linux-kernel - Re: [PATCH AUTOSEL 5.0 011/262] tracing: kdb: Fix ftdump to not sleep

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Message-ID: <20190328161228.24f5aabd@gandalf.local.home>
Date:   Thu, 28 Mar 2019 16:12:28 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Pavel Machek <pavel@....cz>, Sasha Levin <sashal@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.0 011/262] tracing: kdb: Fix ftdump to not
 sleep

On Thu, 28 Mar 2019 12:45:18 -0700
Doug Anderson <dianders@...omium.org> wrote:

> > I see solution is simple, but now we have a loop with GFP_ATOMIC
> > allocations inside. How many "tracing spus" is this expected to loop
> > over? Will not it exhaust atomically available pages and reliably fail
> > in common configurations?
> >                                                                 Pavel  
> 
> Each one of these allocations is ~32 bytes and you do one per CPU.
> Even with systems with a lot of CPUs that's not going to be tons.
> ...and you only do it with GFP_ATOMIC when you're actively dropped
> into kdb and debugging.  It seems like going for simplicity is the
> right call here, but of course if Steven or Daniel say that it has to
> be done a different way then they're the true authorities.

I really don't care. The code in question is only affected when we have
CONFIG_KGDB_KDB enabled. But as it gets called from an atomic context,
is it any different than what it was doing before? Except now with
GFP_ATOMIC it is actually safer.

Now, we could add some helper functions in the ring-buffer code to
allow us to pre-allocate the ring_buffer_iter at boot up. Then we could
pass in the per-allocated iters and use them here.

-- Steve