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From:   Luigi Rizzo <lrizzo@google.com>
Date:   Tue, 18 Feb 2020 01:39:40 -0800
Message-ID: <CAMOZA0L14CjA97UHj7V1tPuOtesrUykYPj3_vgbF3JQWS3bcaw@mail.gmail.com>
Subject: Re: [PATCH v3] kretprobe: percpu support
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     linux-kernel@vger.kernel.org, naveen.n.rao@linux.ibm.com,
        anil.s.keshavamurthy@intel.com, David Miller <davem@davemloft.net>,
        gregkh@linuxfoundation.org, Ingo Molnar <mingo@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
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On Mon, Feb 17, 2020 at 11:55 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Hi Luigi,
>
> On Mon, 17 Feb 2020 16:56:59 -0800
> Luigi Rizzo <lrizzo@google.com> wrote:
>
> > kretprobe uses a list protected by a single lock to allocate a
> > kretprobe_instance in pre_handler_kretprobe(). This works poorly with
> > concurrent calls.
>
> Yes, there are several potential performance issue and the recycle
> instance is one of them. However, I think this spinlock is not so racy,
> but noisy (especially on many core machine) right?

correct, it is especially painful on 2+ sockets and many-core systems
when attaching kretprobes on otherwise uncontended paths.

>
> Racy lock is the kretprobe_hash_lock(), I would like to replace it
> with ftrace's per-task shadow stack. But that will be available
> only if CONFIG_FUNCTION_GRAPH_TRACER=y (and instance has no own
> payload).
>
> > This patch offers a simplified fix: the percpu_instance flag indicates
> > that we allocate one instance per CPU, and the allocation is contention
> > free, but we allow only have one pending entry per CPU (this could be
> > extended to a small constant number without much trouble).
>
> OK, the percpu instance idea is good to me, and I think it should be
> default option. Unless user specifies the number of instances, it should
> choose percpu instance by default.

That was my initial implementation, which would not even need the
percpu_instance
flag in struct kretprobe. However, I felt that changing the default
would have subtle
side effects (e.g., only one outstanding call per CPU) so I thought it
would be better
to leave the default unchanged and make the flag explicit.

> Moreover, this makes things a bit complicated, can you add per-cpu
> instance array? If it is there, we can remove the old recycle rp insn
> code.

Can you clarify what you mean by "per-cpu instance array" ?
Do you mean allowing multiple outstanding entries per cpu?

I will address your code comments in an updated patch.

thanks
luigi