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

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Date:   Wed, 2 Nov 2022 14:02:49 +0000
From:   Leonid Ravich <leonid.ravich@...anetworks.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Steven Rostedt <rostedt@...dmis.org>
CC:     "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

> > > 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 no scheduler here to warn about illegal migration. 

>Leonid what driver are you testing?
mlx5 - (MLNX_OFED-5.3-1.0.0.1)