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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	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: Tue, 18 Feb 2020 17:23:25 -0800
Message-ID: <20200219012324.fibreerdmd2zdzr3@ast-mbp> (raw)
In-Reply-To: <87blpvqu2y.fsf@nanos.tec.linutronix.de>

On Wed, Feb 19, 2020 at 01:49:57AM +0100, Thomas Gleixner wrote:
> Alexei,
> 
> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
> > Overall looks great.
> > Thank you for taking time to write commit logs and detailed cover letter.
> > I think s/__this_cpu_inc/this_cpu_inc/ is the only bit that needs to be
> > addressed for it to be merged.
> > There were few other suggestions from Mathieu and Jakub.
> > Could you address them and resend?
> 
> I have them fixed up already, but I was waiting for further
> comments. I'll send it out tomorrow morning as I'm dead tired by now.
> 
> > I saw patch 1 landing in tip tree, but it needs to be in bpf-next as well
> > along with the rest of the series. Does it really need to be in the tip?
> > I would prefer to take the whole thing and avoid conflicts around
> > migrate_disable() especially if nothing in tip is going to use it in this
> > development cycle. So just drop patch 1 from the tip?
> 
> I'll add patch 2 to a tip branch as well and I'll give you a tag to pull
> into BPF (which has only those two commits). 

That works too.

> > Regarding
> > union {
> >    raw_spinlock_t  raw_lock;
> >    spinlock_t      lock;
> > };
> > yeah. it's not pretty, but I also don't have better ideas.
> 
> Yeah. I really tried hard to avoid it, but the alternative solution was
> code duplication which was even more horrible.
> 
> > Regarding migrate_disable()... can you enable it without the rest of RT?
> > I haven't seen its implementation. I suspect it's scheduler only change?
> > 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.
> 
> You have to talk to Peter Zijlstra about this as this is really
> scheduler relevant stuff. FYI, he undamentaly hates migrate_disable()
> from a schedulabilty POV, but as with the above lock construct the
> amount of better solutions is also close to zero.

I would imagine that migrate_disable is like temporary cpu pinning. The
scheduler has to have all the logic to make scheduling decisions quickly in
presence of pinned tasks and additional migrate_disable shouldn't introduce
slowdowns or complexity to critical path. Anyway, we'll discuss further
when migrate_disable patches hit mailing lists.

  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
2020-02-18 23:36       ` Alexei Starovoitov
2020-02-19  0:49         ` Thomas Gleixner
2020-02-19  1:23           ` Alexei Starovoitov [this message]
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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