linux-kernel - Re: [PATCH 3/3] bpf: Implement prealloc for task_local

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Message-ID: <YXNCfQvOLlT8yXkH@slm.duckdns.org>
Date:   Fri, 22 Oct 2021 13:00:13 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Martin KaFai Lau <kafai@...com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, bpf@...r.kernel.org,
        kernel-team@...com, linux-kernel@...r.kernel.org,
        KP Singh <kpsingh@...nel.org>
Subject: Re: [PATCH 3/3] bpf: Implement prealloc for task_local_storage

Hello,

On Fri, Oct 22, 2021 at 03:47:33PM -0700, Martin KaFai Lau wrote:
...
> > +	for_each_process_thread(g, p) {
> I am thinking if this loop can be done in bpf iter.
> 
> If the bpf_local_storage_map is sleepable safe (not yet done but there is
> an earlier attempt [0]),  bpf_local_storage_update() should be able to
> alloc without GFP_ATOMIC by sleepable bpf prog and this potentially
> will be useful in general for other sleepable use cases.
> 
> For example, if a sleepable bpf iter prog can run in this loop (or the existing
> bpf task iter loop is as good?), the iter bpf prog can call
> bpf_task_storage_get(BPF_SK_STORAGE_GET_F_CREATE) on a sleepable
> bpf_local_storage_map.

Yeah, whatever that can walk all the existing tasks should do, and I think
the locked section can be shrunk too.

        percpu_down_write(&threadgroup_rwsem);
        list_add_tail(&smap->prealloc_node, &prealloc_smaps);
        percpu_up_write(&threadgroup_rwsem);

        // Here, it's guaranteed that all new tasks are guaranteed to
        // prealloc on fork.

        Iterate all tasks in whatever way and allocate if necessary;

Thanks.

-- 
tejun