linux-kernel - Re: [PATCH v8 6/8] cgroup/cpuset: Change cpuset

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Message-ID: <20190701082731.GP3402@hirez.programming.kicks-ass.net>
Date:   Mon, 1 Jul 2019 10:27:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     mingo@...hat.com, rostedt@...dmis.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org, lizefan@...wei.com,
        cgroups@...r.kernel.org, Prateek Sood <prsood@...eaurora.org>
Subject: Re: [PATCH v8 6/8] cgroup/cpuset: Change cpuset_rwsem and hotplug
 lock order

On Mon, Jul 01, 2019 at 08:52:33AM +0200, Juri Lelli wrote:
> Hi,
> 
> On 28/06/19 15:03, Peter Zijlstra wrote:
> > On Fri, Jun 28, 2019 at 10:06:16AM +0200, Juri Lelli wrote:
> > > cpuset_rwsem is going to be acquired from sched_setscheduler() with a
> > > following patch. There are however paths (e.g., spawn_ksoftirqd) in
> > > which sched_scheduler() is eventually called while holding hotplug lock;
> > > this creates a dependecy between hotplug lock (to be always acquired
> > > first) and cpuset_rwsem (to be always acquired after hotplug lock).
> > > 
> > > Fix paths which currently take the two locks in the wrong order (after
> > > a following patch is applied).
> > > Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> > 
> > This all reminds me of this:
> > 
> >   https://lkml.kernel.org/r/1510755615-25906-1-git-send-email-prsood@codeaurora.org
> > 
> > Which sadly got reverted again. If we do this now (I've always been a
> > proponent), then we can make that rebuild synchronous again, which
> > should also help here IIRC.
> 
> Why was that reverted? Perf regression of some type?

IIRC TJ figured it wasn't strictly required to fix the lock invertion at
that time and they sorted it differently. If I (re)read the thread
correctly the other day, he didn't have fundamental objections against
it, but wanted the simpler fix.