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From: Thomas Gleixner <tglx@linutronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	David Miller <davem@davemloft.net>,
	bpf@vger.kernel.org, netdev@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Sebastian Sewior <bigeasy@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Clark Williams <williams@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code
Date: Fri, 14 Feb 2020 20:56:12 +0100
Message-ID: <87imk9t02r.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200214191126.lbiusetaxecdl3of@localhost>

Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:
> On 14-Feb-2020 02:39:31 PM, Thomas Gleixner wrote:
>> Replace the preempt_disable/enable() pairs with migrate_disable/enable()
>> pairs to prepare BPF to work on PREEMPT_RT enabled kernels. On a non-RT
>> kernel this maps to preempt_disable/enable(), i.e. no functional change.

...

> Having all those events randomly and silently discarded might be quite
> unexpected from a user standpoint. This turns the deadlock prevention
> mechanism into a random tracepoint-dropping facility, which is
> unsettling.

Well, it randomly drops events which might be unrelated to the syscall
target today already, this will just drop some more. Shrug.

> One alternative approach we could consider to solve this is to make
> this deadlock prevention nesting counter per-thread rather than
> per-cpu.

That should work both on !RT and RT.

> Also, I don't think using __this_cpu_inc() without preempt-disable or
> irq off is safe. You'll probably want to move to this_cpu_inc/dec
> instead, which can be heavier on some architectures.

Good catch.

Thanks,

        tglx

      

  reply index

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-14 13:39 [RFC patch 00/19] bpf: Make BPF and PREEMPT_RT co-exist Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 01/19] sched: Provide migrate_disable/enable() inlines Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 02/19] sched: Provide cant_migrate() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 03/19] bpf: Update locking comment in hashtab code Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 04/19] bpf/tracing: Remove redundant preempt_disable() in __bpf_trace_run() Thomas Gleixner
2020-02-19 16:54   ` Steven Rostedt
2020-02-19 17:26     ` Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 05/19] perf/bpf: Remove preempt disable around BPF invocation Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 06/19] bpf: Dont iterate over possible CPUs with interrupts disabled Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 07/19] bpf: Provide BPF_PROG_RUN_PIN_ON_CPU() macro Thomas Gleixner
2020-02-14 18:50   ` Mathieu Desnoyers
2020-02-14 19:36     ` Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 08/19] bpf: Replace cant_sleep() with cant_migrate() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 09/19] bpf: Use BPF_PROG_RUN_PIN_ON_CPU() at simple call sites Thomas Gleixner
2020-02-19  1:39   ` Vinicius Costa Gomes
2020-02-19  9:00     ` Thomas Gleixner
2020-02-19 16:38       ` Alexei Starovoitov
2020-02-21  0:20       ` Kees Cook
2020-02-21 14:00         ` Thomas Gleixner
2020-02-21 14:05           ` Peter Zijlstra
2020-02-21 22:15           ` Kees Cook
2020-02-14 13:39 ` [RFC patch 10/19] trace/bpf: Use migrate disable in trace_call_bpf() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 11/19] bpf/tests: Use migrate disable instead of preempt disable Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 12/19] bpf: Use migrate_disable/enabe() in trampoline code Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 13/19] bpf: Use migrate_disable/enable in array macros and cgroup/lirc code Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code Thomas Gleixner
2020-02-14 19:11   ` Mathieu Desnoyers
2020-02-14 19:56     ` Thomas Gleixner [this message]
2020-02-18 23:36       ` Alexei Starovoitov
2020-02-19  0:49         ` Thomas Gleixner
2020-02-19  1:23           ` Alexei Starovoitov
2020-02-19 15:17         ` Mathieu Desnoyers
2020-02-20  4:19           ` Alexei Starovoitov
2020-02-14 13:39 ` [RFC patch 15/19] bpf: Use migrate_disable() in sys_bpf() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 16/19] bpf: Factor out hashtab bucket lock operations Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 17/19] bpf: Prepare hashtab locking for PREEMPT_RT Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 18/19] bpf, lpm: Make locking RT friendly Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 19/19] bpf/stackmap: Dont trylock mmap_sem with PREEMPT_RT and interrupts disabled Thomas Gleixner
2020-02-14 17:53 ` [RFC patch 00/19] bpf: Make BPF and PREEMPT_RT co-exist David Miller
2020-02-14 18:36   ` Thomas Gleixner
2020-02-17 12:59     ` [PATCH] bpf: Enforce map preallocation for all instrumentation programs Thomas Gleixner
2020-02-15 20:09 ` [RFC patch 00/19] bpf: Make BPF and PREEMPT_RT co-exist Jakub Kicinski

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