep would/should warn about this?

Delete the pkey logging from the tracepoint, it can't work, I guess.

Jason