linux-kernel - Re: [PATCH v4] tracepoint: Do not fail unregistering a probe due to memory failure

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Message-ID: <119550773.160.1611771202224.JavaMail.zimbra@efficios.com>
Date:   Wed, 27 Jan 2021 13:13:22 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     paulmck <paulmck@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matt Mullins <mmullins@...x.us>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [PATCH v4] tracepoint: Do not fail unregistering a probe due to
 memory failure

----- On Jan 27, 2021, at 1:07 PM, rostedt rostedt@...dmis.org wrote:

> On Wed, 27 Jan 2021 13:00:46 -0500 (EST)
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> > Instead of allocating a new array for removing a tracepoint, allocate twice
>> > the needed size when adding tracepoints to the array. On removing, use the
>> > second half of the allocated array. This removes the need to allocate memory
>> > for removing a tracepoint, as the allocation for removals will already have
>> > been done.
>> 
>> I don't see how this can work reliably. AFAIU, with RCU, approaches
>> requiring a pre-allocation of twice the size and swapping to the alternate
>> memory area on removal falls apart whenever you remove 2 or more elements
>> back-to-back without waiting for a grace period.
> 
> Good point ;-)
> 
>> 
>> How is this handled by your scheme ?
> 
> I believe we can detect this case using the "prio" part of extra element,
> and force a rcu sync if there's back to back removals on the same
> tracepoint. That case does not happen often, so I'm hoping nobody will
> notice the slowdown with these syncs. I'll take a look at this.
> 
> Thanks for bringing that up.

Requiring an RCU synchronize on element removal is quite intrusive, and can
be problematic if tracepoint removal is called from e.g. preempt-off context.

Thanks,

Mathieu

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