linux-kernel - Re: scheduling while atomic in z3fold

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Message-ID: <20201129112922.db53kmtpu76xxukj@spock.localdomain>
Date:   Sun, 29 Nov 2020 12:29:22 +0100
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Mike Galbraith <efault@....de>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-rt-users@...r.kernel.org
Subject: Re: scheduling while atomic in z3fold

On Sun, Nov 29, 2020 at 11:56:55AM +0100, Mike Galbraith wrote:
> On Sun, 2020-11-29 at 10:21 +0100, Mike Galbraith wrote:
> > On Sun, 2020-11-29 at 08:48 +0100, Mike Galbraith wrote:
> > > On Sun, 2020-11-29 at 07:41 +0100, Mike Galbraith wrote:
> > > > On Sat, 2020-11-28 at 15:27 +0100, Oleksandr Natalenko wrote:
> > > > >
> > > > > > > Shouldn't the list manipulation be protected with
> > > > > > > local_lock+this_cpu_ptr instead of get_cpu_ptr+spin_lock?
> > > > >
> > > > > Totally untested:
> > > >
> > > > Hrm, the thing doesn't seem to care deeply about preemption being
> > > > disabled, so adding another lock may be overkill.  It looks like you
> > > > could get the job done via migrate_disable()+this_cpu_ptr().
> > >
> > > There is however an ever so tiny chance that I'm wrong about that :)
> >
> > Or not, your local_lock+this_cpu_ptr version exploded too.
> >
> > Perhaps there's a bit of non-rt related racy racy going on in zswap
> > thingy that makes swap an even less wonderful idea for RT than usual.
> 
> Raciness seems to be restricted to pool compressor.  "zbud" seems to be
> solid, virgin "zsmalloc" explodes, as does "z3fold" regardless which of
> us puts his grubby fingerprints on it.
> 
> Exploding compressors survived zero runs of runltp -f mm, I declared
> zbud to be at least kinda sorta stable after box survived five runs.

Ummm so do compressors explode under non-rt kernel in your tests as
well, or it is just -rt that triggers this?

I've never seen that on both -rt and non-rt, thus asking.

-- 
  Oleksandr Natalenko (post-factum)