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

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Date:   Thu, 4 Apr 2019 15:22:02 +0200
From:   Pavel Machek <pavel@....cz>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        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 2019-03-28 16:12:28, Steven Rostedt wrote:
> 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.

Ok, I guess 32 bytes is small enough, I somehow imagined it would be bigger...

-- 
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