linux-kernel - Re: [RFC patch 14/19] bpf: Use migrate

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Date:   Wed, 19 Feb 2020 10:17:28 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>, bpf <bpf@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Clark Williams <williams@...hat.com>,
        rostedt <rostedt@...dmis.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code

----- On Feb 18, 2020, at 6:36 PM, Alexei Starovoitov alexei.starovoitov@...il.com wrote:

[...]

> If I can use migrate_disable() without RT it will help my work on sleepable
> BPF programs. I would only have to worry about rcu_read_lock() since
> preempt_disable() is nicely addressed.

Hi Alexei,

You may want to consider using SRCU rather than RCU if you need to sleep while
holding a RCU read-side lock.

This is the synchronization approach I consider for adding the ability to take page
faults when doing syscall tracing.

Then you'll be able to replace preempt_disable() by combining SRCU and
migrate_disable():

AFAIU eBPF currently uses preempt_disable() for two reasons:

- Ensure the thread is not migrated,
  -> can be replaced by migrate_disable() in RT
- Provide RCU existence guarantee through sched-RCU
  -> can be replaced by SRCU, which allows sleeping and taking page faults.

I wonder if it would be acceptable to take a page fault while migration is
disabled though ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com