linux-kernel - Re: [BUG] rcu-tasks : should take care of sparse cpu masks

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Date:   Tue, 12 Apr 2022 10:13:15 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     KP Singh <kpsingh@...nel.org>
Cc:     Neeraj Upadhyay <quic_neeraju@...cinc.com>, paulmck@...nel.org,
        Eric Dumazet <edumazet@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stanislav Fomichev <sdf@...gle.com>, andrii@...nel.org,
        ast@...nel.org, bpf <bpf@...r.kernel.org>
Subject: Re: [BUG] rcu-tasks : should take care of sparse cpu masks

On Tue, Apr 12, 2022 at 08:28:07AM +0200, KP Singh wrote:
> On Wed, Apr 6, 2022 at 5:44 PM KP Singh <kpsingh@...nel.org> wrote:
> >
> > On Tue, Apr 5, 2022 at 10:38 PM Martin KaFai Lau <kafai@...com> wrote:
> > >
> > > On Tue, Apr 05, 2022 at 02:04:34AM +0200, KP Singh wrote:
> > > > > >>> Either way, how frequently is call_rcu_tasks_trace() being invoked in
> > > > > >>> your setup?  If it is being invoked frequently, increasing delays would
> > > > > >>> allow multiple call_rcu_tasks_trace() instances to be served by a single
> > > > > >>> tasklist scan.
> > > > > >>>
> > > > > >>>> Given that, I do not think bpf_sk_storage_free() can/should use
> > > > > >>>> call_rcu_tasks_trace(),
> > > > > >>>> we probably will have to fix this soon (or revert from our kernels)
> > > > > >>>
> > > > > >>> Well, you are in luck!!!  This commit added call_rcu_tasks_trace() to
> > > > > >>> bpf_selem_unlink_storage_nolock(), which is invoked in a loop by
> > > > > >>> bpf_sk_storage_free():
> > > > > >>>
> > > > > >>> 0fe4b381a59e ("bpf: Allow bpf_local_storage to be used by sleepable programs")
> > > > > >>>
> > > > > >>> This commit was authored by KP Singh, who I am adding on CC.  Or I would
> > > > > >>> have, except that you beat me to it.  Good show!!!  ;-)
> > > >
> > > > Hello :)
> > > >
> > > > Martin, if this ends up being an issue we might have to go with the
> > > > initial proposed approach
> > > > of marking local storage maps explicitly as sleepable so that not all
> > > > maps are forced to be
> > > > synchronized via trace RCU.
> > > >
> > > > We can make the verifier reject loading programs that try to use
> > > > non-sleepable local storage
> > > > maps in sleepable programs.
> > > >
> > > > Do you think this is a feasible approach we can take or do you have
> > > > other suggestions?
> > > bpf_sk_storage_free() does not need to use call_rcu_tasks_trace().
> > > The same should go for the bpf_{task,inode}_storage_free().
> > > The sk at this point is being destroyed.  No bpf prog (sleepable or not)
> > > can have a hold on this sk.  The only storage reader left is from
> > > bpf_local_storage_map_free() which is under rcu_read_lock(),
> > > so a 'kfree_rcu(selem, rcu)' is enough.
> > > A few lines below in bpf_sk_storage_free(), 'kfree_rcu(sk_storage, rcu)'
> > > is currently used instead of call_rcu_tasks_trace() for the same reason.
> > >
> > > KP, if the above makes sense, can you make a patch for it?
> > > The bpf_local_storage_map_free() code path also does not need
> > > call_rcu_tasks_trace(), so may as well change it together.
> > > The bpf_*_storage_delete() helper and the map_{delete,update}_elem()
> > > syscall still require the call_rcu_tasks_trace().
> >
> > Thanks, I will send a patch.
> 
> Martin, does this work? (I can send it as a patch later today)
LGTM. Thanks.